DEPOSITIONAL CONTROLS OF A GUELPH FORMATION PINNACLE REEF DEBRIS APRON AND THEIR EFFECT ON RESERVOIR QUALITY: A CASE STUDY FROM NORTHERN MICHIGAN
Zachary M.K. Cotter
A Thesis
Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
May 2020
Committee:
James Evans , Advisor
Margaret Yacobucci
Yuning Fu
© 2020
Zachary Cotter
All Rights Reserved iii ABSTRACT
James Evans, Advisor
The Middle Silurian-aged Guelph Formation pinnacle reefs and associated deposits of the
Michigan basin (U.S.A.) are a prolific hydrocarbon play, valued for its potential for enhanced oil recovery (EOR) and carbon sequestration. Recent work has aided in resolving reef growth models and complex architecture, however previous studies have been focused on reef development, largely overlooking depositional controls of the leeward debris apron development and implications for reservoir development. This study hypothesizes that the leeward debris apron of Guelph Formation pinnacle reefs accumulated with depositional controls and architectural elements like those of larger, line-fed slope apron systems of carbonate platform margins. This study utilizes a case study well, which was laterally deviated leeward of the reef pinnacle and captured the leeward slope profile of a Guelph Formation pinnacle reef. This study uses 70 m of whole core, 117 core plugs, 16 mercury injection capillary pressure (MICP) curves,
21 thin sections, in addition to a suite of geophysical wireline logs, including borehole image logs, to build a depositional model for the leeward debris apron and evaluate controls on reservoir quality. Core analysis of sedimentary deposits recovered from the well identified 16 lithofacies, interpreted to have been deposited within six facies associations including reef zone, tempestite, debrite-turbidite, subtidal back-reef, intertidal, and supratidal. Stratigraphic analysis revealed that the leeward debris apron developed within two distinct growth stages: (1) a stage correlative to active reef growth and accumulation of the debris apron and (2) a peritidal stage of deposition. Reef growth deposits (stage one) consisted of deepening upward sedimentary successions comprised of skeletal framestones, floatstones, rudstones, grainstones, wackestones iv and intraclastic conglomerates. The vertical succession of these deposits was interpreted to represent the lateral shift of environments downslope from active carbonate factory settings to more distal segments of the leeward slope. Sediment gravity flows, partial Bouma sequences (Ta and Tde), and well-preserved tempestite successions were present in a significant volume of recovered core, interpreted to suggest that off-bank transport via storm-wave resuspension and sediment gravity flows triggered by events of slope destabilization were the primary sediment transport mechanisms feeding development of the leeward debris apron. The vertical succession of deposits of the leeward debris apron of Guelph Formation reefs was found to resemble those of larger, line-fed slope apron systems associated with carbonate platform margins. Guelph
Formation peritidal successions (stage 2) were observed to unconformably overlie subaerially- exposed reef stage deposits and the bottommost contact was interpreted as transgressive surface of erosion. Guelph Formation peritidal deposits consisted of mudstones, cryptalgal bindstones, and skeletal packstones which were interpreted to represent a shallowing-upward transition from subtidal-to-supratidal environments. The contact between the Guelph Formation and Ruff
Formation was observed as a sharp, erosive unconformity interpreted to be a second transgressive surface of erosion with evidence of micro-karsting occurring just below the contact, suggesting the leeward slope underwent a second period of subaerial exposure. A generalized reservoir characterization effort was conducted on Guelph Formation sediments to characterize reservoir quality. Reservoir characterization revealed capillary behavior that can be generalized by three type curves, and the primary pore architecture and textures of sampled
Guelph Formation sediments were overprinted by diagenetic processes including dolomitization, recrystallization, dissolution, stylolitization and fracturing. Tempestites were found to exhibit the best petrophysical character and highest degrees of petrophysical predictability, interpreted to be v a result of high initial bioclastic content and well-sorted textures generated by wave-suspension processes. vi
I would like to dedicate this text to all of those who have helped me get to the point that I am in
life. To my family, professors, colleagues, mentors, friends, and acquittances… Thank you for
the inspiration in my life. vii ACKNOWLEDGMENTS
I would like to thank my adviser Dr. James Evans for his guidance, teachings, patience
and continuing support throughout this study and my greater term at Bowling Green. I would like
to thank my undergraduate adviser Frank Schwartz for his continued mentoring, friendship, and
for continuously pushing me to be the best scientist that I can be.
I would like to thank Battelle’s energy group for giving me the opportunity to grow and
learn as an intern. I would like to thank Autumn Haagsma, Amber Conner, Valarie Smith, Joel
Main, Neeraj Gupta, Bill Harrison, Wayne Goodman, Charlotte Sullivan, Matt Rine, and the
Core Energy LLC. team for their input and assistance in completing this project. I would also like to thank my committee members Peg Yacobucci and Yuning Fu for their assistance in completing this study.
This project was lucky enough to obtain funding from several supporters including the Midwest Carbon Sequestration Partnership (MRCSP), the Geological Society of America
(GSA), the American Association of Petroleum Geologists (AAPG) under the Fred Dix
Memorial grant, and the Bowling Green State University Geology Department under the Richard
Fox Practical Geophysics Grant. A special thank you to the family of Fred Dix and Richard Fox, memorial grants such as these fund so much great science; thank you! vi
TABLE OF CONTENTS
Page
CHAPTER I: INTRODUCTION….…………………………………………………...... 1
Carbonate Reefs ……………………………………………………………………..……… 4
Silurian Carbonate Reefs……………………………………………………………. 5
Leeward Reef Margins………………………………………………………………. 6
Peritidal Carbonates……..…………………………………………………………..……… 8
Carbonate Slope Systems………..…………………………………………………..……… 10
Debris Apron Models ………………………………………………………………. 11
Bioclastic Submarine Fan Models …………………………………………………. 13
Differentiating Debris Aprons from Calciclastic Submarine Fan……..……………. 16
Bioclastic Sedimentation ………..…………………………………………………..……… 17
Tempestites …………………………………………………………………………. 17
Subaqueous Gravity Flows …………………………………………………………. 18
Carbonate Reservoir Characterization .……………………………………………..……… 21
CHAPTER II: GEOLOGICAL BACKGROUND …….………………………………...... 25
Basin History ………….………..…………………………………………………..……… 25
Silurian Stratigraphy ………..…..…………………………………………………..……… 25
Paleogeography and Environment …………………………………………………..……… 26
Guelph Pinnacle Reef Depositional Models….……………………………………..……… 27
CHAPTER III: METHODOLOGY ……………………………….……………………..... 33
Case Study Field History ………..…………………………………………………..……… 33
Whole Core Analysis ………..…….………………………………………………..……… 33 vii
Thin Section Petrography………..…………………………………………………..……… 34
Geophysical Well Log Analysis……………………………………………………..……… 34
Gamma-Ray Logs …….……………………………………………………………. 35
Bulk-Density Logs …………………………………………………………………. 35
Neutron Porosity Logs ……..………………………………………………………. 36
Resistivity Borehole Image Logs……………………………………………………. 37
Constructing the Geophysical Log Model ……….…………………………………..……… 38
Core Plug Analysis ….….………..…………………………………………………..……… 39
Mercury Intrusion Capillary Pressure (MICP) Analysis ….………………………..……… 41
CHAPTER IV: RESULTS …...... 46
Lithofacies Analysis ……………..…………………………………………………..……… 46
Dolomitic Mottled Mudstone (Mm) ….……………………………………………. 46
Laminated Dolomitic Mudstone (Ml) ………………………………………………. 47
Pale Grey Mudstone (Mpg) ……..…………………………………………………. 48
Dolomitic Skeletal Wackestone (Sw)………………………………………………. 50
Dolomitic Skeletal Packstone (Sp)…….……………………………………………. 51
Dolomitic Skeletal Grainstone (Sg) …..……………………………………………. 52
Dolomitic Mottled Skeletal Floatstone (Sfm)………………………………………. 53
Dolomitic Cryptalgal Intraclastic Floatstone (Ifcr)…………………………………. 55
Dolomitic Intraclastic Boulder Floatstone (IFb) ……………………………………. 56
Coated Grain Dolomitic Rudstone (Rcg)……………………………………………. 58
Massive Amalgamated Skeletal Rudstone (Ram)……………………………………. 59 viii
Cross-bedded Skeletal Rudstone (Rcb) ………..……………………………………. 60
Heterolithic Crypt-Algal Bindstone and Clotted Mudstone (Hcr)….………………. 61
Heterolithic Disturbed Crypt-Algal Bindstone and Skeletal Packstone (Hcrd)..…… 63
Wrinkled Algal Bindstone (Bcr)……………………………………………………. 64
Dolomitic Coral-Stromatoporoid Framestone (Fr)…………………………………. 65
Lithofacies Associations ………..…………………………………………………..……… 66
Supratidal Facies Association (FA1) ……..…………………………………………. 66
Intertidal Facies Association (FA2) ……...…………………………………………. 67
Subtidal Back-Reef (FA3) ………..……...…………………………………………. 68
Debrite-Turbidite Facies Association (FA4) …….…………………………………. 69
Tempestite Facies Association (FA5) …....…………………………………………. 71
Skeletal Reef Facies Association (FA6) …....………………………………………. 72
Depositional Environments ………..………………………………………………..……… 73
Reef Depositional Stage …………..……...…………………………………………. 73
Peritidal Depositional Stage ………..……...……………………………….………. 76
Stratigraphy ……………………..………..……...…………………………………………. 77
The Guelph Dolomite Formation ………...…………………………………………. 77
Ruff and Guelph Dolomite Formation Contact…….………………………………. 80
The Lower Ruff Formation …………..……...………………………………..……. 36
Geophysical Log Analysis ….…..………..……...…………………………………………. 82
Geophysical Log Model …….…..………..……...…………………………………………. 83
Reef Association Geophysical Log Profile ………...………………………………. 84
Proximal Debris Apron Association Geophysical Log Profile …….………………. 85 ix
Peritidal Environment Geophysical Log Profile..……………………………..……. 87
Reservoir Characterization …….…..………..……...………………………………………. 88
Guelph Formation Core Analysis …………………..………………………………. 89
Reef Association Core Analysis ……….…………..………………………………. 92
Proximal Debris Apron Core Analysis ……………..………………………………. 94
Distal Debris Apron Core Analysis ……….………….……………………………. 96
Intertidal Association Core Analysis ………..……..………………………………. 98
CHAPTER V: DISCUSSION ……………………………...... 100
Depositional Environments …….…….……..……...………………………………………. 100
The Reef Core Environment ………………………..………………………………. 100
The Debris Apron Environment ………..…………..………………………………. 103
Role of Storm-Influence on Debris Apron Development ….………………. 104
Role of Sediment Gravity Flows on Debris Apron Development ….………. 106
Comparison to the Mullins and Cook (1986) Slope Apron Model ….……... 110
Sequence Stratigraphic Framework ….……..……...………………………………………. 112
Controls on Reservoir Quality and Development …..………………………………………. 114
Future Work ……………………..………..……...…………………………………………. 119
CHAPTER VI: SUMMARY & CONCLUSIONS …………………………..……………. 121
REFERENCES ……………………………………………………………………………. 129
APPENDIX A: TABLES ………………………………………………………………...... 140
APPENDIX B: FIGURES ……………………………………………………………...... 152
APPENDIX C: PETROPHYSICAL DATA ...……………………………………………. . 198
1
CHAPTER I: INTRODUCTION
The Middle Silurian-aged pinnacle reefs of the Guelph Formation (“Brown Niagaran”) are a unique example of reef growth style within the geologic record, due to their immense vertical profiles (up to or greater than 150 m) occurring as large isolated “pinnacle” paleotopographic structures. Models of pinnacle reef growth have remained controversial for nearly sixty years, however recent work by Rine (2016, 2017) has aided in constraining models of reef growth and complex architecture. Recent findings indicate these reefs served as sources for large amounts of reef-derived debris that accumulated with reef growth, forming debris apron and talus deposits along the leeward and windward reef flanks (Rine 2016, 2017). Previous reef growth models have largely overlooked the importance of windward and leeward slope environments.
Subsequentially, very little is known about the leeward slope apron environment and its sediment transport mechanisms, stratal architecture, reflectance of organic reef growth, or controls on reservoir quality. This study hypothesizes that the debris apron of the Guelph Formation pinnacle reefs features predictable stratal architecture that is comparable to the larger-scale, line-sourced slope apron model developed by Mullins and Cook (1986). Furthermore, this study hypothesizes that slope sedimentation processes known to yield desirable reservoir quality in other basins
(such as the Permian Basin), are also very likely present within the leeward debris apron of the
Guelph Formation reefs. This investigation will use a case study well to develop a geological model for the leeward debris apron environment of Guelph Formation pinnacle reefs, identify major sediment transport mechanisms, and correlate how debris apron sediment transport processes affect reservoir quality. 2
Since discovery of the Guelph Formation pinnacle reefs, they have served as prolific
hydrocarbon plays within the Michigan Basin, producing over 500 million barrels of oil and 2.9
trillion cubic feet of natural gas, making it one of the largest oil producers in the United States
(Rine, 2016). Guelph Formation pinnacle reefs are completely encased within hundreds of
meters of evaporite and carbonate lithologies of the Salina Group, which created localized petroleum systems (Suhaimi 2016; Rine, 2016; Friedman and Kopaska-Merkel, 1991). Recently,
interest in these reefs has been renewed due to their potential as enhanced oil recovery (EOR)
and carbon sequestration targets. While many of the discovered reefs have undergone primary
recovery, only 5% of these reefs have undergone any kind of EOR development, creating a
significant opportunity for post-primary hydrocarbon development. It is estimated that future
EOR operations within the Michigan basin could produce between 180-to-200 million barrels of
hydrocarbon reserves (Grammar et al., 2008).
Despite previous success as a hydrocarbon play, efforts to generate stratigraphic models
of Guelph Formation pinnacle reefs and related strata were controversial due to disagreement
regarding the timing of deposition of inter-reef evaporites in reference to reef growth stages
(Rine 2016; Gill,1973; Huh 1973). Furthermore, the Guelph Formation pinnacle reefs have
undergone a complex diagenetic history, being affected by freshwater leaching, multiple
episodes of dolomitization, and the precipitation of void filling evaporite minerals, which greatly
affected rock lithology, textures, and distributions of porosity and permeability of the reefs
(Friedman and Kopaska-Merkel, 1991). The combination of complex geologic and diagenetic
histories often results in a difficulty correlating geology to reservoir quality (Suhaimi, 2016;
Friedman and Kopaska-Merkel, 1991). Within this study, reservoir quality refers to the relative 3
capacity of a formation, facies or lithology to store and transmit fluids and gases such as
hydrocarbons, CO2 and natural gas.
This study advances our understanding of marginal pinnacle reef environments of the
Guelph Formation by characterization of the leeward slope apron environment of a case-study
reef field. In 2016, a CO2 injection well named the Lawnichak and Meyer 9-33 well (hereby
referred to as the 9-33 well), was drilled by Battelle Memorial Institute and Core Energy LLC.,
under the Midwest Carbon Sequestration Partnership (MRCSP) to support a CO2-driven EOR
and associated carbon storage demonstration within the Dover 33 field, a pinnacle reef field
located in Otsego County, Michigan (Fig.1). This injection well functioned as a characterization
well, featuring approximately 70m (230 ft) of whole core, 117 core plugs, 16 mercury injection
capillary pressure (MICP) samples, and 21 thin sections, in addition to a suite of geophysical
logs including gamma-ray (GR), neutron porosity (NPHI), bulk density (RHOB) and resistivity-
based borehole image logs (Baker Hughes STAR tool). The 9-33 well was drilled along the
leeward profile of a pinnacle reef, deviated roughly-oblique to the pinnacle reef structure itself,
yielding a full lateral profile of the leeward-reef slope environment. During drilling operations of this well, whole rock core was recovered from the Lower Ruff Formation through Guelph
Formation, with deposits thought to represent leeward slope and reef zone deposits (Battelle,
2016). Due to the wealth of data available, this MRCSP characterization well offers a premier opportunity to characterize the leeward slope apron environment of the Guelph Formation pinnacle reefs, gaining insight into the depositional processes, architecture, diagenetic alterations, petrophysical properties and geophysical log profiles that represent the leeward slope
apron environment. 4
This subsurface facies analysis of the leeward debris apron environment of a Guelph
Formation pinnacle reef was conducted with the following objectives: (1) to generate a detailed depositional model for the leeward debris apron environment; (2) to generate a geophysical log model of debris apron successions of Guelph Formation from the 9-33 well; (3) to characterize variations in pore architecture (pore type, pore throat sizes), porosity, and permeability across the debris apron environment and; (4) to correlate controls on reservoir quality to various sediment transport processes observed within the debris apron environment. Study results will contribute to understanding the evolution of localized, pinnacle reef slope apron deposits and aid in the development of EOR, carbon sequestration, and gas-storage strategies within the Michigan basin.
Carbonate Reefs Carbonate reefs are affected by biological, climatological, and oceanographic factors, making them excellent knowledge repositories that are relevant to a variety of scientific disciplines. Ancient reefs are economically significant as prolific hydrocarbon reservoirs.
Biogenic construction of reefs dates to the Archean, where low-relief microbial mounds may be observed. Carbonate reefs are biologically constructed features that grow vertically from the seafloor and can become rigid, wave-resistant structures (James and Wood, 2010). The character of any reef is highly dependent on the types of organisms alive in that interval of time. The initiation and growth styles of reefs is dependent on both autogenic and allogenic conditions. In modern times, autogenic processes include the combination of light intensity, sedimentation rate, salinity, temperature, nutrient levels, and water turbulence with optimal reef growth occurring with a combination of these factors, termed the “carbonate window”. These factors control growth patterns of metazoan colonies which build reef relief. Ancient autogenic conditions may be drastically different from modern times, making the application of “actualism” concepts to 5
ancient reefs problematic. Allogenic conditions affecting reef growth include long-term changes in the carbonate window, presence of antecedent topography, and changes in relative sea-level
(James and Wood, 2010).
Silurian Pinnacle Reefs Pinnacle reefs are defined as localized biogenic marine structures with >10m of original relief. The development of pinnacle reef geometries has been found in numerous geological periods; however, development was most pronounced during the Silurian and Devonian
(McLaughlin et al., 2019). During the Silurian period, reef development was prolific globally, generating major reef structures in North Greenland, Sweden, and Eastern Canada. However, the most concentrated number of Silurian carbonate reefs developed within the Great Lakes region
(Ontario, Michigan, Wisconsin, New York, Ohio, Indiana, Illinois, and Iowa).
The geometry and vertical successions of Silurian reef complexes vary greatly according to regional controls, although the main reef building biota (i.e., stromatoporoids and tabulate corals) are consistent globally (Ritter, 2008). A prominent Silurian reef-builder, stromatoporoids are an extinct group of carbonate sponges which constructed domal, encrusting, or branching calcareous skeletal structures. Another Silurian reef builder, tabulate corals (phylum Cnidaria), were colonial invertebrates which produced calcareous exoskeletons that developed either laminar, massive, mound-like, encrusting or erect-branching geometries that were restricted to the photic zone (Babcock, 1996).
Generally, the extensive development of reefs during the Silurian is thought to record a transitional climate (Ritter, 2008; Copper and Brunton, 1991) following “icehouse” conditions at the end of the Ordovician. Additionally, extensive carbonate production was further enhanced by 6
global eustatic transgressive conditions. However, paleoclimatic models of Silurian marine
environments are still evolving to this day. McLaughlin et al. (2019) examined Silurian pinnacle
reef growth across the Midwest and found a correlation between pinnacle reef growth and
13 δ Ccarb excursions, suggesting that reef proliferation may be linked to perturbations of the
Silurian carbon cycle, and that rapid Silurian sea-level rises may be linked to transient releases of methane and subsequent warming pulses.
Leeward Reef Margins Reef margins are relatively steep slopes adjacent to carbonate factories, and are generally classified as either windward or leeward margins (Playton et al., 2010; Hine et al., 1981).
Windward slope margins (also known as the fore-reef), are typically ramp or escarpment slope geometries that may feature bypass zones, where sediment is transported from shallow to deeper environments without significant deposition along portions of the slope. Reefs also develop leeward margins, which face opposite the dominant wind direction. Leeward margins typically exhibit much shallower slope angles than windward margins, and often form as accretionary carbonate debris aprons (Playton et al., 2010).
Sedimentation within the leeward reef margin environment is generally driven by combination of mechanisms which deposit bioclastic sediment across shallow and deep margin settings (Tournadour et al., 2015; Hine et al., 1981). Leeward reef margin sedimentation mechanisms include: 1) off-bank transport of bioclastic sediment to shallow and deep slope segments via tidal currents and wave resuspension processes (Pilskaln et al., 1989; Hoskin et al.,
1986; Hine et al., 1981); 2) slope destabilization events (slumps and slides) which initiate
downslope transport via sediment gravity flows (debris flows and turbidites) (Tournadour et al.,
2015); 3) bottom-current transport mechanisms (Tournadour et al., 2015; Hine et al., 1981) and 7
4) autochthonous sedimentation generated across the leeward margin by pelagic and benthic
carbonate producers.
This assemblage of sedimentation processes occurring within the leeward reef margin environment occur contemporaneously, however off-bank transport has been found to be the primary mechanism for feeding sediment into the leeward reef margin environment (Tournadour et al., 2015; Hine et al.,1981). In a study investigating modern off-bank transport with leeward bank margins of the Bahamas, Hine et al. (1981) found most of the sediment flux generated through off-bank sediment transport occurred during major storm events. This interpretation was based on the observations of: (1) the presence of seagrass and algal mat patches on sandy leeward substrates; (2) the occurrence of vertically stacked, buried submarine-cemented horizons within winnowed, bioclastic grainstone bodies, observed at both core and seismic scales; (3) negligible volumes of sand transported by large subtidal waves; and (4) critical shear stress values of sediment were rarely exceeded during normal tidal cycles, all of which suggested minimal transport occurred during normal tidal and wave conditions. Hine et al. (1981) interpreted the presence of seagrass colonies and algal mats on sandy substrates, and submarine cement horizons within bioclastic grainstones, to suggest relatively low sedimentation rates and excessive cementation of previous storm deposits during fairweather conditions. Only during major storms do leeward margins experience high sedimentation rates, generating thick accumulations of storm deposits. However, the study by Pilskaln et al. (1989) characterizing off- bank transport to peri-platform environments argued that normal tidal currents and wave action forces are sufficient to export muddy sediment off bank margins. The study by Hine et al. (1981) also observed a large contrast between the number of buried-cemented horizons (16) and the much larger number of hurricane occurrences (~3200 storms on record), suggesting that each 8
unit may not represent individual storms, but rather relative periods of increased storm activity
separated by long periods of quiescence.
Several studies have noted the importance of deep-water contour currents for the development of leeward margins (Tournadour et al., 2015; Hine et al., 1981). When examining the Little Bahaman Bank’s leeward margin, Hine et al. (1981) described a dynamic boundary below the Florida Current (~400m), where little shallow water derived sediment is transported, instead being dominated by lithoherms and hardgrounds. Tournadour et al. (2015) found that longitudinal currents physically rework distal slope environment of the Little Bahama Bank, forming contourite-related drifts, channel “moats” and small-scale sedimentary structures, arguing these currents may influence the development of mass wasting escarpments and sediment gravity flows.
The slopes of the leeward slope apron can be destabilized and generate subaqueous landslides or numerous bypass structures, such as gullies or canyons. Gullies and canyons produced by slope failures have been found to create localized controls, directing debris flows and turbidity flows in addition to providing accommodation space for the accumulation of debrites and turbidites within the slope environments. Potential triggers for slope destabilization landslides include diapirism, large storms, high sedimentation rates, high pore pressures, intensification of bottom currents, or seismic activity. Sediment failures along the leeward slope affect the overall prograding/aggrading trends of the slope system (Tournadour et al., 2015).
Peritidal Carbonates Carbonate tidal flats are excellent records of changes in sea-level and may produce stratigraphic traps with predictable distributions of reservoir properties (Shin, 1983). Peritidal 9 carbonate systems are divided into three sub-environments: supratidal, intertidal, and subtidal.
Peritidal deposits may exhibit shallowing-upward cycles as a response to processes including subsidence, shoreline progradation, accommodation space changes, eustasy, in addition to autogenic processes. Typical Paleozoic peritidal shallowing-upward successions (Fig.2) consist of bioturbated-to-clotted wackestones, packestones, or patch reefs which grade upwards into intertidal cross-laminated bioclastic grainstones, mudstones and domal stromatolites, which are overlain by supratidal microbial laminates interbedded with intraclastic rudstones and may feature desiccation cracks (Pratt, 2010).
The subtidal zone develops as a belt seaward of the intertidal zone, and is predominantly muddy sediment, serving as a line source for the intertidal zone. Depending on geographic setting, the subtidal zone may extend 10’s of kilometers in width, and feature ooid sand bars, patch reefs or grainstone ramps at the seaward terminus (Shin,1983). Subtidal muds are typically pelleted or destratified from benthic burrowing, yielding a lack of primary structures and a characteristic mottled-gray color. Paleozoic subtidal successions feature interbeds of storm deposits (bioturbated, bioclastic rudstone, packstone, and wackestone) which grade upwards into shoreface deposits (peloidal, carbonate sands that feature cross-stratification and ripple marks)
(Pratt, 2010; Shinn, 1983).
The intertidal zone is located between normal high and low tide, accumulating as a belt between the seaward subtidal zone and landward supratidal zone. Generally, intertidal sediments lack preserved lamination (unless the biota are stressed), exhibit more oxidized coloration, lower fossil diversity, and if bioturbation is low-to-absent, may exhibit algal laminites with fenestral porosity (Shin, 1983). Paleozoic intertidal successions begin with a gradational contact from underlying subtidal deposits, which then grades upward into bioturbated, argillaceous 10
wackestone or mudstone, interbedded with occasional bioclastic peloidal grainstones and can be
capped by domal-to-hemipherical stromatolites or supratidal laminites (Fig.2) (Pratt, 2010).
The supratidal zone is located above normal or mean high tide, and is subaerially exposed
the majority of the time. These sediments only become flooded during spring tides and storm
surges. Key diagnostic features of the supratidal zone include mudcracks, lamination, algal
structures, bird-eye or fenestral porosity, and the presence of intraclasts (Fig.2). Arid supratidal
environments also include teepee structures, desiccation cracks, paleokarst, breccias, and
evaporites such as gypsum and anhydrite (Fig.2) (Pratt, 2010; Shin,1983).
Carbonate Slope Systems Carbonate slope environments are a broad category of features (ramps, fans, escarpments) which connect shallow water to deeper water settings. Slope systems provide
stratigraphic records reflecting the growth, evolution, and depositional conditions of the up-slope
carbonate factories. Slope settings are complex to study as they are comprised of systems controlled by coeval intrinsic factors and extrinsic factors (i.e., dynamics of up-dip carbonate factory), making the application of generalized models difficult (Playton et al., 2010; Payros and
Pujalte, 2007). Furthermore, these systems are poorly understood due to the lack of quality outcrops, limitations of seismic imaging and poor hydrocarbon production as opposed to their siliciclastic counterparts (Playton et al., 2010; Cook and Mullins, 1983).
Within carbonate slope settings, sediment is sourced from the platform top and margins, the carbonate slope itself, and the overlying water column. However, sedimentation is primarily driven by the combination of off-bank and down-slope transport processes. The resulting sediments are primarily bioclastic, meaning the original carbonate sediment was formed by 11
primary production and was then removed from an existing source, transported for some distance
and subsequently redeposited (Playton et al., 2010; Payros and Pujalte, 2007; Mullins and Cook,
1986). Slope environments produce deposits which typically fall into one of two depositional
categories, point-sourced calciclastic fan deposits or line-sourced calciclastic apron deposits.
Debris Apron Models Debris aprons are a type of carbonate slope system, first conceptualized by Mullins and
Cook (1986), that differ from submarine fans due to the fact that they are line sourced. Debris aprons develop as largely unchanneled, debris-flow rich environments, fed by multiple feeder systems. The feeder channels typically exhibit well-defined “V”-shaped heads upslope and become more “U” shaped at their mouths. Modern debris aprons exist as relatively smooth, gently seaward dipping (0.5-2.0°) surfaces, fed by numerous small, closely spaced upper slope feeder channels (Mullins and Cook, 1986).
Two end-member debris apron depositional models were created by Mullins and Cook
(1986), the “slope apron” and “base-of-slope apron” models (Fig.3). Slope aprons (Fig.3A) are most abundant along carbonate platform margins that feature gentle (<4°) slope gradients. These slope systems do not exhibit a sediment by-pass zone, meaning that sedimentation occurs along the entirety of the slope system. Sediment gravity flows such as turbidites and debrites are common in these environments, however, sediment gravity flows do not produce systematic vertical cycles. Instead, the mass-transport facies are randomly distributed as extensive broad sheets across the slope.
In contrast, base-of-slope apron models (Fig.3 B), exhibit an upper-slope bypass zone often consisting of calcareous oozes produced by plankton in the overlying water column and 12
small canyons infilled by mass transport deposits, with occasional sheet flow deposits. These
aprons develop along relatively steep (>4°) platform margins and may exhibit thickening-upward
cycles related to progradation of the escarpments and may include numerous submarine slides
and slump events (Mullins and Cook, 1986).
Carbonate slopes, such as those described by Mullins and Cook (1986), may be
subdivided into four environments: a platform margin (only in base-of-slope apron models),
inner (proximal) apron, outer (distal) apron and basin plain. The platform margin is a zone of coarse-sediment representing sediment bypassing via line-sourced canyons, with finer-grained deposits accumulating on inter-canyon highs. The platform margin is recognized by the dominant occurrence of well-bedded carbonate mudstones, and small canyons filled with coarse debris. The platform margin may experience submarine sliding (Mullins and Cook, 1986). The proximal apron is characterized by muddy debrites, proximal turbidites (thick sheets of normally to inversely graded Bouma Ta divisions) and possible megabreccias, depending on the
environment. Proximal aprons are likely to exhibit evidence of broad, shallow upper slope
channels infilled with coarse grained sediment. Distal apron environments are dominated by
turbidites (Bouma Ta divisions interbedded with thinner, finer-grained Tb-e divisions) and clast-
supported reworked conglomerates. Basin plain deposits consist of distal carbonate turbidites
(Tde) interbedded with peri-platform and pelagic oozes. Basinal environments have the potential
for accumulation of episodic coarse-grained sediment acquired during slope/margin collapse
events. Carbonate slopes may be progradational, with these slope environments prograding over
and encapsulating one another (Fig.4). Thus, in both depositional models the vertical successions
of these environments will be similar. However, it is thought that slope apron systems develop in 13
a less-organized manner (Fig.3A) than the steeper base-of-slope aprons due to the presence of the upper slope by-passing zone (Fig.3B) (Mullins and Cook, 1986).
Mullins and Cook (1986) found that sediment gravity flows such as turbidites and debrites were major architectural elements of debris apron systems, and interpreted sediment gravity flows to be the main sediment transport mechanism for distributing sediment across the slope apron. Sediment gravity flow deposits often exhibit turbidites overlying debrites interpreted to show the development of a basal debris flow and an overriding turbidity flow as the gravity flow moved down slope (Fig.5). In the rock record, this relationship produces a succession of thick, mud-supported conglomerates (interpreted to be proximal debrites) that spatially thin and transition downslope into clast-supported textures, while developing coeval overlying turbidite deposits. The overriding turbidity flow deposits transition downslope from graded bioclastic deposits (Ta-b), to more complete Bouma sequences (Ta-e) overlying the clast-
supported distal debrites as the flow moves further downslope to basinal thin bedded turbidites
(Tde) (Fig. 4).
Bioclastic Submarine Fan Models Bioclastic submarine fans are a type of mass-transport deposit which consists of
channelized-to-unchanneled environments developing from a single feeder (point source) system
(Fig.6). Point-sourced fan models are well known in siliciclastic sedimentology, however, bioclastic submarine fans are rare in the geological record, and no modern analogues are known to exist (Payros and Pujalte, 2007). Due to a lack of modern examples, no depositional model exists for bioclastic submarine fans; instead these deposits are interpreted using models developed for siliciclastic fans. Subsequentially, controls on the development of bioclastic submarine fans are poorly understood (Payros and Pujalte, 2007). 14
As Payros and Pujalte (2007) state, submarine fans are constructional seafloor features
which develop seaward of a single sediment source or beyond a main cross-slope supply route
(i.e., canyons, gullies, or troughs) at the base of the slope (Fig.6). Bioclastic submarine fans are
predominantly comprised of interbedded turbidites and debrites, but also consist of other types of
sediment gravity flows and hemipelagic deposits. Lithologies of bioclastic fans consist primarily
of skeletal calcarenites (packstone to grainstone), calcisiltites (silt-sized grains), calcirudites
(rudstone) and calcilutites (mudstones). Bioclastic fan deposits are highly variable, but may be
lumped into four major types: (1) tabular beds of laminated, fine-grained calcarenites and
calcisiltites, interpreted to be deposited by low-density turbidity flows; (2) irregular, erosive-
based and normally-to-inversely graded mixtures of calcirudites and coarse calcarenites,
interpreted to be deposited by high-density turbidity currents beneath which hyper-concentrated
debris flows developed; (3) disorganized, poorly sorted rudstone with calcarenite matrices
formed from intergranular frictional freezing of high-concentration debris flows; and finally (4)
chaotic structureless mixtures of calciclastic and muddy sediment, observed as matrix-supported
floatstones interpreted to be debrite deposits (Payros and Pujalte, 2007).
Bioclastic fan systems feature four environments (Fig.6). The uppermost zone consists of downslope coalescing outer platform-to-upper slope channels and gullies forming a sediment tributary system. Within this zone, debrites, slumps, slides, and turbidites infill gullies/channels.
These tributary channels coalesce into a main feeder conduit, occurring as either a canyon or leveed channel. These channels are infilled by debrites and turbidites, organized into thinning and fining-upward packages along the channel axis. Channel axis deposits may crosscut hemipelagic muddy sediment, accumulated along marginal levees. The main feeder channel zone feeds into unconfined lobes and sheet deposits. Unconfined zone deposits consist of thick 15
accumulations of clustered, amalgamated turbidites separated by hemipelagic and fine-grained
bioclastic deposits. Within the areas proximal to the main feeder channel, the unconfined zone
can be crosscut by minor distributary channels filled with lensoidal, concave-plane coarse-
grained calciclastic deposits. The unconfined zone grades into the peripheral fan fringe,
characterized by unconfined thin-bedded, low-density calciturbidites interbedded with basin
plain deposits (Payros and Pujalte, 2007).
A comprehensive review of existing carbonate fan literature by Payros and Pujalte (2007) found that the size and general character of fan complexes is largely determined by the grain-size from which sediment is sourced. Due to this, three depositional models exist for coarse-grained, medium-grained, and fine-grained carbonate fan systems. With decreasing grainsize, the carbonate fan complex generally become more laterally extensive and sheet-like. Each model
provided by Payros and Pujalte (2007) exhibits its own distinct depositional architecture.
To date, the main controls on the development of carbonate fan complexes are poorly
understood, however some general trends exist. Similar to many siliciclastic submarine fans that
are generally fed across a slope canyon, carbonate fan systems initially form line-sourced upper slope environments, only becoming point-sourced feeder systems downslope upon coalescence of feeder tributaries. This implies that a mechanism is needed to funnel sediment gravity flows to form a main axial channel in order to develop a carbonate fan. Conversely if this intra-slope funneling mechanism is not present, the system will develop as a carbonate apron. Within ancient carbonate fan systems, this funneling mechanism might be provided by tectonically induced, down-slope directed depressions, acting as sediment conduits. However, it is more likely that inherited topography or mass failures may act as funneling mechanisms (Payros and
Pujalte, 2007). Almost all ancient carbonate fan systems developed along low-angle slopes, 16
sourced from distally steepened carbonate ramps subjected to high-energy currents. On leeward
margins these settings are promoted by environmental conditions which hampered reef formation
(i.e., colder water or higher turbidity).
Reefal environments typically do not promote carbonate fan development. This is due to
the coherence of boundstones which favor linear-mass wasting events that produce sediment
gravity flows which “freeze” downslope, coupled with the fact that re-sedimentation of reefal
sediment seldomly produces fluid gravity flows (turbidity currents) that can be easily
incorporated into feeder channels. Finally, ancient carbonate fan complexes developed mainly
during sea level low-stands when changes in pore pressure destabilize sediments and base level
fall brings carbonate factories closer to the slope break (Payros and Pujalte, 2007).
Differentiating Debris Aprons from Submarine Fans Debris aprons occur as laterally continuous, linear-shaped, slope complexes (Fig.3),
which may be readily distinguished from the isolated lobe-like character of carbonate fans
(Fig.6), if enough exposed outcrop or core data is available to resolve the complex geometry.
However, such optimal situations are rare, resulting in facies associations being key to the correct classification of the bioclastic slope complex. Within debris apron models developed by
Mullins and Cook (1986), debris flows are primary architectural components, while in carbonate
fan models debris flows are subordinate to turbidites. Debris aprons that develop with
unchanneled sheet-like deposits will have relatively low stratal organization when compared to carbonate fan systems. Carbonate fan systems feature channelized deposits cross-cutting hemipelagic and pelagic sediments across the entire slope system, resulting in an abundance of coarse-grained deposits being confined to channels, rather than in levee, overbank and peripheral zones (Payros and Pujalte, 2007; Mullins and Cook 1986). 17
To date, the occurrence of reef-derived slope deposits developing along the leeward
flanks of localized Silurian reef pinnacles is an understudied topic, receiving preliminary
examination by Rine (2016, 2017), as an architectural element and depositional assemblage of
the pinnacle reef complex. Due to the localized occurrence (few km2 scale) coupled with the immense topography created by pinnacles (>150m), it can only be assumed that depositional controls and architecture would resemble those of larger-scale line sourced slopes occurring down-dip from platform margins, or atolls. However, due to the discontinuous and localized nature of these pinnacle reefs, there may be a chance that the leeward pinnacle reef slope developed as a source-fed submarine fan. This study will examine whether leeward slope deposits of Silurian pinnacle reefs are line source debris aprons or point source submarine fan environment.
Bioclastic Sedimentation Tempestites (Storm Deposits) Tempestite (storm) deposits are sedimentary deposits that constrain depositional environment (i.e., proximity to shore) and can be used as elements in sequence stratigraphic
analysis (Sageman, 1996). Additionally, due to the abundance of grain-supported fabric,
tempestite successions are promising as oil traps (Perez-Lopez and Perez-Valera, 2012). The
study of tempestite successsions is challenging as the bulk majority of these deposits are not
preserved above fairweather wavebase due to storm-wave reworking, bioturbation, and
diagenetic processes (Zhou et al., 2011).
The base of a tempestite is marked by an erosive contact, followed by grain-rich deposits
that normally grade upward to undulatory bedded or wave-rippled calcarenite. Key features to
recognize tempestites are the presence of features produced by oscillatory flow regimes. Wave 18
processes often generate signature characteristics such as undulatory, swaley, or hummocky
stratification, and symmetrical ripples (Fig.7). At core-scale, hummocky or swaley stratification would appear as undulatory bedding. Other common sedimentary features observed in tempestite successions include: various sole marks (pot and gutter casts), graded bedding, parallel alignment of shelly debris, planar lamination and escape burrows. Various tempesitite features are known to occur as a function of proximity to sediment source (Fig.7) (Perez-Lopez and Perez-Valera,
2012; Flugel, 2004).
Subaqueous Gravity Flows
Sediment gravity flows are important sediment transport processes, creating some of the
largest and thickest sediment accumulations measured within the geological record (Talling et
al., 2012). These deposits are important recorders of organic carbon transport to the deep ocean,
acting as a key agent of the global carbon cycle, in addition to being critical in hydrocarbon
systems. There exist a variety of sediment gravity flows, including debris flows, grain flows and
turbidity flows.
Turbidites are sedimentary deposits formed by turbidity currents, where sediment is
supported by the upward component of fluid turbulance as it moves down a slope. Turbidite
successions vary greatly in observed sedimentary structures, bedding thickness and textural
features. Variations in turbidite successions are controlled by the distance from source areas,
sediment composition and concentration within the flow, and topography of the depositional
area (Flugel, 2004).
Idealized siliciclastic turbidite successions are comprised of a series of sequential units
termed the Bouma sequence (Fig.8). Described stratigraphically, the lower most division (Ta) 19
consists of a graded or massive sandstone unit, followed by a upper plane bed sandstone (Tb),
current ripple laminated sandstone with several sets of unidirectional foreset bedding or convolute bedding (Tc), laminated siltstone (Td) that is often obscure within fine-grained
sediment, capped by a massive, stuctureless mudstone (Te) (Fig.8). Complete Bouma sequences
are typical of proximal turbidite deposits, with the sequence becoming more incomplete with
increasing distance from the source (Flugel, 2004; Bouma, 1962).
Carbonate turbidites differ significantly from their siliclastic counterparts due to the size
and variability of bioclastic grains, abundance of lithified particles from cementation, and the
thixotropy differences between calcareous muds and siliciclastic muds, all of which lead to
incomplete Bouma sequences being observed in carbonate turbidites. Subsequently, carbonate
turbidites are described by the Meischner sequence (Meischner, 1964) (Fig.9).
The Meischer sequence is correlative to the Bouma model and consists of four distinct layers (Fig.9). Underlying deposits are characterized by micrites or marls with scattered small lithoclasts and large rock fragments. The lowest turbidite layer (bed 1) is the main phase and is divided into three parts: bed 1a, bed 1b, and bed 1c. Bed 1a (correlative to Bouma Ta division) is
characterized by distinctly graded limestones with shallow water fossils and lithoclasts, with
potential for reverse grading and grain imbrication to be seen. Bed 1b (correlative to Bouma Ta
division) consists of a fine-grained micrite and bed 1c consists of faintly laminated micrite that
contains angular limestone clasts and micrite pebbles (correlative to Bouma Tb division). Bed 2
(correlative to Bouma Tc division) is divided into two parts, bed 2a and bed 2b, where bed 2a consists of planar bedded micrite and is overlain by bed 2b with current ripple laminations and sometimes convolute bedding. Bed 3 (correlative to Bouma Td division) is characterized by 20
marls and may exhibit flaser textures, gradually merging into overlying marly or pelagic
sediment (Meischner, 1964).
The Bouma and Meischner models are similar but some key differences exist. One key
difference is carbonate turbidites are gravel-rich compared to siliciclastic turbidites.
Additionally, the possibility of reverse grading near the base of bed 1 is more common in
carbonates than in the siliciclastic Boumas Ta divison. Furthermore the differentation between
bedform laminations of Bouma divsions Tbd are much less developed in bed 2 of the Meischner
model (Flugel, 2004).
Debris flow deposits are another type of sediment gravity flow, where debris is supported
by a cohesive matrix (mixture of interstital water and muddy sediment), which has a finite yield
strength. The cohesive nature of the flow is the main discriminating factor of debris flows from
other types of sediment gravity flows (Dasgupta, 2003). Subaqeous debrites are tabular-to-
lenticular bodies with erosional basal contacts overlying muddy sediments. Many debrites are
coarse-grained conglomerates that exhibit poor sorting, a lack of stratification, and are often
observed with random-to-chaotic clast fabrics. There are no preferred depositional fabrics within
carbonate debrite deposits with the exception of crude grading at the base, the possibility for
large clasts projecting from the top of beds, and the occurrence of either fossiliferous and
nonfossiliferous matrices supporting clasts (Flugel, 2004).
Grain flow deposits are a type of sediment gravity flow where the flow is supported by
direct grain-to-grain interactions acting within a matrix consisting of interstitial water and sand-
sized sediment. Grain flow deposits are massive-to-thinly bedded deposits, consisting of
inversely graded sand-sized sediment that drapes irregular topography and thins upwards along 21 channel flanks. Beds exhibit flat-tops and flat-bases with potential for flute marks, scours, and injection structures to be observed at the base of beds. Grain flow deposits may be fully-to- partially clast supported, poorly sorted with clasts floating in sandy or muddy matrix, and may exhibit parallel grain orientation and dish structures.
Carbonate Reservoir Characterization Reservoir characterization methods aim to map the distribution of rock properties such as porosity, permeability, and fluid saturation. Porosity is a scalar property that is defined as the capacity of a rock or material to store fluids, calculated as the fractional or percentage ratio of the pore volume divided by the bulk volume of the material (Cannon, 2016; Lucia, 2007). Porosity is described in the equation below:
= [Equation 1] 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 Φ 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 Where:
Volumepore is the volume of pore space,
VolumeTotal is the total volume of the rock
Φ is porosity
Porosity occurs within rocks as 3-dimensional voids which are connected through small apertures (or pore throats) (Suhaimi, 2016). The porosity of a carbonate rock sample is comprised of two elements: primary (depositional) and secondary (diagenetic) porosity. Primary porosity forms prior to lithification, while secondary porosity forms and both may be subsequently destroyed during diagenesis (Flugel, 2004). 22
Permeability is the rate that a fluid moves through the pore spaces within a rock.
Permeability is a vector property where horizontal and vertical permeability values often differ
significantly, and values change according to the scale of the sample measurement (core plug,
well log or production test). Permeability is quantified using Darcy’s law and can be reported in
millidarcies (mD), as shown in equation 2 (Lucia, 2007) below:
= [Equation 2] 𝜇𝜇𝜇𝜇𝜇𝜇 𝑘𝑘 𝐴𝐴∆𝑝𝑝 Where:
k is intrinsic permeability (mD)
µ denotes fluid viscosity (m2/sec)
L length of pipe (or flow distance) (m)
Q is rate of flow (mD)
A is area (m2)
Δp is the pressure differential (pa)
Permeability is dynamic as it is dependent on both rock and fluid properties. Darcy’s law
calculates the permeability of a single-phase fluid (known as intrinsic permeability). In contrast,
effective permeability is the permeability of one fluid phase in the presence of another, and relative permeability is the ratio of effective to intrinsic permeability for a given fluid saturation.
Relative permeability yields information expressing the relative contribution of each fluid phase to the total flow capacity of the rock (Cannon, 2016). 23
Permeability and porosity control the storage and flow properties of rocks. Both are
dependent on individual-scale pore geometry (pore type, shape, and pore-throat size) within the
pore network (distribution of pore throats, pore shapes and whether the connecting throats are
straight or tortuous) (Cannon, 2016; Lucia, 2007). Diagenetic modifications may significantly
alter these properties creating a lack of correlation between porosity and other petrophysical attributes such as permeability (Suhaimi, 2016; Cannon, 2016).
Pore spaces of a reservoir were initially filled with water that is either free, capillary
bound (held by surface tension) or bound by hydroscopic forces. Water is then displaced by
hydrocarbons during migration due to differential fluid density. Capillary pressure occurs at the
microscopic scale within a rock wherever two immiscible fluids come into contact within the
pore space of a rock and is defined as the measured pressure difference between the phases
(Suhaimi, 2016; Cannon, 2016). This property determines fluid distribution and saturation within
a rock and defines how a reservoir will behave dynamically. Water saturation (Sw) is the
proportion of total pore volume occupied by formation water, while hydrocarbon saturation (Sh)
is the remaining portion of total pore volume not occupied by water. Water saturation is denoted
by equation 3 below:
= [Equation 3] 𝑉𝑉𝑉𝑉𝑉𝑉𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑆𝑆𝑤𝑤 𝑉𝑉𝑉𝑉𝑉𝑉𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 Where:
Sw is water saturation,
Volwater is pore volume filled with water,
Voltotal is the total pore volume. 24
Hydrocarbon saturation is denoted by equation 4 below:
= 1 [Equation 4]
𝑆𝑆ℎ − 𝑆𝑆𝑤𝑤 Where:
Sh is hydrocarbon saturation
Sw is water saturation.
The zone where water saturation is 100% and the capillary pressure is zero is called the free water level (FWL). The contact of the FWL may be sharp or transitional depending on the connectivity of the pore system and the size of pore throats (Cannon, 2016) 25
CHAPTER II: GEOLOGICAL BACKGROUND Basin History The Michigan Basin is a shallow, circular intracratonic basin covering an area of 207,200 km2. The basin extends from the upper and lower peninsulas of Michigan into adjacent areas of
Wisconsin, Illinois, Ohio, Indiana, and Ontario. The basin is bounded by structurally stable areas including the Canadian Shield to the north-northeast, the Findlay arch to the southeast, the
Kankakee arch to the southwest, and the Wisconsin arch to the west; and the basin is slightly tilted asymmetrically to the north (Fig.10). Some controversy exists as to whether basin subsidence was uniform or discontinuous as evidenced from the differential elevation between the northern and southern Guelph Formation pinnacle reef trends (Sears and Lucia, 1979; Gill,
1977). The basin originated during the Precambrian, with maximum subsidence rates occurring during Late Silurian to Middle Devonian, resulting in the Silurian strata comprising over 30% of the basin fill (Rine, 2016; Catacosinos et al., 1990).
During the Niagaran North American Stage (~422-418 Ma), the Michigan basin can be divided into three main depositional settings: (1) a shallow, broad carbonate platform with barrier reefs, back-reef lagoons and patch reefs; (2) a distally-steepening platform slope in which most pinnacle reef development occurred and; (3) a deep central basin consisting of carbonate micrites (Fig.10) (Friedman and Kopaska-Merkel, 1991).
Silurian Stratigraphy The geologic units of the Silurian Niagara Group and lower Salina Group are the focus of this study. Common usage of the geological nomenclature of producing units within the basin are often mixed between proper geological names and industry-derived terms; within this study, industry-derived terms are listed in parentheses and all stratigraphic nomenclature corresponds to 26
subsurface nomenclature (Fig.11). The Niagara Group is Niagaran (~422-418 Ma) in age,
overlies the Manistique Group, and is divided between the Lockport (Gray Niagaran) Formation,
and the Guelph (Brown Niagaran) Formation. The Salina Group overlies the Niagara Group, is
Cayugan (~421-416 Ma) in age, and is comprised of (in ascending order) the Cain Formation (A-
0 carbonate), Salina A-1 evaporite, the Ruff Formation (A-1 Carbonate), A-2 evaporite, A-2
carbonate, and Salina B-unit, C-unit, D-unit, E-unit, F-unit, and G-unit (Fig.11) (Rine, 2016).
The Guelph Formation (Brown Niagaran) the focus of this study.
Paleogeography and Environment A combination of paleomagnetic, paleoclimatic and biogeographic data indicate that, from the
Ordovician through the Devonian time, the North America continent was located near the
paleoequator and the landmass was rotated approximately 45° clockwise, with the Michigan
basin located approximately 25⁰ south of the paleoequator (Van der Voo, 1988). During Silurian
time, the Michigan basin was one of several shallow-marine embayments within the North
American intracratonic seaway. Cercone (1986) interpreted that the basin experienced freshwater influx via several inlet channels (Fig.10). The presence of shallow intracratonic seaways coupled with the paleogeographic location is thought to have provided favorable environmental conditions for carbonate reef growth until the Ludlovian (Rine, 2016; Friedman and Kopaska-Merkel, 1991).
According to Gill (1977), near the end of Niagaran time the Michigan basin became decoupled from adjacent epicontinental seas on the surrounding platforms through the extensive lateral development of basin-rimming barrier reefs. Additionally, it is interpreted that the basin was tectonically transported northward, marking a paleoclimate transition from humid tropical to arid tropical conditions. Consequently, extensive evaporites of the Salina Group were deposited, 27 sea-level within the basin fell, and the pinnacle reefs were subjected to one or more periods of subaerial exposure and freshwater leaching. Lack of fossil content and prevalence of cyanobacterial mats within the Ruff Formation suggest that marine conditions never returned to fully open-marine settings during the time of the Salina Group (Rine, 2016, 2017). By the end of the Salina Group deposition, the Michigan basin was filled with the Salina Group cyclic evaporites intercalated with thin limestone units, representing alternating conditions of low and high relative sea-levels (Friedman and Kopaska-Merkel, 1991; Cercone and Lohann, 1987).
Guelph Formation Pinnacle Reef Depositional Models The Silurian pinnacle reefs of the Michigan basin are isolated, topographically high carbonate buildups that exist in two well defined belts along the margins of the basin (Fig.10).
Both reef trends are completely encased in younger, impermeable evaporites and mudstones of the Salina Group (Fig.12) (Rine, 2016; Friedman and Kopaska-Merkel, 1991; Gill, 1977).
Michigan basin pinnacle reefs vary in size according to their location in the basin, but average reef height is 213 meters (698 ft.) (Rine, 2016). Reefs in the northern trend of the basin are generally smaller in diameter but are vertically taller than the reefs in the southern trend.
Generally, reef height increases basinward in both northern and southern trends. The Niagaran pinnacle reefs developed along the platform margin within stable subtidal conditions, between the barrier reef complex on the outer platform and the deeper water setting of the basin’s depositional center (Suhaimi, 2016; Rine 2016).
Models for Guelph Formation (Brown Niagaran) pinnacle reef growth generally divide each reef complex into three major stages: bioherm stage, organic reef stage, and a supratidal stage (Freidman and Kopaska-Merkal,1991). The Guelph Formation pinnacle reefs originated as crinoidal-bryozoan bioherms (basal mounds) developing along the platform margin on 28 antecedent topography where water clarity and light penetration were greater than adjacent areas.
The biohermal stage represents the initial stabilization and build up from low-energy, deep- subtidal conditions below the storm weather wave base (SWWB). Guelph Formation reefs accumulated vertically and laterally, eventually reaching the SWWB. Once at SWWB, bioherm mounds were colonized by tabulate coral and stromatoporoids, which then built upward to reach the fairweather wave base (FWWB), eventually undergoing a complete autogenic ecological succession, shifting reef growth into a skeletal reef stage dominated by frame-building metazoans (Rine, 2016; Suhaimi, 2016; Freidman and Kopaska-Merkal,1991).
Growth above FWWB marked the transition from bioherm facies assemblage to reef core facies assemblage, according to Suhaimi (2016) and Rine (2016). Reef core facies was dominated by wave-resistant fauna including stromatoporoids, tabulate corals, coralline algae, crinoids, bryozoans, and brachiopods (Freidman and Kopaska-Merkal,1991). Vertical “keep up” growth ensued as the reef kept pace with relative rises in sea level, resulting in vertical
“pinnacle” reef morphologies. Depending on which reef growth model is considered, at the peak of growth, the northern-trend pinnacles could have towered 90 to 210 meters above the sea floor
(Freidman and Kopaska-Merkal, 1991). Several studies (Rine, 2016; Balogh, 1981; Gill, 1973;
Huh 1973) have interpreted the deposition of reef-shed detritus along reef flanks into low-lying inter-reef areas to be coeval with reef growth.
The third stage of Guelph Formation pinnacle development was the supratidal island stage, where peritidal deposits (most notably cyanobacteria stromatolites) draped the reef pinnacles. The supratidal island stage consists of stromatolites, burrowed mudstones, peloidal wackestones, and flat-pebble conglomerates, interpreted to represent intertidal-to-supratidal conditions (Freidman and Kopaska-Merkal, 1991; Gill, 1975). Several disconformities and 29
different facies exist within the supratidal island stage itself, suggesting fluctuating sea level. The
stromatolitic cap facies marks the end of the Guelph Formation deposition (Suhaimi 2016; Rine
2016, 2017). Following the supratidal island stage of the Guelph Formation, the Cain Formation
(A-0 carbonate) of the Lower Salina Group was unconformably deposited within inter-reef areas, and then evaporitic-carbonate deposition of the Salina Group ensued.
There exist four contrasting depositional models for the development of the Guelph
Formation pinnacle reefs, with each model differing on interpretation of the uppermost
peritidal/reefal deposits of the Guelph Formation as either being Niagaran or Cayugan in age
(Rine, 2016). Three of these depositional models were generated before the 1980’s (Mesolella et
al., 1974; Huh, 1973; Gill 1973) while the fourth and most recent growth model was generated in
2015 by Rine (2016). As summarized by Mesolella et al. (1974), the first model states that the entire reef sequence, from bioherm stage to the top of the stromatolitic cap at the reef crest, was conformable and deposited prior to the A-1 evaporite and A-1 Carbonate (Ruff Formation), putting the Niagaran-Cayugan contact at the base of the A-2 evaporite overlying the stromatolitic cap (Gill, 1975; Briggs et al., 1978). The second model suggests that there was contemporaneous deposition of anhydrite and halite occurring within deeper inter-reef areas during pinnacle reef development. However, as summarized by Rine (2016), biostratigraphic evidence shows that the Ruff Formation is Ludlovian and Pridolian in age, compared to coral- bearing zones of the reefs are from the Wenlockian or older, disqualifying this model.
The third reef growth model argues that there were two or three discontinuity-bounded episodes of carbonate deposition on the reef crests, and that during the Wenlockian, massive stromatoporoid/coral pinnacle reefs were actively building along the shelf. In this model, deposition of the A-1 evaporite occurred following a sea-level regression resulting in exposed 30
reef core, but upon sea-level rise, carbonate sedimentation was reinitiated on reef tops with
restricted marine and evaporative settings, producing the algal stromatolite accumulations
(Huh,1973; Mesolella, 1974; Sears and Lucia, 1979).
The fourth model by Rine (2016) argued that during Guelph Formation deposition, paleo- wind direction was the main controlling factor in depositional profile and reef complex geometry, where southeasterly-dominant wind directions resulted in an asymmetrical distribution of depositional facies assemblages (Fig.12, Table 1). The model identified four major depositional facies associations for Guelph Formation reef development, representing sediment accumulation along various positions of the reef complex as a function of paleo-wind direction.
Guelph Formation depositional facies associations interpreted in the work by Rine (2016) include the reef core, bioherm, reef apron and windward reef talus (Fig.12). The lithofacies observed by Rine (2016) for each depositional association of the Guelph Formation and the
Salina Group are shown in Tables 1 and 2.
After identifying depositional associations, Rine (2016) then used sequence stratigraphic analysis to generate a reef complex growth model to resolve the long-standing problem regarding the timing of Guelph Formation and Salina Group deposition. Rine (2016) divided lithologic successions from the Guelph Formation and Salina Group into seven depositional stages, representing three complete system tract cycles (shown in Table 3). Due to the scope of the present study, being focused only on Guelph Formation sediments, all seven depositional stages of the Guelph Formation-Salina Group complex will not be reviewed, however these stages are summarized in Table 3 and are described in more detail by Rine (2016).
The model developed by Rine (2016) states that the first depositional stage of the Guelph
Formation-Salina Group pinnacle reef complex comprised the bioherm-to-reef core successions 31 of the Guelph Formation. Rine (2016) interpreted these successions to have been deposited during the transgressive systems tract (TST) to early high-stand systems tract (HST) conditions, with the TST/HST contact being observed as a thin crystalline dolomite bed (bioherm cap facies,
Table 1), occurring between the two successions. Rine (2016) interpreted this crystalline dolomite to be either a transgressive surface of erosion (TSE) or submarine hardground surface representing a small disconformity or transgressive flooding event. The autogenic ecological succession from crinoids to tabulate corals and stromatoporoids was interpreted to mark early
HST conditions, where metazoans built to-and-above the FWWB, “keeping-up” with Niagaran sea-level rise and basin subsidence throughout the early HST.
The second depositional stage is marked by the deposition of the stromatolitic cap where algae-rich intertidal deposits draped only the reef core and uppermost debris apron associations.
Rine (2016) interpreted this depositional shift to mark the slowing of sea-level rise and transition from early-to-late HST conditions, where shallower intertidal stromatolitic lithologies were deposited up to 9 m (30 ft) thick. Rine (2016) noted that the significant thickness of the stromatolitic cap suggested that rises in sea-level most likely occurred during the late-HST creating the accommodation space for the intertidal package, citing that preservation of a 9 m thick succession of intertidal sediment was possible but not probable. Rine (2016) observed an unconformity overlying the intertidal stromatolitic cap association located along reef crests, which the author interpreted to represent cessation of stromatolitic cap growth within the reef crest location of the complex.
Rine (2016) observed the intertidal stromatolitic cap association overlying not only the reef core association but also the distal and proximal segments of the leeward debris apron, located approximately 61 m (200’ft) down dip of the shallowest intertidal package overlying the 32
reef core. This was interpreted to suggest that once sea-level dropped below the reef crest,
subaerially exposing late-HST stromatolitic cap deposits, deposition of the intertidal
stromatolitic cap continued along the reef flanks, moving down the leeward slope as sea-levels
dropped. The termination of the stromatolitic cap along the reef crest is marked by an
unconformity, interpreted to mark a third-order sequence boundary occurring along the entire
reef complex topography (inter-reef topographic lows through the reef crest), marking the end of
the HST. Rine (2016) interpreted the deposition of the intertidal stromatolitic cap facies along the
reef flanks to be falling stage system tract (FSST) deposits, as they directly overlie HST deposits and the interpreted third-order sequence boundary.
Rine (2016) interpreted the FSST deposits to encompass not only intertidal stromatolitic cap packages occurring along the reef flanks, but also the A-0 carbonate deposited within inter-
reef topographic lows, suggesting deposition between the two units occurred either coeval or
sequential. After the second depositional stage of the Guelph Formation and A-0 carbonate, Rine
(2016) interpreted sea-level draw down and extensive subaerial exposure of the Guelph
Formation pinnacle reefs to have taken place, with sea level dropping until basin-decoupling
from global oceans occurred, initializing the evaporitic deposition of the remaining portion of
the A-0 carbonate and Salina Group. The sequence-stratigraphic based model generated by Rine
(2016) is argued to generally support the third depositional model previously described. For
more information regarding the remaining Guelph Formation-Salina Group complex depositional
stages, please refer to Table 3 and the work conducted by Rine (2016).
33
CHAPTER III: METHODOLOGY Case Study Field History and the 9-33 Well This investigation is based on core data obtained from the Lawnichak & Meyer 9-33 well
(hereby referred to as the 9-33 well). The 9-33 well was drilled within the Dover 33 reef field, along the northern pinnacle reef trend located in Otsego County, Michigan. The Dover 33 field was discovered in the late 1970’s and after undergoing primary recovery, underwent water flooding EOR operations during the 1990’s. Within the past 15 years, Dover 33 field began CO2- driven EOR and carbon sequestration operations, serving as a carbon capture and utilization field demonstration for MRCSP. The 9-33 well was drilled in 2016, along the leeward flank of the reef and was deviated from the Salina Group through the Guelph Formation at an angle oriented perpendicular to the reef complex (Fig 1). Drilling operations recovered approximately 70 m
(200 ft) of whole core, split into seven core runs. After core recovery, open-hole geophysical logging operations were performed, obtaining the well logs used in this study. This project utilizes whole core, core analysis, and geophysical data that was provided by Battelle, Core
Energy LLC., and the Midwest Carbon Sequestration Partnership.
Whole Core Analysis Core analysis was conducted over a five-day period at the Core Lab facility located in
Houston, Texas. Core analysis involved macroscopic evaluation of lithology, texture, sedimentary structures, descriptions of diagenetic modifications (salt plugging, fracturing, dolomitization), and pore types visible at the macro-scale. Core was described on a millimeter scale using a 10x hand lens and 2% dilute hydrochloric acid. Lithologic classification is based on
Dunham (1962) and was later interpreted offsite using the modified-Dunham classification scheme by Embry and Klovan (1971). Professional quality, high resolution core photographs 34
were taken by Core Laboratories and provided by Battelle Memorial Institute. Additional feature-specific core photographs were taken onsite in Houston using a Nikon camera.
Thin Section Petrography A total of 21 thin sections were prepared; 16 samples were selected to represent specific lithofacies and the remaining five samples were taken from intervals of interest (surfaces, fracture fills etc.). Thin section preparation was completed by Core Laboratories in Houston,
Texas. Thin sections were stained with alizarin red-S and potassium ferricyanide to distinguish carbonate minerals and with sodium cobalt nitrite to identify potassium feldspars (Core
Laboratories, 2017). Thin sections were described using a Zeiss microscope in plane and polarized light and the lithology was described according the Folk (1959, 1962) classification scheme. Microscopic descriptions noted mineralogy, allochem types, textures, sedimentary structures, pore types, and cement types; these observations were used to verify macroscopic interpretations, aid in explaining petrophysical character, and aid in future work.
Geophysical Well Log Analysis Geophysical well logs are tools which utilize a device that is lowered into a wellbore, to obtain continuous measurements of the response of rock formations to a variety of geophysical analysis techniques (acoustic, nuclear, electrical, and chemical). Data obtained from well logs are then used to map subsurface geology, steer well trajectories, interpret depositional environments, obtain or estimate petrophysical values and enhance seismic analysis, serving as the primary subsurface data source in place of recovered rock cores (Cannon, 2016; Ellis and Singer, 2008).
Several geophysical well logs were available for the 9-33 well, obtained by Baker Hughes and provided by Battelle. Open-hole wireline logs used in this study included formation bulk density
(ZDEN), compensated neutron porosity (NHPI), gamma-ray (GR), resistivity-based borehole 35
image (RBHI) log (STAR-tool). The resolution of these geophysical logs is approximately 2.54 cm (1 inch) for the RBHI log and 0.15m (0.5 ft) for all other well logs.
Gamma-Ray Logs
The gamma ray tool measures the natural radioactivity of a formation, responding to the presence of uranium (U)-, potassium (K)- and thorium (Th)-rich minerals (Cannon, 2016).
Gamma ray logs can be used to estimate lithology, relative mud-content and the depositional environment of rocks within the subsurface. Rock fabrics which exhibit high amounts of clay minerals typically yield more positive gamma ray signatures, suggestive that a muddy fabric is present, while more grain-rich rocks yield lower gamma ray signatures (Suhaimi, 2016; Cannon,
2016). Due to the gamma-ray log response to rock fabrics, relative coarsening-upward or fining- upward stratigraphic trends may be interpreted from the shape of the gamma-ray curve (Fig.13) and mapped across space. The relative shape of the gamma-ray log signature, forming as a response to the succession of coarsening-upward or fining-upward stratigraphic trends can be used for interpreting the depositional environment of sedimentary successions (Fig.14) (Walker,
1984). This study utilizes the gamma-ray log to pick formation tops and generate a geophysical
log model of the recovered core obtained from the 9-33 well. Gamma-ray log analysis will
consist of the curve-shape analysis described by Walker (1984) (Figs 13 and 14) to identify
geophysical signatures of depositional successions observed within the 9-33 well core.
Bulk Density Logs
The bulk density log is a type of geophysical well log that measures the electron density
of rock units surrounding the borehole (Cannon, 2016). These logs utilize a radioactive source
such as cobalt (60Co) or cesium (137Cs), to emit medium-energy gamma rays which collide with 36
the electrons of surrounding rock, resulting in a loss of energy and scattering of the gamma ray
particles (a process known as Compton scattering). The number of scattered high-energy range
gamma-ray particles that return to tool’s detection sonde are proportional to the density of the
rock formation (Cannon, 2016). The bulk density log is often used for lithology identification
based on density differences and used to calculate average porosity of the rock matrix. This study will use the density log to interpret lithology and supplement neutron porosity log interpretations.
Neutron Porosity Logs
Neutron porosity logs are a type of geophysical log which uses a chemical source such as
americium or beryllium, which continuously emits neutron particles that collide with the atomic
nuclei of the formation (Cannon, 2016). When each collision occurs, neutron particles lose energy, and eventually the neutron particle is absorbed by a nucleus resulting in the emittance of
a gamma ray. Due to similar atomic mass values, a maximum amount of energy is lost when a
neutron collides with a hydrogen atom. The amount of neutron particles lost is measured by the
tool’s sonde, and the bulk amount of hydrogen is then calculated, generating a value known as
the hydrogen index (HI). HI is defined as the number of hydrogen atoms per unit volume of the
rock divided by the number of hydrogen protons per unit volume of pure water.
This tool operates under the assumption that the main source of hydrogen within a
formation is either water or hydrocarbons, and therefore the total amount of hydrogen measured
is proportional to the total volume of porosity present, because pore space are fully saturated.
Certain properties present in rock formations such as clay volume and presence of natural gas
can offset neutron porosity readings and must be considered when interpreting the neutron
porosity log (Cannon, 2016). This study will use the neutron porosity log to identify relative 37
zones of high porosity and corroborate the electrical responses of Guelph Formation sediments
observed within the RBHI log.
Resistivity-based Borehole Image Logs
Resistivity-based borehole image (RBHI) logs are a type of geophysical device which
measures the electrical potential of rock formations, using variations in electrical resistivity to
generate high-resolution images of rock fabrics, fractures and faults surrounding the borehole
wall. RBHI logging tools consist of an elongate mandrel device that features multiple arms protruding from its center axis, where each arm consists of an array of closely spaced microelectrodes. These pads propagate an electrical current into the formation measuring the current density across the pad while keeping a constant electric potential relative to the return electrode. Any variation measured in current density is due to resistivity of the formation. The resistivity measurement is a function of mineralogy, porosity, pore fluid, pore geometry, cementation, and clay content (Lagraba et al., 2010; Ellis and Singer, 2008; Prensky, 1999).
As the RBHI logging tool is pulled upwards from the bottom of the borehole, the tool records continuous helical scans. Once field measurements are acquired, algorithms are then used to normalize helical scans into circumferential scans, turning the measurement matrix at each depth into pixels. A grey scale or color scale is then applied to represent the range of resistivity values measured within a depth interval. Once the circumferential image is generated, the image undergoes various digital processing techniques, such as signal conditioning and image enhancement that correct the raw signals for noise and distortion during the acquisition process. Image analysis techniques (such as filtering, sharpening, normalization, and false coloring) are then used to improve the overall image quality, emphasize specific features, and 38 quantitatively extract orientation data (strike and dip) of faults, fractures and bedding (Hansen and Buczak, 2010; Prensky, 1999).
RBHI logs are presented as 2D visualizations, but the displayed track itself represents the full 360° profile of the borehole unrolled and orientated to magnetic north. RBHI logs are presented in both static and dynamic views, where each view differs by the normalization technique used to generate the images. In static normalization, a histogram equalization technique is used to obtain the maximum quality of the image and the range of resistivity values obtained over the entire logged interval is computed and partitioned into 256 color levels. Static image logs are best used for the identification of large-scale resistivity variations and features.
The dynamic view is generated by rescaling color intensities over a fixed depth interval (such as
2 m), instead of the entire logged interval as with static normalization techniques (Hansen and
Buczak, 2010; Prensky 1999).
RBHI logs may be used to interpret stratigraphic trends, identify various structural features and resolve variations in rock fabrics identified by contrasts in electrical textures
(electrofacies) (Prensky 1999). RBHI logs are best used to supplement core analysis, however, it may also be used cautiously where no core data is present. RBHI logs are used in this study to aid in the constructing a geophysical log model and also correlate distinct fabrics observed in whole core to depth-matched responses of various geophysical logs.
Constructing the Geophysical Log Model
The first step in constructing a geophysical log model is correlating geophysical log to the core. Discrepancies in depth correlation may occur due to the stretching of the geophysical log tool cable and missing core cables. Typically, gamma ray (GR) logs are used and the 39
correlation is confirmed using core gamma-ray measurements, textures, core photographs and, if
available, borehole image logs. However, core gamma-ray measurements were too low (<10
GAPI) and discontinuous, due to the seven core breaks, to directly compare to wireline gamma-
ray log. RBHI log-based core depth calibrations were performed by first calibrating the depth of the RBHI log to the wireline gamma-ray (these two logs were collected on separate tools). In this study, there was a 1.3m (4 ft) difference between the RBHI log and other well logs. Once depth calibrated, the RBHI log matched distinctive textural features observed from core photographs
(such as strong rock texture contacts and large dissolution features). This process showed that the first four core run intervals did not need any depth shifting. Depths from the remaining three core run intervals were indistinguishable (using these methods) and are assumed to be relatively in place, with a unavoidable degree of uncertainty.
Core Plug Analysis A total of 117 core plugs were drilled from the 9-33 core, at a sample spacing of 0.45m
(1.5 ft). Core plugs analysis was conducted by Core Laboratories in Houston, Texas. Samples were cleaned and dried within an oven until fully dried. Core plugs were then placed into Core
Lab’s CMS-300™ core holder and a pre-selected stress was applied. Pressurized helium is discharged into one end of the sample from a reference cell and vented into the atmosphere at the other end. Pressure decay was monitored over time to determine the Klinkenberg permeability
(Kinf), which may be converted to permeability to air using the following equation (Core
Laboratories, 2017; Yang, 2017; Cannon 2016):
b K = K 1 + (P + P ) [Equation 5] air a inf � � � m a �� Where: 40
Pm = mean pressure
Pa = atmospheric pressure
bair = the slip factor to air
Kinf = Klinkenberg permeability
Ka = permeability to air.
After permeability analysis, samples were air dried at 95°C (~203°F) and grain volumes were determined by placing samples into stainless steel matrix cups, and then helium was injected into the samples from a reference cell at a known pressure using Core Lab’s autoporosimeter. Grain volume (g/cm3) was resolved using Boyle’s Law of Gas Expansion.
Grain density was then calculated by dividing sample’s dry weight by grain volume. The direct pore volume was measured using Boyle’s Law of Gas Expansion listed below:
[Equation 6] 1 𝑃𝑃 ∝ 𝑉𝑉 Where:
P= Pressure (Pa)
V= Volume (Cm3)
Porosity was calculated as the pore volume fraction of the summation of bulk volume (Core
Laboratories, 2017).
Core plug analysis results were utilized by this study to perform descriptive statistics and
generate a series of scatterplots to examine correlations between the distribution of porosity,
permeability and depositional processes at the lithofacies scale and permeability-porosity 41 transforms were generated to model permeability as a function of porosity. Power functions were chosen to model porosity and permeability data, as these yielded the best prediction values compared to linear transforms which yielded correlation coefficients below 10%.
Mercury Intrusion Capillary Pressure (MICP) Analysis MICP analysis is used to obtain capillary pressure-saturation relationships and gain insight into the pore-architecture within a rock sample. A total of sixteen MICP experiments were conducted on 2.5cm (1”) trim from selected core plugs by Core Laboratories in Houston,
Texas. The core plugs trim used in the MICP experiments are from samples also used for core plug analysis and thin section generation (previous sections).
Mercury is a non-wetting fluid (>90° wetting angle), and the principal theory behind mercury injection is that mercury cannot spontaneously enter a small pore which has a wetting angle > 90° due to surface tension, however this resistance may become overcome by exerting an external pressure on the mercury (Yang, 2017; Cannon, 2016; Webb, 2001). The volume of mercury injected at each measured pressure determines the non-wetting phase saturation. The relationship between mercury saturation and capillary pressure is given by the Washburn equation below:
cos P = (A) [Equation 7] r c 2γ θ p Where:
Pc = capillary pressure (psi)
γ = interfacial tension (dynes/cm),
θ = contact angle (degrees), 42
rp = pore throat radius (microns),
A = psi conversion constant.
In a water-wet system, oil migrating to the water saturated pores under a given pressure would only enter pores larger than a given rp (i.e., capillary pressure is inversely proportional to the pore
throat radius for any given pore). Various conversion relationships are typically implemented
using various surface tensions between fluids, or for fluids-to-air, to obtain more accurate
estimates of oil-to-water and gas-to-water systems. In addition to the saturation of the wetting
phase or the non-wetting phase, other factors affecting the capillarity curve include pore size,
pore distribution, composition of the fluid, and pore wetting-drying hysteresis (Yang, 2017).
The capillary pressure (Pc) curve may be divided into three segments: the initial segment,
intermediate segment, and end segment. During the initial period, the saturation of the wetting
phase (air) decreases slowly with increasing capillary pressure, while the saturation of the non-
wetting phase (Hg) increases slowly. At this point, the core has not been penetrated by the non-
wetting phase fluid, but instead the non-wetting fluid is entering both small pits on the surface
and dissected large pore openings. In the intermediate segment, the non-wetting fluid enters
much of the pore network, and capillary pressure curves have various shapes indicating pore size
variability. The final segment displays the highest pressures needed to enter the smallest pore
networks, which are still holding the wetting phase fluid (Yang, 2017).
Various qualitative and quantitative indices are used to describe a capillary curve.
Qualitative information may be derived by examining the intermediate section of the Pc curve;
this is due to the fact that the Pc curve shape is indicative of the pore structure including the size,
shape, and sorting of the pores (Yang, 2017). Longer intermediate segments (with respect to the 43
horizontal axis) suggest good sorting of pore throat size distributions, and intermediate sections
that trend parallel to the horizontal axis feature larger pore throat sizes that require lower entry
pressures. Therefore, curves that feature intermediate sections that trend non-parallel (i.e., diagonal) to the horizontal axis are characterized by smaller pore throats that require higher entry pressures. Additionally, the skewness of the Pc curve yields insights into the size and abundance of pores, where the more large pores and pore throats present within a sample, the more skewed to the right the curve will be and vice versa; assuming a predictable relationship between pore throats and pore-throat size, which is not a valid assumption in carbonate lithologies that feature secondary pore systems (Yang, 2017; Lucia, 2006). Figure 15 displays six idealized unimodal Pc curves with varying shapes reflecting differences in sorting and skewness of the pore throat size populations.
Quantitative observations on capillary pressure/saturation relationships include threshold
pressure (PT), Median saturation pressure (Pc50), and the minimum wetting phase saturation
(Smin). Threshold pressure (also known as entry or displacement pressure) is the minimum
pressure needed to force the non-wetting phase into the core, which is determined by the largest
pore of the network (Yang, 2017). By knowing the entry pressure, one may then have a relative
idea of the pressure needed to displace the wetting fluid using a non-wetting fluid. The pressure
of median saturation (Pc50) is the capillary pressure at 50% saturation, corresponding to the invasion into the average pore throat radius (assuming a normal pore distribution). The minimum wetting phase saturation (Smin) is the volume percentage of pores that have not been
intruded by the non-wetting phase when wetting phase displacement reaches its highest value.
With respect of a water-wet rock, Smin represents the connate water saturation. In contrast, if the
reservoir is an oil-wet system, Smin represents residual oil saturation. The Smin index reflects the 44
pore structure, where the lower value suggests better petrophysical quality. Unfortunately, due
to the high injection pressures, the potential Smin values within this study would bear significant error, and therefore will not be examined (Yang, 2017).
This study will utilize an improvised classification system based on the qualitative and quantitative interpretation methods described by Yang (2017) to classify capillary behavior across the depositional environments of the Guelph Formation (Table 4). For simplicity, the classification system features three types of capillary behavior (type I, type II and type III) which represent either unimodal or multimodal pore system behaviors. Each Pc curve was classified as
one of the three generalized classes based on the pressure of median saturation (Pc50), and the
presence of kinks within the intermediate section of the Pc curve (Table 4).
Class I capillary profiles are characterized by Pc type curves with long, uniform (unkinked) intermediate segments and low-median saturation pressures (<100 psi) (Table 4). Class I Pc type
curves are interpreted to represent relatively unimodal pore systems with well-sorted pore-throat
size distributions (PTSDs), moderate-to-large sized pore throats, and an absence of kinks along its intermediate segment; this Pc class is considered to have good apparent reservoir quality.
Class II Pc type curves exhibit kinks within the intermediate segment and may display
various median saturation pressures (Table 4). Class II capillary curves are interpreted to
represent bimodal-to-multimodal pore systems that may exhibit a range of pore throat sizes,
median saturation pressures, and feature poor-to-good PTSD sorting; this type of Pc type curve is
considered to exhibit complex quick-look reservoir quality.
Class III Pc profiles are characterized by Pc curves with long, uniform (unkinked) intermediate segments and high median saturation pressures (<100 psi) (Table 4). Class III Pc 45 type curves are interpreted to represent relatively unimodal pore systems with well-sorted
PTSD’s, and small-to-moderate sized pore throats; this Pc class is considered to have poor quick- look reservoir quality.
In addition to classifying general capillary behavior, pore-throat size frequency distributions were generated to assess changes in pore-throat size distributions (PTSDs) across each depositional environment and the formation, collectively. Due to the large number of samples and scope of the study, PTSDs will not be reviewed for each depositional environment; these distributions may be found in Appendix A. Pore throat aperture sizes were classified according to the scheme developed by Doveton (1995).
46
CHAPTER IV: RESULTS
Lithofacies Analysis Sixteen lithofacies were observed with the 9-33 well core, interpreted based on macroscopic changes in lithology, composition, texture, fossil content and sedimentary structures
(Table 5). Each lithofacies was given a code to differentiate lithologies of different interpreted depositional environments. Microscopic observations were also obtained, describing Folk (1962) lithology classification, fossil content, mineralogy, pore types, cements and diagenetic features.
Microscopic observations were used to corroborate and or elaborate macroscopic observations made at the core scale.
Dolomitic Mottled Mudstone (Mm) Macroscopic Observations: Facies Mm is a massive-to-medium bedded, often mottled, dark black-to-grey, dolomitic mudstone with no visible observable skeletal material (Fig.16A).
Mottled intervals within facies Mm are subtle, identified as geometrically irregular discolorations from the darker mud-matrix. No skeletal material is identifiable from Mm sections of core; however, its presence may be indicated by the sparse distribution of isolated vugs. Diagenetic features observed at macroscale include natural fractures and dissolution channels healed by granular dolomitic cement, with the development of fracture-localized vuggy porosity.
Microscopic Observations: Facies Mm is a tan fossiliferous to non-fossiliferous micrite with rare fossil fragments of echinoderm plates and brachiopod fragments. Facies Mm is comprised mainly of dolomite (98-99%) with minor amounts of bladed-anhydrite (1-2%).
Inequigranular contact-rhomb dolomite fabrics are observed with a planar-e crystal shapes and
boundaries, suggesting the unit has undergone a low-to-moderate amount of recrystallization.
Observed cements include pore-filling blocky subhedral dolomite rhombs, in addition to a 47
distinct discontinuous dolomitic cement occurring within localized zones of the matrix. This
cement is only resolvable under cross-polarized light, with a rotating stage, and features
sweeping extinction across dolomite crystals, with the extinction showing radial, isopachous
geometric character. This cement is interpreted to be a micritized and recrystallized remnant of a
marine crustal cement, such as radial fibrous calcite (RFC) cement (Fig.17). Dissolution processes are prominent, with localized zones of matrix leaching creating early stage microporosity and late stage vugs. Pore types observed at a microscopic-scale include intercrystalline, interparticle, mudstone microporosity, and vuggy porosities; all of which are
often reduced by pore filling subhedral dolomite rhombs.
Interpretation: The massive nature and lack of any observable structures within this
mudstone suggests either (1) uniform depositional conditions; (2) rapid sedimentation; or (3)
destratification by bioturbation agents or via fluid escape. Destruction of bedding lamination and
generation of mottled textures is often associated with benthic biota activity within deposits from
lower intertidal to subtidal environments (Coniglio et al., 2004; Shin, 1983). Facies (Mm) is
therefore interpreted to be deposited in subtidal-to-lower intertidal conditions.
Laminated Dolomitic Mudstone (Ml) Macroscopic Observations: At core scale, facies Ml (Fig.16B) is observed as a planar-
to-wavy laminated, dark grey-to-blue, dolomitic mudstone. Lamination varies between planar to
wavy, generally becoming undulate moving up section. Diagenetic features observed at core
scale included the development of bedding parallel hopper casts at 1687.2 m (5535.5 ft).
Microscopic Observations: Not Sampled. 48
Interpretation: Lamination is a common feature found in a variety of deposits.
However, as noted by Shin (1983) and Pratt (2010), the preservation of horizontal lamination is a
commonly feature within upper-intertidal to supratidal conditions (Fig.1). Within the intertidal- to-subtidal zone, the preservation potential of bedding lamination is extremely low due to benthic organism bioturbation. Thus, when it is preserved, this is suggestive of either (1) a low number of burrowing organisms or (2) stressed biotic conditions possibly from periods of hyper salinity or oceanic restriction (Pratt, 2010; Shin, 1983). The combination of hopper casts and preservation of laminations is interpreted to suggest facies Ml was deposited in a restricted environment, most likely in intertidal conditions.
Pale Grey Mudstone (Mpg)
Macroscopic Observations: Facies Mpg is a thin-to-medium bedded, pale grey-to-blue, dolomitic mudstone with undulatory-to-flat bedding contacts and no observable fossil content
(Fig. 16C). Facies Mpg almost always overlies skeletal grainstones, forming pale, normally graded interbedded successions. When present, facies Mpg is easily identifiable due to its distinct pale color, contrasting with surrounding darker facies. Bedding contacts are flat-to- undulatory. Additionally, when identifying facies Mpg at core scale, thin dark wisps may be observed and misinterpreted as mud drapes; these were later identified to be stylolites from thin section analysis. Diagenetic alterations witnessed at core scale include the sparse development of pinpoint to moderate sized vugs.
Microscopic Observations: Facies Mpg, is a tan micrite, comprised almost entirely of subhedral-to-anhedral dolomite, with minor amounts of calcite (<1%) occurring as fine-sized inclusions within dolomite crystals. Bright white, subhedral, sutured equigranular dolomite fabrics are observed with non-planar crystal shape and boundaries, suggestive that facies Mpg 49 underwent a moderate to high amount of recrystallization. A large amount of matrix leaching occurred near the contact of the overlying grainstone, leaching away finer dolomicrite and replacing it with coarser dolomite crystals, sometimes witnessed as floating euhedral rhombs.
The interpreted remnant-recrystallized crustal cement (see description in facies Mm section, Fig
17) is present lining dissolution vugs. Pore types observed within facies Mpg include intercrystalline porosity, mudstone microporosity and vuggy porosities.
Interpretation: The stark pale color of facies Mpg and underlying skeletal grainstone, occurring interbedded with distinctly darker facies suggests either: (1) facies Mpg was deposited in non-normal conditions, possibly in conjunction of a rapid flux of “clean” carbonate mud from shallower, more oxygen rich environments (Mullins and Cook, 1986; Ager,1974) or; (2) the color may have a diagenetic origin by means of recrystallization or porosity development. Thin section analysis shows that facies Mpg has undergone a moderate amount of recrystallization, suggesting a possible diagenetic origin for the pale color of the facies. The lack of bioclasts and sharp basal contact suggests rapid deposition. The occurrence of normally graded interbeds of grainstone and mudstone suggests that facies Mwb was deposited by either by suspension fallout during waning storm energy conditions or by turbidity currents. The undulated-to-wavy upper contacts of the facies may suggest either: (1) erosive scouring at the base of overlying deposits;
(2) that an oscillatory regime remained for a period after fine-grained suspension fallout (Perez-
Lopez and Perez-Valera, 2012); (3) mud deposited from suspension draped wave ripples; or (4) possibly it was an effect of subsurface compaction and consolidation processes.
Based on the evidence for rapid sedimentation, and presence of a strong grading mechanism, facies Mpg is interpreted to be either 1) mud that was resuspended by storm conditions, then deposited via fallout during waning storm conditions, making it part of a larger 50
tempestite succession; or 2) mud deposited by a turbidity current, representing the Te division of
the turbidite succession (bed 2a of the Meischner sequence). Due to the similarity between
tempestite and turbidite successions and the limitations geologic observations made at core scale,
the absolute origin of facies Mpg cannot be resolved. However, it should be noted that some mud
volumes may correspond to normal background sedimentation (pelagic rain). The possibility of
facies Mpg being part of a larger tempestite sequence, suggests this unit was deposited above
storm weather wave-base.
Dolomitic Skeletal Wackestone (Sw)
Macroscopic Observations: Facies Sw is a massive, mottled, dark grey-brown or black skeletal wackestone with fossil fragments of bivalves and bryozoans (Fig.18A). Macroscopic diagenetic features observed included medium-to-large vugs (> 2.5 cm) and healed fractures.
Microscopic Observations: At thin section scale, facies Sw is a fossiliferous micrite
(biomicrite) with fragments of echinoderm plates, gastropods, and bivalves. Mineralogically, the lithofacies is comprised entirely of dolomite with no other minerals observed. Observed porphyrotopic dolomite textures consist of floating subhedral rhombs with planar-e crystal boundaries and shapes, suggesting a moderate amount of recrystallization occurred. Localized zones of aphanotropic groundmasses were observed, suggesting that recrystallization was not homogenous within this lithofacies. Grain diminishment of fossil fragments is prominent. Drusy dolomitic cement is observed throughout the sample. Matrix leaching is prominent within intervals where low amounts of recrystallization was observed, creating vuggy porosity. Pore types observed include interparticle, intercrystalline, and vuggy. Vugs are reduced-to-occluded by pore-filling drusy cement. 51
Interpretation: The massive texture of facies could be produced by either (1) uniform
depositional conditions; (2) rapid sedimentation; or (3) destratification by bioturbation or via
fluid escape. The destruction of bedding lamination and generation of mottled textures is often
associated with benthic biota activity within deposits from lower intertidal to subtidal
environments (Coniglio et al., 2004; Shin, 1983). Echinoderms, gastropods, brachiopods, and
bivalves are all organisms that may be found within intertidal-subtidal conditions. Based on its fossil content and its massive nature, facies Sw is interpreted to be deposited in subtidal-to-lower intertidal conditions.
Dolomitic Skeletal Packstone (Sp) Macroscopic observations: Lithofacies Sp is observed as a thin-to-medium, wavy bedded, dark-to-light grey, dolomitic skeletal packstone with erosive lower contacts and preferential orientation of bioclasts (Fig.18B). Skeletal debris is observable at macroscopic as
small whitish speckles. Lower bedding contacts are sharply erosive and upper bedding contacts
are wavy to gradational.
Microscopic Observations: Not sampled.
Interpretation: Preferential orientation of skeletal debris could be associated with either
sediment gravity flows, currents or wave transport processes (Flugel, 2004). The erosive lower
contacts of facies Sp indicates it is an event layer. The incorporation of intraclasts with cryptalgal
lamination suggests erosion and reworking of algal mats from the upper intertidal or supratidal
zone. It cannot be directly determined which process generated facies Sp, however it is
interpreted that skeletal fragments found in facies Sp sourced from subtidal to intertidal
environments. Thus, facies Sp is interpreted to be part of either a sediment gravity flow (debrite) 52
or a storm deposit (tempestite), consisting of bioclastic material sourced from intertidal-to-
subtidal settings.
Dolomitic Skeletal Grainstone (Sg) Macroscopic Observations: Facies Sg is observed as a thin-to-medium bedded, normally graded, pale white-to-grey, dolomitic grainstone with undulatory-to-flat bedding contacts, disarticulated skeletal debris, and burrows (Fig.18C). At macroscopic scale, skeletal
material is unrecognizable apart from brachiopod fragments. A single vertical, sinusoidally-
shaped burrow (~ 1 mm wide and 2.5 cm long) is observed to crosscut through the grain fabric
and bedding contact between the facies Sg and the overlying pale-grey mudstone (facies Mpg) at
1745.59 m (5727 ft). The internal structures of burrows cannot be discerned, presumably because
of dolomitization. Facies Sg always occurs underlying facies Mpg, forming fining-upward
successions.
Microscopic Observations: Facies Sg is a tan-to-white, well-to-poorly sorted biosparite
with skeletal fragments consisting of brachiopods, crinoid ossicles, branching bryozoans, and
echinoderm plates. Mineralogically, the unit is comprised entirely of dolomite. Aphanotopic
dolomite textures dominate sampled sections, suggesting the unit has undergone a low amount of
recrystallization. Diagenetic cements observed in thin section samples included micritized and
recrystallized, remnant RFC cement (Fig.17) in addition to granular and drusy dolomitic
cements. Pore types observed included interparticle, intercrystalline, moldic, shelter, and vug
porosities. Enlarged vugs are leached and lined with pore-filling remnant crustal cement. Shelter
pores and molds are infilled by granular to drusy cements forming drusy mosaics and are lined
with RFC cement. The bedding plane contact between facies Sg and overlying pale-grey mudstone (facies Mpg) features bedding parallel RFC cement. 53
Interpretation: Normal grading may be produced by suspension driven fallout during waning-energy conditions during storms or turbidity currents. The undulated-to-wavy upper contact of the facies may suggest either: (1) erosive scouring at base of overlying bed; (2) an oscillatory flow regime remained for a period after fine-grained suspension fallout during waning storm conditions (Perez-Lopez and Perez-Valera, 2012); (3) it is an effect of subsurface compaction and consolidation processes; or (4) it is part of a larger feature not discernable at core scale (such as hummocky or swaley stratification). Idealized tempestite successions commonly thin with increasing distance from source or shoreface and are separated by erosional bases and muddy sediment from background sedimentation (Perez-Lopez and Perez-Valera,
2012; Brant and Elias, 1989). However, an allodaptic turbidite origin cannot be ruled out, as these two facies are similar to the idealized profile of bed 1ab and 2a of the Meischner sequence
(divisions Ta and Te of the Bouma sequence). Facies Sg is interpreted either to be part of tempestite succession (most likely located distally from its source) or, possibly in some intervals part of a turbidite succession.
The vertical branching burrow observed within facies Sg appear is interpreted to be
Chondrites or Ballonoglossites triadicus, resembling many of the morphologies attributed to burrowing worms (Gingras et al., 2008). Ballonoglossites triadicus ichnofacies have been found in tempestite deposits (Perez-Lopez and Perez-Valera, 2012). This burrow is interpreted to possibly due to be opportunistic predatory behavior occurring post-deposition.
Massive Dolomitic Skeletal Floatstone Facies (Sfm)
Macroscopic Observations: Facies Sfm is a massive, mottled, grey-to-brown, dolomitic floatstone with bioclasts of intact-to-fragmented pentamerid brachiopods, rugose coral, crinoids, 54
echinoderms and bryozoans (Fig.18D). Mottled textures appear as irregularly shaped
discolorations within a darker background matrix.
Microscopic Scale: Facies Sfm is a tan sorted dolomitic biosparite with brachiopods,
crinoid spines and echinoderm plates. Aphanotopic dolomite textures are dominant, with
localized zones of porphyrotopic contact-rhomb textures with crystals exhibiting non-planar to planar-e shape and contacts, suggesting a low amount of recrystallization has occurred within the unit. Pore-filling cements observed include remnant-recrystallized crustal cement and blocky
euhedral-to-anhedral dolomite cements forming granular and drusy cement mosaics.
Stylolitization is excessive, creating stylobreccoid textures. Dissolution within the matrix and
along stylolite sutures is prevalent creating vugs and microchannels. Pore types observed within
facies Sfm include intercrystalline, microporosity, vug, and microchannel porosity. Euhedral to
anhedral dolomitic cement reduces-to-occludes stylolite-focused microchannels and vugs. Vugs
are most likely dissolution-enhanced fossil ghosts, and many are lined with remnant-
recrystallized RFC cement, suggesting these originated as fossil molds (Fig.17).
Interpretation: The presence of mottled textures and massive bedform suggests either:
(1) uniform depositional conditions; (2) rapid sedimentation; or (3) destratification by bioturbation agents or fluid escape. The destruction of bedding lamination and generation of mottled textures is often associated with benthic biota activity. Furthermore, the presence of benthic biota (echinoderms, brachiopods, crinoids) are limited to lower intertidal to subtidal zones (Flugel, 2004; Coniglio et al., 2004; Shin, 1983). The presence of articulated brachiopods may indicate the organism died in-place, suggesting autochthonous sedimentation. Facies Sfm is
interpreted to be a skeletal floatstone deposited in subtidal-to-intertidal settings.
55
Dolomitic Cryptalgal Intraclastic Floatstone (Facies Ifcr)
Macroscopic Observations: Facies Ifcr is a thinly bedded, tan-to-dark brown, dolomitic floatstone with imbricated, pebble-sized, cryptalgal-laminated mud intraclasts held within a grey mudstone matrix (Fig.19A). Intraclasts appear to be sourced from underlying cryptalgal bindstones, sometimes appearing partly detached from underlying strata. Clast imbrication is rare, being seen in only a few thin beds.
Microscopic Observations: Facies Ifcr is a brecciated biolithite with rounded cryptalgal laminated intraclasts. The lithofacies is comprised mainly of dolomite with minor amounts of euhedral bladed anhydrite (~5%). Aphanotopic dolomite fabrics dominate the sample along with a few isolated zones consisting of floating euhedral dolomite rhombs, suggesting a low amount of recrystallization. Subhedral blocky dolomitic cement is present, infilling pores throughout the sample. Matrix groundmass is heavily stylolitized with irregular and hummocky swarms with localized areas of matrix dissolution creating microporosity and small vugs. Pores types observed include microporosity, intercrystalline, vug and fenestrae porosities. Pore filling blocky subhedral dolomite cement and anhydrite infill dissolved matrix zones and vugs.
Interpretation: Intraclasts are commonly observed in peritidal environments. Intraclasts are produced by storm-wave erosion and often occur in supratidal and subtidal channels.
Hardening and cementation is most prominent in supratidal zones, which therefore produce most tidal flat intraclasts. Supratidal intraclasts may also include diagnostic features such as cryptalgal lamination or bird’s-eye vugs (Shin, 1983). Facies Ifcr is interpreted to be an intraclastic breccia, composed of microbial bindstones that were eroded and transported by storm waves, and redistributed into upper intertidal-to-supratidal depths. 56
Dolomitic Intraclastic Boulder Floatstone (Ifb)
Macroscopic Observations: At core scale, facies Ifb is a massive-to-bedded (very thickly, medium), poorly sorted, inversely-to-crudely graded, pale-to-dark grey and black, dolomitic floatstone with boulder-to-cobble sized, rounded-to-subangular intraclasts held within a fossiliferous-to-non fossiliferous mud matrix (Fig.19B-D). Identifiable bioclasts included articulated-to-disarticulated crinoid ossicles, brachiopods, bryozoans, and tabulate coral. Facies
IFb has three types of occurrence: (1) as floatstone with rounded, boulder-sized, dark intraclasts of cemented material floating in a non-fossiliferous mudstone matrix, located at 1708.4 m (5605 ft) (Fig.19D); (2) as a massive, chaotic floatstone with subangular-to-rounded, cobble-sized intraclasts of pale mudstone and dark cemented material floating in a fossiliferous mudstone matrix, observed first at 1734.3 m (5690 ft) (Fig.19B); and (3) as a medium bedded, inversely graded floatstone with sub-angular, cobble-sized intraclasts of bioclastic floatstone held in a non- fossiliferous mudstone matrix, located at 1739 m (5705.5 ft) (Fig.19C). Diagenetic alterations observed at core scale include fracturing, salt plugging, and the sparse development of vuggy porosity.
Microscopic Observations: Thin section samples were taken from type 1 and 2 occurrence textures. Within the type 1 texture, facies Ifb is a fossiliferous intra-micrite with no other allochems except for intraclasts. This unit is comprised entirely of dolomite with equigranular sutured to aphanotopic dolomite fabrics with planar-e to nonplanar crystal shape and contacts, suggesting a moderate amount of recrystallization occurred within the muddy matrix. Within the intraclasts, matrix leaching of fine grains and replacement dolomite rhombs yield significant intercrystalline porosity and some reduced vuggy porosity. Diagenetic cements observed include blocky dolomitic cement and remnant-recrystallized crustal cement, occurring 57
exclusively within the intraclast. Observed pore types include interparticle, intercrystalline and
vuggy porosities.
Within the type 2 texture, the facies is a sorted biosparite with fossil fragments of
crinoids and brachiopods. This texture is comprised mainly of dolomite with minor amounts
(<5%) of anhydrite. Sutured equigranular dolomite fabrics dominate this texture with nonplanar
crystal shape and contacts. Stylolitization is excessive with oil-saturated columnar-to-irregular
stylolite swarms brecciating the sample. Dissolution along stylolite swarm sutures is excessive,
creating open microchannels. Blocky-to-granular and remnant-recrystallized marine crustal dolomitic cements are observed throughout the sample. Pore types observed include microchannel, vug, and intercrystalline porosities. Blocky to granular cements reduce to occlude stylolite-focused microchannels and remnant crustal occurs within background matrix of non- brecciated zones. Stylolites are observed to cross-cut remnant-recrystallized crustal cement.
Interpretation: Intraclasts are products of erosion that could have been generated by either (1) wave reworking of underlying beds or (2) subaqueous debris flows. Sediment gravity flows are major sedimentation processes occurring on carbonate debris aprons and fans
(Tournadour et al., 2015; Mullins and Cook, 1986). The appearance of polymictic intraclastic floatstones where the intraclasts are similar in appearance to other lithofacies described within this study, both of which occur in proximity to this unit, suggest an origin by downslope transport, with probable erosional reworking of underlying deposits. Very thick-to-massive deposits of poorly-sorted, crudely-graded debris with very-large intraclasts seemingly “frozen” within a muddy matrix are characteristic of subaqueous debrites. Unfortunately, the overlying strata were not observed due to a break in core recovery, eliminating the opportunity for further 58
verification by interbedded relationships with turbidites. Facies Ifb is interpreted to be a debrite,
consisting of bioclastic, mud-rich sediment sourced from up-slope environments.
Coated Grained Dolomitic Rudstone (Rcg)
Macroscopic Observations: Facies (Rcg) is a thin-to-medium, bedded, brown-to-black, dolomitic rudstone consisting of pebbles (up to 12mm in diameter), that are intact-and-broken micrite-coated grains (Fig.20A). The coated grains exhibit a range of sizes, having both fossil and non-fossil fragments. Fossil fragments appear to have been brachiopods. No observable structures exist within micritic envelopes and grading of grains is absent. Additionally, the fabric of this rudstone facies is often chaotic, fractured, and salt plugged, with different colored muddy sediment infilling cavities between grains.
Microscopic Observations: Not sampled.
Interpretation: The identification of structures, composition and size are important elements for identifying and deciphering the origin of various coated spherical grains such as oncolites, pisoids, ooids and cortoids (Flugel 2004). These coated grains do not exhibit internal laminations or structure, suggesting a peloidal or cortoidic origin. Furthermore, these grains are larger than typical ooids. Cortoids are coated grains which feature micritic envelopes formed as a result of various micritization processes (microboring and infilling by of photosynthetic and non- photosynthetic organisms). Micritization typically takes place at or near the sediment-water surface. Peloids are coated grains that vary in size, lack internal structure, and are generated by accumulation of fecal pellets, disintegration of calcified microbes/algae, bio-erosional processes
(such as microboring or rasping), reworking of micritic clasts, and a variety of other processes
(Flugel, 2004). Coated grains are often associated with high energy conditions with constant 59
wave action, at or near-wave base. Broken or fragmented coated grains often indicate
allochthonous transport and re-sedimentation (Flugel, 2004). Due to lack of internal structure,
fossil nuclei and size, pebble constituents observed within facies Rcg are interpreted to have
either a peloidal or cortoidic origin. The broken-to-fragmented state of the coated-grains, coupled with the shallow water origin of cortoids and peloids, suggests facies Rcg represents sediment gravity flows, most likely debrites comprised of shallow water, wave-influenced grains sourced from up-slope environments, at or above the FWWB.
Massive Amalgamated Skeletal Rudstone (Ram)
Macroscopic observations: Facies Ram is an inclined, massive, crudely graded, heavily cemented, dolomitic rudstone with amalgamated beds of mudstone and skeletal grainstone, and laterally aligned clasts, as measured with respect to the rock core (Fig.20B). Cementation is extensive, visible by eye, as thick, dark, isopachous crusts enveloping skeletal debris. Observed fossils include brachiopods, echinoderms, crinoids, tabulate coral and bryozoans.
Microscopic Observations: Facies Ram is a unsorted biosparite with allochems of brachiopods, bryozoans, and echinoderms. This lithofacies is comprised entirely of dolomite and features porphyrotopic contact-rhomb to sutured dolomite fabrics, suggesting a moderate amount of recrystallization occurred. Remnant-recrystallized crustal cement is prominent throughout the sample in addition to block and granular dolomitic cements. Stylolitization is excessive with high amplitude-to-irregular stylolite swarms brecciating the sample. Dissolution along stylolite swarms is prominent, creating microchannels along suture zones. Pore types observed include intercrystalline, channel, moldic and vuggy porosities. Pore filling subhedral-to-anhedral dolomitic cement infilled micro-channels, vugs, and moldic porosities. 60
Interpretation: Crude-grading and laterally aligned clasts are interpreted to suggest
fluctuating energy conditions and transport by either waves or currents. These features are
commonly observed in storm deposits (tempestites). Bedding amalgamation is also a common
feature observed in tempestites, occurring when the erosional surfaces between consecutive
storm beds is not discernable, making beds appear as thick single units that lack mud (Huck,
2013; Compton, 1985). Due to its coarse bioclastic nature, crude grading, lateral alignment of
fossils and amalagmated beds, facies Ram is interpreted to be part of a tempestite sequence.
Proximal tempestites are often amalgamated, and exhibit fair-weather wave influence such as
winnowing and lamination (Huck, 2013; Perez-Lopez and Perez-Valera, 2012; Brand and Elias,
1989). Furthermore, the occurrence of thick, amalgamated, and heavily cemented storm-
generated bioclastic deposits have been described in leeward slope environments of the Bahamas by Hine et al, (1981). The extensive cementation is interpreted to represent times of relative quiescence between periods of intense storm activity (Hine et al., 1981). The best preservation of tempestite deposits is between the SWWB and FWWB (Perez-Lopez and Perez-Valera, 2012).
Due to the evidence of wave-reworking, facies Ram is interpreted to be a proximal tempestite deposit (deposited at or near FWWB), with its features being best explained by constant wave- action condensing skeletal grains, removing mud, and amalgamating bioclastic storm deposits.
Cross-bedded Skeletal Rudstone (Rcb) Macroscopic Observations: Facies Rcb is observed as a medium bedded, cross- stratified, dark-to-light grey, dolomitic, skeletal rudstone with flat-to-undulatory bedding contacts. Cross-bedding consists of coarse (>2mm) bioclasts inclined forming forsets (Fig. 20C).
The bottom bedding contact is flat, with the upper contact being undulatory.
Microscopic Observations: Not sampled 61
Interpretation: Cross-stratification may be produced from unidirectional and bidirectional flow regimes (Boggs, 2006). Within marine environments it is more commonly
associated with bi-directional flow regimes from wave action, however, cross bedded
conglomerates may also be formed from unidirectional bottom currents (Tournadour et al.,
2015). Facies Rcb occurs only once at 1738.6 m (5704 ft), overlying facies Ifb, an interpreted
debrite, suggesting this facies may be the reworked upper part of the debrite. Due to the evidence
of wave-reworking and the close proximity of facies Rcb to tempestite successions (~1.5 m),
facies Rcb is interpreted to be a wave-reworked, uppermost segment of a debrite, deposited
above the SWWB.
Heterolithic Crypto-Algal Dolomitic Bindstone and Mudstone (Hcr) Macroscopic Observations: Facies Hcr is observed as apparently inclined, heterolithic sediment consisting of wavy-to-planar bedded, grey, dolomitic mudstone thinly interbedded with brown-white dolomitic bindstone. The bindstone features planar-to-wavy cryptalgal lamination and rare laterally linked hemispherical stromatolites (Fig. 21A-C). The algal bindstones are distinct from the mudstones due to their brown or white color and presence of stratiform fenestral pores. This facies occurs in two distinct fabric types: (1) as grey mudstone is interbedded with high amplitude (<15cm), laterally linked stromatolites and (2) as brown mudstone is interbedded with high amplitude (>15cm) hemispherical stromatolites (Fig.21A and
C).
Microscopic Observations: Facies Hcr is a tan biolithite with cryptalgal lamination (Fig.
22), interlaminated with fossiliferous, clotted micrite (Figs.22 and 23). The lithofacies is entirely dolomitic and features aphanotopic (bindstone) to equigranular sieve (clotted mudstone) dolomitic fabrics. Granular to blocky dolomitic cement is observed forming drusy cement 62
mosaics. Excessive leaching is witnessed within and around isopachous micritic envelopes of the
clotted texture. Irregularly shaped stylolite swarms are observed close to bedding contacts, with
excessive dissolution and cementation between swarms. Many clots appear to be enlarged via dissolution and subsequently reduced-to-occluded by granular cements. Pore types observed include intercrystalline, interparticle, vug and fenestral porosities.
Interpretation: Microbial laminites and stromatolites are biogenic deposits formed as a result of sediment trapping, binding, microbial calcification and/or precipitation induced by microbial activity (Coniglio et al., 2004). Clotted fabrics are commonly observed in reefal and peritidal environments, occurring as clotted interbeds as the result of microbial activity (Flugel,
2004). Microbial laminites may be found at any depth across the peritidal zone, however the presence of laterally linked hemispherical stromatolites is most often observed in upper subtidal-
to-intertidal depths (Pratt, 2010). In Silurian reefs located in Ontario, alternating interbeds of clotted mudstone and cryptalgal bindstone has been linked to: (1) variations in sediment flux caused by changes in current strength, wave action, or tidal pulses; (2) changes in microbial community structure; or (3) variations in the relative importance of microbial mat calcification vs. accumulation of detrital sediment (Coniglio et al., 2004). The low-relief (~30 cm), domal and hemispherical stromatolites observed may have formed from a combination of mat doming and preferential accretion on elevated surfaces (Pratt, 2010). The excellent preservation of lamination suggests a lack of bioturbation. Facies Hcr is interpreted to be tidally influenced, alternating layers of microbial mats and clotted mudstone deposited in the upper subtidal-to-intertidal conditions.
63
Heterolithic Disturbed Crypto-Algal Bindstone and Skeletal Packstone (Hcrd)
Macroscopic Observations: Facies Hcrd is a medium bedded, grey-brown, heterolithic
unit consisting of partial to completely brecciated, brown, dolomitic cryptalgal bindstones,
alternating with grey, dolomitic skeletal packstones (Fig. 22 D). The lamination of the bindstone ranges from complete to “disturbed” (wrinkled) to discontinuous (brecciated). Skeletal debris found in the packstone is preferentially orientated to bedding, being observed as white specks, with fossil fragments not being resolvable at macroscopic scale.
Microscopic Observations: In thin section, facies Hcrd is a tan, unsorted biosparite with coarse fragments of cyanobacterial mats, brachiopods, mollusks, echinoderms, and crinoids.
Most micritic components of the original floatstone appear leached out, extensively
recrystallized or replaced by dolomitic cement. The lithofacies is comprised entirely dolomite
and exhibits equigranular, subhedral-to-anhederal, sieved-to-sutured dolomite fabrics. Anhedral
granular and blocky subhedral dolomitic cements are observed between and within skeletal
grains, forming granular to drusy mosaics. Multiple generations of cementation are present with
finer, tan, granular dolomitic cements being leached out and replaced by coarser bright white,
blocky dolomitic cement. Pore types observed include intercrystalline, intraparticle, and vuggy
porosities.
Interpretation: Algal mats are typically deposited in upper subtidal-to-intertidal
conditions. The preferred clast orientation of bioclasts suggests transport and alignment by either
current or wave transport during storms, or as a subaqueous debris flow. This facies is
interpreted to be originally deposited as a tidally influenced microbial laminite within the upper
intertidal to supratidal zone, that was later disturbed by an influx of bioclastic material. This
influx of bioclastic material is interpreted to most likely be transported by storm waves or 64 sediment gravity flows. Facies Hcrd is interpreted to be a subtidal-intertidal microbial laminite that was disturbed by during storm-wave action or a slope destabilization event.
Wrinkled Algal Bindstone Facies (Bcr)
Macroscopic Observations: Facies Bcr is a thin-to-medium bedded, grey-to-brown or black-to-white, dolomitic bindstone with wrinkled-to-serrated cryptalgal laminations, teepee structures and desiccation cracks (Fig. 24A-B). This facies occurs in two separate intervals; the lower most interval (1686.5 m or 5533.2 ft) is observed as a single thinly bedded, wrinkled mat, occurring only once (Fig.24 B). The second interval, occurring from 1684.0-1685.8 m (5525-
5531 ft) (Fig. 24A) begins as wrinkled mats with teepee structures, transitioning into more serrated mat geometries with desiccation structures.
Microscopic Observations: In this section, facies Bcr is a biolithite with cryptalgal lamination. The lithofacies is comprised almost entirely of dolomite (93%) with localized zones of calcite inclusions within dolomite crystals (<2%) and minor amounts (<5%) of anhydrite and halite. Subhedral, sutured equigranular dolomite fabrics with nonplanar crystal contacts are laminated with zones of coarser subhedral-to-anhedral, sutured equigranular crystals with nonplanar contacts, suggestive that this unit has undergone a moderate to high amount of recrystallization. No pore filling cement is observed, presumably from the extensive recrystallization. Pore types observed include intercrystalline and fenestral pores.
Interpretation: Microbial mats are often observed from upper subtidal-to-supratidal depths within Paleozoic successions. Supratidal mats undergo frequent subaerial exposure, forming desiccation features such as teepee structures, wrinkled/serrated lamination, and desiccation cracks. Due to the restricted nature of the supratidal zone, bioturbating biota are in 65
lower abundance, leading to higher preservation potential of lamination (Pratt, 2010; Shin,
1983). The combination of preserved lamination with wrinkled-to-serrated cryptalgal geometries, teepee structures and desiccation cracks are interpreted to show facies Bcr was deposited in supratidal conditions.
Coral-Stromatoporoid Reef Framestone (Fr)
Macroscopic Observations: Facies Fr is observed as tan-to-greyish white framestone
with encrusting stromatoporoids and branching tabular coral colonies (Fig.24C-D).
Stromatoporoids often exhibit an anastomosing laminar geometry and are continuous-to- discontinuous with cavities infilled by muddy internal sediment (Fig.24C). Tabulate corals are observed as branching colony “thickets” with growth structures parallel with the core.
Macroscopic diagenetic alterations include salt plugging, natural fractures, and the development of vugs.
Microscopic Observations: In thin section, facies Fr is a tan biolithite, with in-place fossil frameworks consisting of stromatoporoids and tabulate corals. The lithofacies is almost entirely dolomite with trace amounts of calcite (<2%), occurring as inclusions within dolomite crystals, and minor amounts of anhydrite (<2%). Aphanotopic, anhedral dolomite fabrics dominate all samples taken, suggesting the unit underwent low amounts of recrystallization.
However, stromatoporoid crusts exhibit equigranular sutured dolomite fabrics with nonplanar crystal shapes and contacts, suggesting preferential recrystallization of stromatoporoid laminae.
Sparse amounts of blocky subhedral and granular anhedral dolomitic cements are observed, creating drusy mosaics. Pore types observed include growth framework, intercrystalline, vug and interparticle. Dissolution of matrix and growth framework pores is prominent. 66
Interpretation: Stromatoporoid sponges often grow in laminar forms, growing and anchoring sediment (James and Wood, 2010; Flugel 2004). Tabular stromatoporoids often occur at high-energy reef crests being locally bound by calcimicrobes, with automicrite, micrite, or marine cements. In back-reef conditions, growth forms range from domal, bulbous, and dendroid geometries. Several of these forms are less wave-resistant, so they tend to colonize the back reef and peritidal environments. The variation of stromatoporoid geometry as a function of energy conditions within Silurian patch reefs has been debated. Sandstrom and Kershaw (2008) found that stromatoporoids only grew in an anastomosing (repeated laminar), coalescent geometries across sampled Silurian patch reefs in Greenland. In contrast, tabulate corals are interpreted to have formed as dome-shaped, erect-branched, or chain-like geometries that varied as a function of wave energy conditions. Other studies interpreted delicate branching types of coral geometries with lower-energy conditions, such as back-reef areas (James and Wood, 2010; Flugel, 2004). In this study, facies Fr is interpreted to be a skeletal framestone formed within reef zone, above the
FWWB.
Lithofacies Associations A total of six lithofacies associations were observed from the vertical succession of deposits from the 9-33 core. The lithofacies (facies) associations are interpreted by the sequential succession of lithofacies and the nature of their bedding relations. The stratigraphic succession of lithology and facies is shown in Figure 26 with an explanation of symbology provided in Figure
25.
Supratidal Facies Association (FA1)
The supratidal facies association (FA1) consists of lithofacies Bcr and Ifcr (Table 6). At the base of FA1, bindstone (lithofacies Bcr) occurs interbedded with microbial laminite 67
floatstone breccias (Ifcr), interpreted to be periodic storm lag deposits. FA1 is interpreted to be
deposited within supratidal conditions due to the occurrence of serrated-to-wrinkled cryptalgal
geometries, teepee structures and desiccation cracks. The frequency and thickness of storm
breccias decreases moving upward with the laminites becoming increasingly desiccated and
transitioning from wrinkled-to-serrated algal geometries. This stratigraphic trend is interpreted to
represent a shallowing-upward transition (Figs.26 and 27).
This association ranges from appoximately 0.30 m-to-1.5 m thick. However, the maxium
thickness is probably greater, as this section is captured at the top of the cored interval. FA1
appears twice within the cored interval: (1) as a 15cm thick supratidal facies association of
planar to crinkled microbial laminites with teepee structures, interbedded with storm breccias
that is overlain by intertidal bindstone facies Hcr; and (2) as a 1.5 m thick succesion of
interbedded storm breccias and microbial mats, transiting to desiccated microbial mats, to the
very top of the cored interval. The first occurrence this facies association is both overlain and
underlain by intertidal deposits (FA2); the second appearance occurs at the top of the cored
interval (Figs.26 and 27).
Intertidal Facies Associations (FA2)
The intertidal facies association (FA2) consists of laminated dolomitic mudstone
(lithofacies Ml), mottled dolomitic mudstone (lithofacies Mm), heterolithic dolomitic clotted
mudstone cryptalgal bindstone (lithofacies Hcr), disturbed heterothic dolomitic mudstone- cryptagal bindstone (lithofacies Hcrd), and dolomitic skeletal packestone (lithofacies Sp) (Table
6). At the base, dark mottled mudstones (lithofacies Mm) are sharply overlain by lighter colored
laminated mudstones (lithofacies Ml); these facies then alternate until facies Mm is absent.
Facies Ml is periodically interbedded with erosive skeletal packestones (lithofacies Sp), 68
gradationally transitioning into flat-to-wavy heterolithic clotted mudstones and cryptalgal
laminites-or-stromatolites (lithofacies Hcr). Facies Hcr then gradually transitions upward into a wrinkled, cryptalgal bindstone with teepee structures (lithofacies Bwr) (Fig.26). The upward transition from mottled mudstone to laminated mudstone, then wavy cryptalgal laminites and domal/hemispherical stromatolites, which become episodically interbedded with storm-lag breccias in the uppermost intervals is interpreted to indicate FA2 is an subtidal-to-intertidal
sedimentary succession (Figs.26 and 27).
The preservation of lamination suggests a relatively low amount of benthic burrowing
organisms, which is commonly observed within upper intertidal conditions, according to Pratt
(2010). The occurrence of erosive, grain-rich skeletal packestones (lithofacies Sp) and
floatstones (lithofacies Hcrd) with preferential clast orientation, occuring periodically within the
uppermost occurences of the association, is interpreted to be records of storm deposits. This
lithofacies succession is interpreted to represent a shallowing-upward succesion, transitioning
from the subtidal zone to the upper intertidal zone (Figs.26 and 27). FA2 occurs twice within the cored interval, and ranges from 0.6 m-to-7 m thick. Subtidal back-reef deposits (FA3) underly
FA2 and FA2 is overlain by supratidial deposits (FA1).
Subtidal Back-Reef Association (FA3)
The subtidal back-reef association (FA3) consists of massive mottled mudstone
(lithofacies Mm), skeletal wackestone (lithofacies Sw), and very thickly-bedded skeletal
floatstone (lithofacies Sfm) (Table 6). From base-to-top of FA3, dark-colored skeletal wackestones and floatstone transition into mottled mudstone. These lithologies are interpreted to
be deposited mainly through normal marine background sedimentation processes (such as
palegic rain and authocthoneous production from benthic organisms). The number of observed 69
fossils and bioclastic material decreases dramatically moving up section. FA3 is interpreted to represent a deepening-upward succession, where the reference location is accumulating along slope, becoming more distally-located from the skeletal debris-shedding carbonate factory of the pinnacle reef structure. Due to the lack of any evidence of wave-influence, FA3 is interpreted to
be deposited below wave-base along the leeward slope.
This facies association is observed once throughout the cored interval and is
approximately 5.5 m thick. This lithofacies association is most likely thicker, however due to
poor core recovery, a signficant section immediately below this interval (above FA4) was not
recovered. Resistivity-based image logs taken from the missing core interval, suggest that the missing interval consists of mud-supported textures similar to FA3. FA3 deposits are overlain by
intertidal deposits (FA2), and overlies debrite association deposits (FA4) (Figs.26 and 27).
Debrite/Turbidite Association (FA4)
The debrite and turbidite association (FA4) consists of massive intraclastic floatstone
conglomerate (Ifb), medium-bedded cross-stratified bioclastic rudstone (lithofaices Rcb), pebble-
cobble sized peloidal rudstone (lithofacies Rcg), normally graded skeletal grainstone (lithofacies
Sg), and convoluted grey mudstone (lithofacies Mpg)(Table 6). From base-to-top of FA4,
alternating beds of thickly-bedded chaotic intraclastic floatstone conglomerate and peloidal rudstone are sharply overlain by medium-bedded interbeds of normally-graded skeletal grainstone and grey mudstone with convoluted upper contacts. This section is overlain by a thick interval of chaotic, inversely-to-normal-to-crudely graded intraclastic floatstone conglomerate
with interbeds of pelodial rudstone and mudstone, with one rare interbed of undulose, cross-
stratified bioclastic rudstone. Due to poor core recovery, a large break in recovered core (~25 m) 70
occurred, immediately followed by a 0.5 m thick, intraclastic floatstone conglomerate with a
single boulder sized intraclast floating in nonfossiliferous muddy texture.
The debrite-turbidite succession is interpreted to be comprised of both distally and
proximally sourced sediment gravity flow events, stemming from destablization events occuring
in upper, middle and lower slope segments. Due to the variation in grading of bioclasts (normal-
to-inverse-to-crude) and its mud-supported intraclastic texture, the intraclastic floatstone
conglomerate (lithofacies Ifb) is interpreted to be proximally-sourced debrite. The sporadic
occurrence of grain-supported pelodial rudstones are interpreted to be more distally sourced
debrites, generated from destablization events in up-slope environments. Intervals of pelodial
rudstones are often capped by normally graded interbeds of skeletal grainstone and mudstone
with convoluted upper contacts and are interpreted to be carbonate turbidites corresponding to 1a and 2ab beds of the Meischer sequence (Bouma Ta and Tde divisions). The rare occurrence of
cross-stratified bioclatic rudstone immediately overlying an interpreted debrite deposit is interpreted to suggest the uppermost segments of these debris flows were reworked by wave-
action or bottom currents, most likley placing the location of first debrite succession above
wavebase. The second occurrence of debrites occurs just below the 25 m thick break in
recovered core. This single occurrence of a intraclastic boulder conglomerate is immediately overlain by subtidal back-reef deposits and is interpreted to be a proximal debrite most likely consisting of previously remobilized muddy intraclastic sediment that was subjected to a subsequent destablization event in lower slope settings.
The debrite-turbidite succession occurs in two intervals with the largest interval being 8 m (26 ft) thick, followed by a break in the core (~25 m), then immediately following the missing section, a 0.5 m thick succession featuring only the intraclastic floatstone conglomerate. 71
Subsequently, the true thickness of the debrite-turbidite association is not resolvable, however it may be up to 35 m thick, most likely interfingering with the subtidal back-reef successions. No
stratigraphic trends are observed within the debrite-turbidite succession. FA4 overlies the tempesite association (FA5) and underlies the subtidal back-reef association (FA3).
Tempestite Association (FA5)
The tempestite association (FA5) consists of pale grey undulate-to-flat mudstone
(lithofacies Mpg), undulatory normally-graded skeletal grainstone (lithofacies Sg), and
amalgamated skeletal rudstone (lithofacies Ram) with laterally aligned clasts. Starting at base of
FA4, alterations between medium bedded mudstone (lithofacies Mpg) and amalgamated skeletal
rudstone (Ram) with extensively-cementation are sharply overlain by well-preserved, medium
sized interbeds of undulatory to flat normally graded bioclastic grainstone (lithofacies Sg) and
grey mudstone (lithofacies Mpg) (Fig.26).
The transition from the interbedded amalgamated skeletal rudstone (lithofacies Ram) and
grey mudstone (lithofacies Mpg) interval into interbedded cycles of skeletal grainstone
(lithofacies Sg) and grey mudstone (lithofacies Mpg) is interpreted to record the shift from
proximal to distal tempestite associations, indicating a relative shift in water depth to deeper
settings above wave base (Fig.26 and 27). Within the tempestite succession there are four distinct
intervals with extensive cementation, which can be readily identified from a dark grey coloration
and visible isopachous micritic rims enveloping the bioclasts. These dark colored, extensively
cemented intervals which alternate with lighter colored brown-grey intervals are interpreted to
have an origin related to either 1) extensive recrystallization causing discoloration; or 2)
generation of marine-cemented horizons corresponding to periods of relative quiescence between
periods of increased storm activity (Hine et al., 1981). 72
The tempesite association (FA5) is 5.1 m thick and interpreted to represent a deepening-
upward transition. The large abunance of skeletal material is interpreted to suggest close-
proximity to active carbonate factory/reefal settings. FA5 directly overlies the reef association
(FA6) and underlies the debrite-turbidite association (FA4).
Skeletal Reef Facies Association (FA6)
The skeletal reef facies association consists of delicate branching colonial tabulate corals and coalesced laminar stromatoporoid framestones (lithofacies Fr), in addition to internal sediment consisting of bedded, mottled skeletal floatstones (lithofacies Sfu) and massive, mottled mudstone (lithofacies Mm) (Table 6). From base to top, mottled mudstone and skeletal floatstones are sharply interbedded with fossil framestones (Fig.26 and 27).The presence of articulated bioclastic packestones, in between metazoan skeletons, indicates these mud-supported deposits are internal to the reef framework, most likely being deposited due to in-place autocthoneous reef-dweller life cycles, and potentially suspension fallout. Other studies have noted the signficant volumes of internal sediment within Guelph Formation pinnacle reef frameworks (Rine, 2016).
Within this lithofacies association, no depth-related trends were observed, however growth patterns within coral colonies indicate reef growth upward, perpendicular to the orientation of the rock core. The skeletal reef lithofacies association is observed once within the cored interval is 7.9 m thick. However, this facies assoication is presumably thicker, as this interval occupies the lowest core depths acquired. This facies association is overlain by FA4
deposits and represents the bottom most section of the 9-33 well core.
73
Depositional Environments
The succession of depositional environments observed within the 9-33 well core may be
divided into two depositional stages: (1) active reef and associated reef-derived debris apron
deposits (reef stage); and (2) a peritidal stage consisting of subtidal-to-intertidal and supratidal
sedimentary successions developing on top of the underlying reef stage deposits (Fig.28 and 29).
Reef Depositional Stage
Reef zone deposits (FA6, Table 6) consist of coalesced laminar stromatoporoids and
“branching” colonial tabulate coral framestones, which built rigid, wave-resistant, topographic
structures (Figs. 27 and 28). Internal cavities between metazoan framestones are infilled by
fossiliferous muddy sediment. Fossil integrity suggests reef cavity-infilling sediment was
deposited by normal reef activities (bioeroson, secretion, grazing, etc.) with influences of wave-
transport. Growth patterns within in-place tabulate coral colonies suggest vertical reef growth
occurred perpendicular with respect to the core. The delicate branching nature of tabulate coral
colonies suggests reefal deposits recovered within the rock core were built leeward of the reef
crest, where wave energy was relatively lower, compared to that of the fore-reef and reef crest environments, encountered at reef zone (James and Wood, 2010). Reef zone successions are
observed to be onlapped by bioclastic deposits of the leeward-reef slope (FA5-3, Table 6).
The leeward-reef slope environment is comprised of bioclastic debris and muddy
sediment, interpreted to have been transported from reef zone settings into the slope environment
via storm wave resuspension (FA5) and sediment gravity flows (FA4), in addition to muddy
sediment deposited during normal marine conditions (FA3)(Fig.28). Due to variation in bioclastic content observed between facies associations, the leeward slope environment may be divided 74
into a proximal and distal slope segment (Figs.27 and 28). Within this study, the boundary between proximal and distal slope segments is placed at the SWWB, but may also be approximated by a sharp decline in the relative volume of skeletal content, as in other studies
(Rine, 2016).
The proximal leeward slope setting consists of amalgamated skeletal rudstones and mudstones, well-bedded skeletal grainstones and mudstones, and massive chaotic floatstone conglomerates and bedded peloidal rudstones, all of which are interpreted to be deposited by
storms (FA5) or mass-transport processes (FA4) (Fig.27 and 28). Amalgamated skeletal rudstones, well-bedded skeletal grainstones and mudstones are interpreted to be proximal and
distal tempestite associations (FA5, Table 6). Proximal amalgamated tempesites are interpreted to
be deposited closer to (or above) the FWWB, where fairweather waves and subsequent storm
events align skeletal clasts and amalgamate bedforms. Distal well-bedded tempestite successions
are interpeted to be deposited between the FWWB and the SWWB, where a higher preservation
potential is likely, and no wave amalgamation occurs (Perez-Lopez and Perez-Valera, 2012)
(Fig.27 and 28). The transition from proximal to distal tempestites observed in FA5 is interpreted
to represent a deepening-upward transition, indicative of increasing deeper depths from the
FWWB to closer to the SWWB (Fig.28)
Within the proximal leeward slope segment, overlying the tempestite association (FA5),
is a succession comprised of chaotic intraclastic floatstone conglomerates, peloidal rudstones,
cross-stratified bioclastic rudstones, skeletal grainstones and grey mudstones, interpreted to represent a combination of distal-to-proximally sourced debris flows and turbidites (FA4) (Fig.27 and 28). Chaotic intraclastic floatstone conglomerates feature mud-supported textures and cobble sized intraclasts of grey mudstone, interpreted to suggest the facies is a proximally-sourced 75
debrite (Mullins and Cook, 1986), with intraclastic sediment most likely sourced from the
underlying tempestite succession (FA5). A rare cross-stratified bioclastic rudstone occuring at the top of a proximally-sourced debrite is interpreted to suggest reworking by wave-action or bottom currents. The evidence for wave influence in addition to the presence of single intraclastic
conglomerate 25m above the main debrite-turbidite succession and just below the subtidal back- reef association is interpreted to suggest the debrite-turbidite succesion spans from above-to- below wave-base (into the distal slope apron).
Interbedded with the proximal debrites are medium-bedded, grain-supported pelodial
rudstones. The thinner bedding, well-sorted and grain-supported textures observed in this facies
are interpreted to suggest these debrites represent the distal end of their run-out zone, therefore
are distally sourced (Mullins and Cook, 1986). Often overlying (or in close proximity) to
distally-sourced debrites are interbeds of medium-bedded normally-graded grainstone and
mudstone with convoluted upper contacts. These grainstone and mudstone interbeds are
interpreted to be turbidites representing 1a and 2ab beds of the Meischer sequence (Bouma Ta and
Tde divisions), most likely indicative of the overlying turbidty flow which develops as debris
flows reach the end of their run-outs (Mullins and Cook, 1986).
The distal leeward slope environment consists of the bottom most floatstone
conglomerate observed in the debrite-turbidite association (FA4) in addition to the autochthonous
skeletal floatstones, wackestones, and mudstones observed within the subtidal back-reef
association (FA3) (Fig.28). Many sediments exhibit mottled color textures, interpreted to suggest
bioturbation by benthic biota, and some fossils appear fully articulated. The rapid decrease in
skeletal fossil content from the base of the distal leeward-slope to the top (FA4-FA3), is interpreted represent a deepening-upward trend (Fig.28 and 27). 76
Reef stage facies associations captured within the 9-33 well core are interpeted to represent the lateral succession of sub-environments encountered leeward of the pinnacle reef, shifting the reference location downslope, further away from the active carbonate factory setting of the pinnacle reef. The bulk volume of sedimentary deposits observed within the leeward slope environment were comprised of tempesites and sediment gravity flows, interpreted to suggest wave-resuspension and downslope transport are the primary sediment transport mechanisms within the leeward slope environment.
Peritidal Depositional Stage
The uppermost section of the rock cored recovered from the 9-33 well is interpreted to be sedimentary successions deposited from a peritidal environment including intertidal and supratidal environments (FA1 and FA2, Table 6). Peritidal deposits are observed to directly
overly leeward-slope deposits, which may indicate peritidal deposits blanket all reef stage
deposits, moving down-the leeward slope. Directly overlying distal leeward reef slope deposits
(deposited below the wavebase), upper subtidal-to-intertidal successions (FA2, Table 6) occur,
consisting of laminated mudstones and mottled mudstones, erosive skeletal packstones and
floatstones, and heterolithic clotted mudstone with cryptalgal bindstone/stromatolites. Skeletal
packstones overlying erosive basal contacts and floatstones are interperted to be either fossil-rich
mass-transport deposits, possibly debrites, or storm-lag deposits; these lithofacies occur
episodically interbedded with normal-marine intertidal deposits.
Throughout the intertidal facies association, lithofacies transition from mottled
mudstones to laminated mudstone, to heterolithic clotted mudstone-cryptalgal bindstones; this
transition is interpreted to represent a transition from upper-subtidal to intertidal conditions. The transition into overlying supratidal facies successions is marked by the occurrence of wrinkled 77
cryptalgal geometries and teepee structures. Supratidal successions (FA1, Table 6) exhibit wrinkled-to-serrated cryptalgal geometries, teepee structures, occur interbedded with storm floatstone breccias and show evidence for increasing desiccation. The succession from intertidal to supratidal sediments is interpreted to represent a shallowing-upward trend, and there are two observed shallowing-upward packages (Fig.27).
The occurrence of peritidal shallowing-upward packages above leeward-reef slope sediments suggests fluctuating water depths and a relative fall in sea-level after the reef depositional stage. Hopper crystal casts observed within intertidal sediments indicate hypersaline conditions, before the end of the first shallowing-upward cycle. Additionally, fossil abundance was very low within the peritidal successsions, with the only visible fossil content being deposited during storms/or mass-flows.
Stratigraphy A 71.4 m (234 ft) section of core was studied from the 9-33 well, located in Otsego
County, Michigan. The section of studied core consisted of the Silurian-aged Guelph Formation
(Brown Niagaran) and Ruff Formation (A-1 Carbonate).
The Guelph Dolomite Formation The stratigraphy of the Guelph Formation was interpreted to be lithologic successions deposited within reefal, debris apron (proximal and distal), and peritidal environments.
Recovered Guelph Formation successions exist from 1755.6-1686.4 m (5760-5533 ft), measured depth, including gaps within the core record, and comprises 69 m of the 71.4 m of rock core.
Guelph Formation successions are split into two depositional stages separated by an unconformity: (1) reef stage and associated reef-derived debris apron deposits; and (2) a peritidal 78
stage. Reef stage sediments exist from 1693.1-1755.6 m (5555-5760 ft) and consist of reefal and
associated leeward debris apron successions.
Reef zone deposits (FA6, Table 6) exist at the base of the cored interval, from 1755.6-
1747.4 m (5760-5733 ft). These sediments consist of interbedded-to-massive deposits of tabulate
coral-stromatoporoid framestones with skeletal floatstones, packstones, and mudstones infilling internal cavities within the reef framework. No stratigraphic trends are observed within the reef
zone successions to suggest variations in relative water depth or carbonate production.
The debris apron succession of the Guelph Formation consists of bioclastic sediment
sourced from the carbonate factory settings (reef zone) located up-dip. The debris apron
succession exists from 1747-1693.1 m (5733-5555 ft), measured depth, and onlaps (stratal
contact visible at core scale) with underlying reef zone lithologies. The leeward debris apron
environment is split into proximal and distal segments, separated by wave base. However, due to
poor core recovery, the exact contact between proximal and distal debris apron environments is
not recorded within the 9-33 core.
The proximal leeward debris apron exists from approximately 1747.4-1734.3 m (5733-
5690 ft). From the base of the proximal leeward apron, an 8 m thick succession of interbedded
tempestites (proximal and distal) successions (FA5, Table 6) are observed to be sharply overlain
by 8-35(?) m succession of debris flows and turbidites (FA4, Table 6). The distal leeward debris
apron is observed from 1709.3-1693.1 m (5608-5555ft). From its base, the distal debris apron
consists of a single debrite, sharply overlain by autochthonous massive skeletal wackestones,
packstones, and floatstones of the subtidal back-reef association (FA3, Table 6). Storm wave-
resuspension is interpreted to be the primary off-bank sediment transport mechanism, feeding sediment to the proximal debris apron environment. This sediment is later remobilized during 79
slope-destabilization events, and redistributed downslope into more distal segments of the
leeward slope.
An unconformity was observed between reef stage and peritidal stage deposits of the
Guelph Formation at a depth of 1693 m (5554.5 feet) (Fig.29). The unconformity was observed
as a darkly colored, wavy, sharp erosive boundary occurring between massive mottled mudstone
facies (facies Mm) and laminated mudstone (facies Ml) and is interpreted to be a sequence
boundary (Fig.29). Below the unconformity, massive non-fossiliferous mudstone, interpreted to
be deposited in the distal debris apron (subtidal back reef association FA3, Table 6), is observed
to become pervasively vuggy just below the unconformity surface. The development of vuggy
porosity just below this contact is interpreted to suggest reef stage deposits (the pinnacle reef and
associated debris apron) underwent subaerial exposure. Overlying the sequence boundary is a
darker, vuggy mudstone approximately 2.5 cm thick. The dark, vuggy mudstone is capped by
another wavy erosive surface observed at the bottom contact of the lighter colored laminated
mudstone (facies Ml). This surface was interpreted to be a transgressive surface of erosion
(TSE), representing a relative rise in sea-level above subaerially-exposed reef stage deposits,
initiating deposition of peritidal deposits (Fig.29). This interpreted TSE surface marks the first
occurrence of peritidal deposition in the 9-33 core, and all overlying deposits were interpreted to
be of a peritidal origin.
Peritidal stage successions of the Guelph Formation consist of intertidal and supratidal
lithofacies associations (FA1, FA2, Table 6). Peritidal successions within the Guelph Formation are observed from 1686.4-1693 m (5533-5554.5 ft). From base, Guelph Formation peritidal deposits consist of laminated mudstone, which transition into flat-to-wavy microbial laminites and stromatolites, that are interbedded with periodic storm deposits. This succession is 80
interpreted to represent upper subtidal to intertidal environments, occurring from 1686.6-
1693.1m (5533.5-5555 ft). This succession is overlain by wavy microbial laminites that feature
evidence of desiccation (teepee structures) and stronger storm influences (wave-aligned floatstone breccias) at its uppermost interval; this succession is interpreted to represent a transition from upper intertidal-to-supratidal environments and is observed from 1686.45-
1686.64 m (5533.5-5333 ft).
From base, Guelph Formation reef stage successions are interpreted to represent the lateral shift of environments downslope from the active carbonate factory (reef zone, crest), along the leeward margin (Fig.29). This lateral shift of environments is observed as a relative deepening-upward transition within the reef stage succession of the Guelph Formation. A period of subaerial exposure ensued after deposition of the reef stage deposits, as evidenced by the presence of micro-karst and vuggy porosity development in the uppermost interval of reef stage deposits. Subsequently, a relative rise in sea-level occurred which initiated deposition of peritidal
stage deposits at approximately 1693 m (5554.5 ft) measured depth, where at least one
shallowing-upward package was observed, terminating at the Guelph Formation-Lower Ruff
Formation contact (Fig.30).
Ruff Formation-Guelph Formation Contact
The contact between the lower Ruff Formation (A-1 carbonate) and upper Guelph
Formation (Brown Niagaran) (Fig.30), is observed as sharp and erosive marked by the distinct change in color between the white/tan Ruff Formation microbial laminites and the brown/grey microbial laminites of the upper Guelph Formation. Just below the contact, wavy-to-flat heterolithic algal laminites-clotted mudstone are observed to become wrinkled and increasingly interbedded with storm-generated algal breccia floatstones, and immediately above these beds 81
are teepee structures. This final succession is roughly 0.3 m (1 foot) thick and is interpreted to
mark the transition from upper intertidal to supratidal conditions within the upper Guelph
Formation. The observed supratidal succession of the Guelph Formation is only about 5 cm
thick, interpreted to suggest some of the stratigraphic record may be missing, or possible
sediment starvation occurred due to complete shutdown of the carbonate factory. The overlying
Ruff Formation laminite disconformably overlies this paleo-topography, and exhibits low-
amplitude domal microbial geometries, interpreted to be suggestive shallow subtidal to intertidal
settings. Based on the evidence, the contact between the supratidal sediments of the Guelph
Formation into overlying subtidal-intertidal sediments of the Ruff Formation is interpreted as a second transgressive surface of erosion. The presence of micro-karst and development of vuggy porosity within uppermost Guelph Formation sediments suggests a period of subaerial exposure
occurred before deposition of the Ruff Formation (Fig.30).
The Lower Ruff Formation The Ruff Formation occupies only 2.4 m (8 ft.) of the 71m (234ft) section of rock core from the 9-33 well. Due to this fact, the interpretation of the lower Ruff Formation is limited.
The intervals observed of the unit are dominated by intertidal-to-subtidal and supratidal
microbial laminites. Additionally, relatively thin (2.5 cm thick) beds of brecciated algal floatstones indicate storm influence during deposition, and the preservation of these deposits along stratified beds suggests a lack of bioturbation, which may be related to hyper-salinity. In
the 9-33 well core, the Ruff Formation is comprised of approximately 0.45m (1.5 ft) of
interpreted subtidal/intertidal sediments while the remaining 1.95 m of sediment is interpreted to
represent supratidal conditions, with evidence of increasing desiccation from the occurrence of 82 mud cracks and serrated microbial geometries within the uppermost intervals. This succession is interpreted to be evidence of a shallowing-upward trend (Fig.27).
Geophysical Well Log Analysis The 9-33 well core is the basis of the geophysical well log model built for the debris apron and adjacent pinnacle reef environment of the Guelph Formation. Due to the thin occurrence of the Ruff Formation (<3 m) in this core, no interpretations of the geophysical well log signatures within this formation will be provided, as there is no core data to base interpretations from. The geophysical log model will be based mainly on gamma-ray signature of logged sediments but will corroborated using neutron porosity and formation bulk density logs to verify relative changes in relative rock fabrics, such as relative volumetric drops in fossil content.
Core-to-log interpretations of GR, NPHI and RHOB log signatures will be further enhanced using resistivity-based image logs, which may measure centimeter-scale variations in conductivity and produce electro-textures comparable to rock textures observed in the 9-33 core.
The section of interest within the cored interval of the 9-33 well occurs between 1755.6-
1684.0 m (5760-5525 ft) (Fig. 31). Some specific trends within the cored interval are easily discerned via well log analysis including a thick interval displaying very low (<15 API) gamma ray values which form cylindrical-to-irregular log patterns. Cylindrical and irregular log patterns are commonly associated with clean (low amounts of radiogenic material) carbonate successions found in reef and carbonate slope environments (Walker, 1984), both of which are observed in the 9-33 core. This clean carbonate succession is overlain by an interval with steadily increasing gamma ray values from its base, forming a funnel-shaped curve. Funnel-shaped curves are associated with fining-upward successions from a variety of environments (marine and nonmarine), one of which are tidally influenced environments. Sedimentary successions 83 recovered from this interval consists of shallowing-upward peritidal deposits of the Ruff
Formation and the Guelph Formation.
The top of the Guelph Formation was picked at an unconformity visible in the 9-33 core at 1686.4 (5533 ft) (Fig.31). In the geophysical log model, this depth pick places this unconformity contact two feet above the peak API value of a funnel trend (shoaling upward pattern) in the gamma ray (Fig.31). Furthermore, this 1686.4 m (5533 ft) pick is accompanied by low porosity measurements from the NPHI log which is not corroborated by the visible shows of porosity below the Guelph Formation-Ruff Formation contact within the core. Ideally, the
Guelph Formation-Ruff Formation contact should manifest as gradually increasing gamma ray signatures creating a funnel shape pattern (showing transition from subtidal to supratidal), with peak GR values (at the very top of funnel) corresponding to the top of the supratidal sediments and show a porosity kick in the NPHI log denoting the visible open porosity observed in the core. This discrepancy between uncharacteristic GR and NPHI log signatures is interpreted to suggest the Guelph Formation-Ruff Formation contact should be placed at1690.4 m (5536 ft), where geophysical log signatures are more in agreement with the trends observed at core scale.
The depth discrepancy between core and geophysical logs is interpreted be explain by either: 1) sampling rate issues as the geophysical logs sample at 0.45 m (1.5 foot) increments compared to the core which is described a mm scale; 2) depth-matching issues between the core and geophysical logs; or 3) stretching of the open-hole wireline cable of the geophysical log tool due to tension, resulting in mismatch depth readings.
Geophysical Log Model Core analysis has revealed that Guelph Formation successions recovered from the 9-33 well may be split into two depositional stages comprised of four major depositional 84
environments (Fig.27). The geophysical log signatures of these four depositional environments
are shown in (Fig.31). GR shape analysis revealed only two distinct trends (irregular/cylindrical
and funnel-shaped), across these environments. The lowermost section 1755.6-1693.1 m (5760-
5555 ft) encompasses reef, proximal debris apron, and distal debris apron environments, all which are characterized by an irregular to cylindrical-shaped gamma ray pattern and correspond
to the reef stage of deposition. It is extremely difficult to differentiate these associations from
GR, NPHI, and ZDEN signatures alone. The uppermost section 1693.1-1686.4 m (5555-5533 ft)
consists of peritidal successions of the Guelph Formation, including both intertidal and supratidal
associations, and is characterized by a funnel-shaped gamma ray pattern (Fig.31). The geophysical profiles of the distal debris apron association will not be reviewed to any further detail, due to the absence of core data to validate any interpretations made from the RBHI, neutron or density logs. However, resistivity-based electrotextures observed within this interval are characterized as largely resistive with heterogeneously distributed conductive features, interpreted to be consistent with the mud-rich bioclastic fabrics observed from the core recovered within this interval.
Reef Association Geophysical Profile
Geophysical signatures of the reef zone association (FA6, Table 6) are observed from
1747.4-1755.6 m (5733-5760 ft) (Figs. 31 and 32). The gamma-ray signatures of the reef zone
are very low, ranging from approximately 12-15 API (Fig.32). Gamma-ray log patterns are
observed as irregular to cylindrically shaped, and interpreted to represent clean, interbedded
carbonate successions. Total measured porosity values obtained from the NPHI log within the
Guelph Formation reef association range from approximately 7-to-15%, with most intervals
indicating porosity shows greater than 10% (as noted by red infill, via a >10% porosity cutoff), 85
occurring with some relatively thin (~1.8m or less), tighter intervals with porosity values
reaching < 7% (Fig. 32). Resistivity textures displayed in the image log, exhibit a relatively
heterogenous succession of lithologies with varying degrees of conductivity. No correlative
trends are resolvable between rock textures observed from the core and electric textures observed
from the image log (Fig.32). This is most likely due to either a low amount of electrical contrast
between distinct lithologies and-or the relatively massive nature of the Guelph Formation reef
association.
Proximal Debris Apron Association Geophysical Profile
Geophysical signatures of the proximal debris apron association (FA5-4, Table 6) are
observed from approximately 1747.4-1734.3 m (5733-5690 ft) (Figs. 31 and 33). Gamma-ray
signatures of the proximal debris are very low, ranging from approximately 12-16 API, and
exhibit irregularly shaped log patterns, which are interpreted to represent clean, interbedded
carbonate successions. Total measured porosity values obtained from the NPHI log within the
proximal debris apron association range from approximately 10-to-27%, with most intervals
indicating porosity shows greater than or around 15%, (Fig. 33). A large porosity kick is
recorded from 1744.9-1741.9 m (5725-5715 feet), with NPHI porosity peaking to above 23% at
1744 m (5722 feet) then gradually decreasing to approximately 10% at 1741.9 m (5715 feet).
This large porosity kick is correlative in depth to the tempestite association (FA5, Table 6), and corroborated by the well bedded conductive-resistive signatures within the RHBI log. This porosity kick in the NPHI log is interpreted to be a result of alternating beds of skeletal grainstone (lithofacies Sg) and grey mudstone (facies Mpg).
The proximal debris apron association is recorded by the image log as a heterogenous succession of lithologies with two distinct electro-textural zones: (1) a well bedded, alternating 86
resistive-conductive zone occurring from 1445.2-1741.0 m (5726-5712 ft); and (2) a zone
exhibiting massive bedding with discontinuous resistive (yellow) features occurring within a
more conductive (brown) matrix material occurring from 1739.7-1734.3 (5708-5690 ft.)
(Fig.33).
Zone 1 is interpreted to represent the tempestite successions (lithofacies Mpg, Sg, and
Ram), due to the good correlation in depth and by the fact that the tempestite successions are one
of the few well-bedded successions within the 9-33 core (Fig. 33). It is interpreted that the
tempestite succession yields such a distinct electric texture within the image log due to the
interbedded nature between non-fossiliferous mudstone (lithofacies Mpg) and grain-rich
grainstone and rudstone (lithofacies Sg and Ram), which yield excellent contrasting electrical
measurements. This tempestite succession (Zone 1) displays the highest range in total measured porosity (NPHI log) of the proximal debris apron succession, ranging from 12%-27% porosity
(Figs. 31 and 33). However, no distinct changes in the gamma ray signature is resolvable within the interpreted tempestite succession, making identification without an image log very difficult.
Zone 2 observed within the image log is interpreted to represent debrite deposits
(lithofacies Ifb, Rcb, and Rcg), due to an excellent correlation in depths and the occurrence of distinct, discontinuous and semi-circular resistive features appearing within a more conductive matrix (Fig.33). As observed in core, these debrite deposits are massive-to-thickly bedded, chaotic floastone breccias with abundant cobble-to-boulder sized intraclasts of mudstone and cemented material, floating within a fossil-rich muddy matrix, and is interpreted to have the ability to yield a electro-texture comparable to electro-texture zone 2. Total measured porosity values recorded by the NPHI log within the electro-texture zone 2 feature moderate porosity values between 10% and ~15% and is the lowest porosity show in the proximal debris apron 87
association. No changes are observed within the gamma ray signature of the debrite deposits
(electro-texture zone 2), making the identification of these deposits without an image log unresolvable (Fig.33).
Peritidal Environment Geophysical Log Profile Due to the relatively thin occurrence of peritidal sediments within the 9-33 core (9.1 m), the geophysical signatures of intertidal (FA2, Table 6) and supratidal (FA1, Table 6) facies
associations will be lumped into one section. Geophysical signatures of the Guelph Formation
peritidal sediments are observed from approximately 1693.1-1686.4 m (5555-5533 ft) (Figs. 31
and 34). Gamma-ray signatures of Guelph Formation peritidal deposits are the highest
encountered within the formation, ranging from approximately 12-to-nearly 30 API. Within this
interval gamma ray log patterns steadily increase from base, forming a funnel-shaped curve
(Figs. 31 and 34). This curve shape represents the transition from intertidal (FA2) to supratidal
(FA1) facies associations, forming a shallowing-upward carbonate package within the core, and
is commonly observed in tidal environments (Walker, 1984). Total measured porosity values
obtained from the NPHI log within the Guelph Formation peritidal deposits range from
approximately 8-to-18%, and represent the tightest signatures recorded from the cored interval.
Guelph Formation peritidal deposits are recorded by the image log as alternating zones of
apparent-interbedded resistive/conductive deposits occurring with zones of electrically
heterogeneous sediment.
Within peritidal deposits are two distinct electro-textural zones: (1) an interbedded,
alternating resistive-conductive zone occurring from 1693.4-1691.3 m (5556-5549 ft); and (2)
another interbedded, alternating resistive-conductive zone occurring from 1688.2-1685.5 m
(5539-5530 ft) (Fig.34). Both zones exhibit similar electro-textures, however core analysis 88
reveals the lowermost interval (electro-texture zone 1), most likely represents interbeds of massive mudstone (lithofacies Mm), laminated mudstone (facies Ml), and microbial laminites
(lithofacies Hcr) with increasing presence of grain-rich storm deposits (lithofacies Sp and Hcrd).
Within this zone, interbedded mudstones are observed to exhibit alternating occurrences of vuggy porosity development. The occurrence of stratiform vuggy porosity in conjunction with
increasing presence of episodic grain-rich storm deposits is interpreted to yield the alternating
resistive-conductive profiles displayed by the image log (Fig.34). The second interval (electro-
texture zone 2) occurs approximately at the same depths as heterolithic microbial bindstones and
clotted mudstones (lithofacies Hcr). It is interpreted that the image log displayed this facies well
within this interval due the occurrence of this facies close to the Guelph Formation-Ruff
Formation contact displays alternating beds of vuggy development, that would account for the
alternating resistive-conductive electro-texture witnessed within the image log. The remainder of
the image log zones exist as a heterogeneously conductive texture that is not distinctive from
other sections of the image log.
Reservoir Characterization A basic investigation was performed to quantify reservoir characteristics of Guelph
Formation deposits within the reef (FA5, Table 6), proximal debris apron (FA4), distal debris apron (FA3) and intertidal (FA2) facies environments. Due to the thin occurrence of the supratidal association (FA1) (<0.15 m) within the Guelph Formation, and the overlying intertidal
and supratidal Ruff Formation deposits (~2.4 m), these intervals will be excluded from
petrophysical analysis. This section characterizes the distribution of porosity and permeability,
assesses petrophysical predictability through the generation of permeability-porosity transforms,
and classifies capillary behavior across the leeward end of the Guelph Formation pinnacle reef 89
complex, ultimately to provide an enhanced foundational knowledge of petrophysical profiles of
the leeward slope.
Guelph Formation Core Analysis
Intervals within the Guelph Formation selected for reservoir quality analysis included
successions deposited within reef (FA6), proximal debris apron (FA4-FA5), distal debris apron
(FA3), and intertidal (FA2) environments. Within this formation, rock lithologies observed from
core analysis range from mudstones, skeletal wackestones, packestones, grainstones, rudstones,
floatstones, to framestones. Thin section analysis of these lithologies revealed that primary pore
types observed included interparticle, fenestrae, moldic, and growth-framework pores (Table 7).
These lithologies have undergone significant amounts of diagenetic alteration from processes
including cementation, dolomitization, dissolution and replacement, fracturing, and
stylolitization. The occurrence of these diagenetic processes resulted in the generation of
secondary porosity including the development of vugs, channels, fracture and intercrystalline
pores.
Core Plug Analysis: A total of 80 analysis-quality core plugs were recovered from the
Guelph Formation. Descriptive statistical analysis was performed on porosity and permeability measurements obtained from core plug analysis results within Guelph Formation (Table 8). The mean measured core plug permeability value of the Guelph Formation was 2.7 ±5.8 mD with a
range of 0.001-27.6 mD. The mean total porosity measured from core plugs recovered from the
Guelph Formation was 7.1% ±2.9% with a range of 1.5-14.2%.
A porosity to permeability transform was generated to assess petrophysical predictability
within the Guelph Formation (Fig. 35), regardless of depositional environment. Guelph 90
Formation core plug permeability and porosity data was fitted to a power function (Fig.35), resulting in a R2 value of 0.6548, indicating that the fitted power function described population moderately well. The success of the power function fitting to Guelph Formation petrophysical data is best explained by the large variance in measured core plug permeability, which varies between five orders of magnitude. As porosity increases, permeability is observed to increase, by nearly an order of magnitude for every porosity increase of 2%. However, upon reaching approximately the 8-10% porosity range, this relationship tapers off, where increases in porosity do not predict changes in permeability as well above 1mD.
MICP Analysis: A total of 14 trim samples were collected from core plugs sampled across the Guelph Formation and were subjected to MICP experimentation (Fig.36). Within associations of the Guelph Formation, three capillary classes can be observed (Table 4, Fig.37), including: class I (yellow), class II (blue), and class III (red). The capillary behaviors and possible implications for each Guelph Formation association will be broken down in greater detail in subsequent sections.
Incremental mercury intrusion results for each sample obtained within the intertidal, debris apron (distal and proximal), and reef environments (FA2-FA6, Table 6) of the Guelph
Formation are shown in Figure 38. Overall, many samples exhibited bimodal pore-throat distributions, with significant volumes of mercury being intruded into pore-throats ranging from
0.01-10 µm (micro-to macro-sized pores), across all environments. Samples obtained from the
intertidal environment exhibited the most uniform behavior with most incremental mercury
volumes being intruded into pore-throats measured at 5 µm (macro-sized) and 0.03 µm (nano-
sized), yielding a strongly bimodal signature. 91
Distal debris apron samples also exhibited bimodal pore-throat distributions with peaks occurring at approximately 1-5 µm (meso-to macro-sized) and 0.02 µm (nano-sized). Samples
obtained from the proximal debris apron (FA4-FA5, Table 6) featured bimodal pore-throat populations with incremental mercury intrusion peaks at approximately 3-5 µm (macro-sized) and 0.05 µm (nano-sized). Reef association samples (FA6, Table 6) were observed to exhibit the
most complex behavior, featuring multimodal pore-throat distributions with incremental mercury
intrusion peaks occurring at approximately 0.09 µm, 0.1 µm, 0.7 µm, and 7 µm, which are
classified as nano, micro, meso and macro-sized pore throats, respectively according to the
Doveton (1995) pore-throat size classification scheme.
Pore-throat size distribution plots for each environment sampled from the Guelph
Formation will not be discussed in subsequent sections, however a brief interpretation of the
behaviors witnessed will be discussed. The strong bimodal behavior observed in most of the
samples is interpreted to be fabric-selective porosity based on an abundance of muddy-bioclastic fabrics within environments leeward of the pinnacle reef, yielding bimodal primary pore populations. It is interpreted that the bimodal primary-pore distributions are enhanced through preferential dissolution of bioclasts, generating vuggy textures as witnessed at core and thin section scales. Deviations from strongly bimodal pore-throat populations are most likely
attributed to non-fabric selective porosity, such as solution-enhanced fractures, and channels.
With regards to the reef association, the multimodal pore-throat distributions observed are
interpreted to arise from the interplay of interparticle porosity, growth-framework porosity, and
non-fabric selective porosity from the development of vugs, channels and fractures, witnessed to
be largely constrained to bioclastic-muddy sediment infilling cavities within the reef framework.
92
Reef Association Core Analysis
Reef association deposits (FA6, Table 6) consist of interbedded-to-massive deposits of
tabulate coral-stromatoporoid framestones (facies Fr) with skeletal floatstones (facies Sfm),
packstones (facies Sp), and mottled mudstones (facies Mm) infilling internal cavities within the
reef framework. Thin section analysis of reef association lithologies revealed that primary pore
types observed included interparticle, moldic, and growth-framework porosity (Table 7). These
lithologies have undergone significant amounts of diagenetic alteration from processes including
cementation, dolomitization, dissolution and replacement, recrystallization, stylolitization, and to
a lesser extent, fracturing. Dissolution and stylolitization were observed to be the most extensive
diagenetic alterations observed within the reef facies association. The occurrence of these
diagenetic processes resulted in the generation of secondary porosities including the
development of vugs, channels and recrystallization-driven intercrystalline pores (Table 7).
Core Plug Analysis: 15 analysis-quality core plugs were recovered from reef association
(FA6, Table 6). Exploratory statistical analysis was performed on porosity and permeability
results obtained from the reef association (Table 9). The mean permeability to air value was 7.20 mD ±9.88mD, with a range of <0.01 to 27.62 mD. Mean total porosity value measured from the reef association was 8.36 % ± 3.32 %, with a range of 2.75-to-14.1%.
A porosity-permeability transform was created to assess petrophysical predictability of the reef association environment. A power function was fitted to core plug data collected from the reef environment to model core plug permeability as a function of measured porosity
(Fig.39). The applied power function modeled reef association petrophysical data very well, as indicated the R2 value of 0.8155. Within the reef association, measured permeability is best
modelled as a function of porosity below the value of 1mD. As permeability increases beyond 93
the 1mD threshold, permeability values become more dispersed and do not correlate as well with
measured values of porosity. This behavior is interpreted to be a product of diagenetic overprint,
where larger pore-throat sizes contributing to higher values of permeability, do not correlate with
the pore-body sizes of bioclasts, vugs, or interparticle pores yielding the porosity values.
MICP Analysis: Four samples were taken from reef association sediments and subjected
to MICP analysis. Of the four samples collected, two samples were obtained from
stromatoporoid/tabulate coral framestone (lithofacies Fr), and two samples were collected from
framework-cavity infilling sediment (lithofacies Sp and Mm). Reef association capillary profiles
are interpreted to show both class I and class II capillary curves (Fig.40). Both framestone
samples (lithofacies Fr) are interpreted as class II curves, however one framestone (lower green
line), exhibits a notably lower median saturation pressure than the other (upper green line). This
suggests that one sample exhibits larger pore-throats, and therefore higher permeability than the
other. Both samples from framestone facies Fr display kinks within the curve, suggesting a
bimodal-to-multimodal pore-throat system is present. The discrepancy between median
saturation pressures and presumed pore-throat size distributions observed between the framestone samples is interpreted to most likely be related to differences in primary depositional fabric (occurrence of muddy internal sediment) or variation in the degree of diagenetic alteration between the two samples.
Two MICP curves were obtained from lithofacies interpreted to be framework cavity- infilling sediment (lithofacies Mm and Sp). The mottled mudstone (lithofacies Mm), denoted by the purple line, is interpreted to be a class I capillary curve, exhibiting a relatively low median
saturation pressure and a general absence of kinks in the curve, with the exception of a slight
variation between saturation-to-pressure at 90% Hg saturation (Fig.40). The low median 94
saturation pressure indicates a moderate to large-sized pore-throat distribution and therefore
potentially moderate-to-high reservoir quality. The skeletal packstone sample (lithofacies Sp),
denoted by the pink line, is observed to be a class II Pc curve, due to its steeper slopes, and
presence of kinks within the MICP curve. This sample exhibits a much higher median saturation
pressure than the mudstone sample, suggesting smaller sized pore-throats controlled most of the
mercury intruded into the packstone sample.
Proximal Debris Apron Core Plug Analysis
The proximal leeward debris apron facies association (FA5-4, Table 6) consists of
undulate-to-flat mudstones (facies Mpg), skeletal floatstones (facies Sfu), normally-graded skeletal grainstones (facies Sg), skeletal rudstones (facies Rcg, Rcb, and Ram), and chaotic floatstone conglomerates interpreted to be debrites (facies Ifb). These deposits are interpeted to represent off-bank and downslope sedimenation along the proximal debris apron, and are interpreted to be tempestites (Ram, Mpg, Sg) (FA5, Table 6), debrites (facies Sfu, Rcg, Rcb, and
IFb) and turbidites (facies Mpg and Sg) (FA4, Table 6). Primary pore types observed from thin
section analysis include interparticle and shelter porosity (Table 7). These deposits have
undergone significant diagenetic alterations including cementation, dolomitization, dissolution and replacement, recrystallization and stylolitzation. Diagenetic processes have resulted in the development of moldic, intercrystalline, fracture, vug and channel porosities (Table 7).
Core Plug Analysis: 35 analysis-quality core plugs were recovered from the proximal debris apron (FA5, Table 6), and exploratory statistical analysis was performed on porosity and permeability results (Table 10). The mean permeability to air value obtained from the proximal debris apron environment was 1.48 ± 4 mD, with ranges in values between <0.01 to 20.5 mD.
The mean porosity value was found to be 6.37% ± 2.66%, with a range of 2.5-to-13.3%. 95
A porosity-permeability transform was created to assess petrophysical predictability of
the proximal debris apron environment. Core plug permeability data was modeled as a function
of total porosity by applying a series of power functions to data obtained from the tempestite
association, debrite-turbidite associations (FA4-FA5, Table 6), in addition to the proximal debris
apron environment as a whole (Fig.41). Application of a power function to the tempestite
deposits generated a R2 value of 0.81, indicating the function described the petrophysical
population very well. However, the power function fit to the debrite-turbidite association
generated a R2 value of 0.57, suggesting the model generated exhibited moderate success. The
power function to fit the entire petrophysical population of the proximal debris apron
environment generated a R2 value of 0.72, indicating the model described most of the measured
population well. The high degree of petrophysical predictability within tempestite deposits and to
a degree the entire proximal debris apron environment is attributed to the well-sorted, grain-rich textures generated by wave-resuspension processes. Debrite deposits also feature high amounts of bioclastic content, however, are poorly sorted due to most of the sampled intervals being proximally sourced debrites. In theory, debrite samples obtained from the deeper slope segments, where their average flow position is located farther into the run-out zone, may yield more grain- supported textures and potentially more petrophysical predictability.
MICP Analysis: Five samples from the proximal debris apron deposits were subjected to
MICP analysis (Fig.42). These samples consisted of mottled skeletal floatstone (facies Sfm),
Skeletal grainstone (facies Sg), undulatory pale grey mudstone (facies Mpg), and amalgamated
skeletal rudstone (facies Ram). Samples from the proximal debris apron environment were
classified as class I, II, and III capillary curves (Fig. 42). MICP data obtained from the mottled
skeletal floatstone (lithofacies Sfm), shown in blue (Fig.42), is observed to exhibit a kinked- 96
capillary pressure curve with a moderate median saturation pressure of 100 psi, and was
interpreted to be class II Pc curve, most likely with a multimodal pore-throat distribution and
complex reservoir quality.
Two MICP samples were obtained from well-graded skeletal grainstone (lithofacies Sg), both of which exhibited Pc curves of a similar unkinked-smooth geometry, however differed by values of median saturation pressure. The lowest curve exhibits a median saturation pressure of
50 psi, while the second curve exhibits a median saturation pressure of approximately 100 psi.
This discrepancy between the two samples is interpreted to illustrate the effects of differing degrees of diagenetic alteration, potentially related to the extents of cementation, dissolution or recrystallization within the sample.
Once MICP sample was collected from the grey mudstone lithofacies Mpg, which was interpreted to exhibit class III capillary behavior suggesting a unimodal, well-sorted pore-throat size distribution, dominated by small-sized pore-throat apertures.
Distal Debris Apron Core Plug Analysis
The distal debris apron environment consists of massive mottled mudstone (facies Mm), massive mottled skeletal wackestones (facies Sw), very thickly-bedded mottled skeletal floatstones (facies Sfm) and thickly-bedded, inversely-graded intraclastic floatstones (facies IFb)
(Table 6). These deposits were interpreted to have been deposited below wave base by predominately normal background sedimentation and subaqeous gravity flows triggered caused by periodic slope destablization. These deposits all featured interparticle primary porosity, and featured moldic, vug, intercrystalline and fracture type secondary porosity. Prominent diagenetic proccesses observed in this association included dissolution, recrystallization and fracturing. 97
Core Plug Analysis: 7 analysis-quality core plugs were recovered from the distal debris apron association (FA3, Table 6), and exploratory statistical analysis was performed on porosity and permeability results (Table 11). The mean value of permeability measured within the distal
debris apron environment was 1.1 ±1.79 mD, with a range of <0.01 to 5.03 mD. The mean value
of total porosity was 6% ± 3%, with a range of 1.5 to 11.3% (Table 11).
A porosity-permeability transform was created to assess petrophysical predictability of
the distal debris apron environment. A power function was fitted to the petrophysical population
measured from the distal debris apron environment generating a R2 value of 0.48, indicating the
model described the population to a moderate degree (Fig.43). The low degree of confidence in
modeling the petrophysical data from the distal debris apron environment is interpreted to be
heavily influenced by the low sample population size.
MICP Analysis: Two samples from the distal debris apron association were subjected to
MICP experimentation, including mottled mudstone and skeletal wackestone (lithofacies Mm
and Sw) (Fig.44). Lithofacies Mm was observed to feature kinked Pc curve geometry with a high
median saturation pressure (~2000 psi) and was classified as a class II Pc curve (Fig.44). This
suggests that the pore-throat network of lithofacies Mm is bimodal with micro- and macro-sized
pore throat apertures both controlling permeability within the lithology.
Lithofacies Sw was classified as a type I curve with a smooth un-kinked curve with a
median saturation pressure of 100 psi, suggesting a well sorted unimodal pore distribution with
moderate to large sized pore throat distributions (Fig.44).
98
Intertidal Facies Association
The intertidal facies association (FA2) consists of laminated dolomitic mudstone (facies
Ml), mottled dolomitic mudstone (facies Mm), heterolithic dolomitic clotted mudstone cryptalgal
bindstone (facies Hcr), disturbed heterothic dolomitic mudstone-cryptagal bindstone (facies
Hcrd), and dolomitic skeletal packestone (facies Sp). Primary pore types observed at the microscopic and macroscopic scale include fenestrae and interparticle, while secondary pore types observed included vug, intraparticle, intercrystalline, and channel porosity. Prominent diagenetic proccesses observed in this association included dissolution and replacement, recrystallization, and stylolitization.
Core Plug Analysis: 24 analysis-quality core plugs were recovered from the intertidal facies association (FA3, Table 6), and exploratory statistical analysis was performed on porosity
and permeability results (Table 12). Lithofacies sampled for core plug analysis included mottled
mudstone (facies Mm), cryptalgal bindstones (facies Hcr), disturbed cryptalgal bindstones (facies
Hcrd), Intraclastic floatstone breccia (facies Ifb), skeletal packstone (facies Sp), laminated mudstone (facies Ml) and skeletal wackestone (facies Sw). The mean permeability value measured within intertidal association was 2.27 ± 4.40 mD, with a range of <0.01 to 15.7 mD.
The mean value of total porosity was found to be 7.25 ± 2.85%, with a range of 3.16-to-12.82%.
A porosity-permeability transform was created to assess petrophysical predictability of the proximal debris apron environment. Application of a power function to the intertidal facies association petrophysical population generated a R2 value of 0.55, suggesting the model was able
to explain a moderate amount of variance within the population (Fig.45). 99
MICP Analysis: Three samples from the intertidal facies association were subjected to
MICP experimentation including mottled mudstone (facies Mm), microbial laminites (facies
Hcr), and disturbed microbial laminites (facies Hcrd) (Fig.46). All three of the samples exhibited kinked curves, only differing by values of median saturation pressure, where facies Hcrd and Hcr both exhibited median saturation pressures greater than 100 psi. All lithofacies are interpreted to display type II capillary behavior and display multimodal pore-throat networks. This multimodal behavior is interpreted to most likely be related to the combination of primary pore systems
(interparticle and fenestrae), overprinted with diagenetic signatures of dissolution leaching out matrix/bioclastic material, and recrystallization creating intercrystalline porosity. 100
CHAPTER V: DISCUSSION
Depositional Environments
Core analysis was performed on the 9-33 well core, revealing that the recovered interval from 1684-1755 m (5525-5760 ft) was comprised of sedimentary successions deposited within reef zone, leeward debris apron and peritidal environments. The sedimentary succession recovered by the core was interpreted to consist of an initially deepening-upward transition that
was unconformably overlain by two shallowing-upward peritidal successions. Due to the
deviated nature of the 9-33 well, the general deepening-upward transition of reef stage deposits
(from reef zone to the distal leeward debris apron slope segment) is interpreted to most likely
correspond to the series of laterally adjacent environments along the leeward slope margin,
which formed coeval with pinnacle reef growth. Unconformably overlying reef stage deposits
are two shallowing-upward peritidal successions. Peritidal deposits are observed directly
overlying debris apron deposits, however, are presumed to drape the entirety of the pinnacle
complex, as reported by previous investigators (Rine, 2016, 2016; Suhami 2016, Freidman and
Kopel, 1991).
The Reef Core Environment
Reef deposits typically consist of four components: 1) rocks produced by framework-
building metazoans; 2) internal sediment which infills framework cavities created by voids in
primary growth or bioerosion; 3) porosity produced by bioerosion due to biota which contribute
to erosion of the reef framework via boring, rasping, and grazing; and 4) abiotic-to-microbial
generated cements, contributing to early-stage lithification of reef sediments (Moore and Wade,
2013). Each of these components occur synchronously producing a wide spectrum of rock
fabrics encountered within reef successions (Rine, 2016; Moore and Wade, 2013). 101
Reef association successions (FA6, Table 6) observed within the 9-33 core occupy 7.9 m of the recovered core and consist of two major lithology types: 1) stromatoporoid and tabulate coral framestones and 2) cavity-infilling bioclastic sediment including irregularly bedded, mottled blue-grey skeletal floatstones (facies Sfm) and mottled mudstones (facies Mm). Due to the relatively thin interval of core recovery within this depositional environment, dolomitization- driven destruction of internal structure of biological fabrics, and absence of core data obtained from the reef crest, interpretation of this environment is limited.
Metazoan growth habits are thought to be controlled by metazoan response to slight variations in wave energy, water depth or turbidity (James and Wood, 2010; Balogh 1981).
Guelph Formation pinnacle reefs are known to feature a wide distribution of growth habits observed within reef-building metazoans (tabulate corals and stromatoporoids) (Balogh, 1981).
Tabulate coral colonies have been observed to exhibit either massive, branching, or solitary forms while stromatoporoids often feature bulbous, hemispherical, irregular-encrusting, massive- laminated, tabular, debris, and other forms (Rine, 2016; Balogh, 1981). Within the 9-33 core, only laminar-coalesced stromatoporoid and branching tabulate coral growth habits were observed. Several studies on Michigan-basin Guelph Formation reef pinnacles have observed these branching tabulate coral colonies (Rine, 2016; Balogh, 1981). Balogh (1981) interpreted
tabulate coral colonies of such habit to have grown in fairly quiet, less turbulent, or sheltered
zones of the reef. The interpretation by Balogh (1981) is supported by the general metazoan
growth patterns described in James and Wood (2010), and is in good agreement with intperetations made by this study, stating these tabulate coral colonies may indicate growth leeward of the high-energy reef crest. The relationship between stromatoporoid growth geometries and environment is more problematic, however. Other investigations of the Guelph 102
Formation pinnacle reefs (Rine, 2016; Huh, 1973; Balogh, 1981) have noted more diverse stromatoporoid growth habits (such as domal or hemispherical shapes) than what was observed in this study. The general lack of stromatoporoid growth diversity may indicate a relatively uniform stromatoporoid response to conditions of the reef enviroment leeward of the crest. This notion may be contested, however, by findings of a study by Kershaw and Motus (2016), where evaluating Late Silurian biostromes of Estonia, the authors observed very little diversity in stromatoporoid growth habits across the entire reef. Due to the limitations of core data, these observations cannot be elaborated on further.
Within the 9-33 core, stromatoporoids were generally observed in relatively higher abundances than tabulate coral colonies. In a separate study conducted by Trout (2012), this realtive trend in stromatoporoid dominance was also observed within reef crest and leeward slopes, suggesting the overall conditions were more favorable for stromatoporoid growth. Within this study, stromatoporoids occurred either as the sole framebuilders or occurred alongside encrusting tabulate coral colonies. This observed exclusivity and cohabitation with tabulate corals was also observed by Balogh (1981), and interpreted to suggest competition between the two metazoans.
Cavity-infilling bioclastic sediment was observed to comprise the bulk majority of reef zone deposits within the 9-33 core. Several other investigations into Guelph Formation pinnacle reefs of the Michigan basin (Rine, 2016, 2016; Balogh, 1981; Gill, 1973) have noted significant volumes of detrital sediment infilling reef cavities, yeilding numbers that reef framestones only occupy 10-50% of total reef successions. Core analysis results of 9-33 core reefal deposits corroborate the findings of previous studies, revealing that framestones only occupy approximately 33% of the reef zone deposits. 103
Cavity-infilling sediment is comprised of blotchy-to-mottled blue-grey skeletal floatstones and mudstones which feature reef dwelling biota. Reef dwelling biota included: crinoids, echinoderms, brachiopods, bryozoans, in addition to other unidentifiable bioclastic debris. Generally reef-dweller fossils appear more articulated than in debris apron environments, however significant volumes of bioclasts appear disarticulated. Bioclastic internal sediment is
sometimes sharply overlain by pale grey mudstones which lack observable fossil content. These
fabrics resemble the storm deposits observed within the proximal debris apron, which may
suggest storm-wave resuspension and subsequent fallout exert an influence in the infilling of reef
cavities in conjuction with background reef sedimentation (boring, rasping, grazing, etc.).
The Debris Apron Environment The 9-33 well was drilled laterally through the leeward debris apron and into
contemporaneous pinnacle reef deposits of the Guelph Formation, capturing rock core from a
significant amount of the leeward depositional zone. Debris apron sediments observed with the
9-33 well were segmented into proximal and distal environments separated at wave-base
(Fig.29). Deposits of the Guelph Formation proximal debris apron consisted of muddy, bioclastic
successions featuring deposits interpreted to be tempestites (lithofacies Mpg, Sg, Ram),
proximally-to-distally sourced debrites (lithofacies Ifb, Rcb, and Rcg) and associated turbidites
(lithofacies Sg and Mpg). Deposits of the distal debris apron predominately consist of muddy
deposits with relatively low bioclastic content and exhibit no evidence of wave influence,
interpreted to be deposited in subtidal back-reef conditions, below wavebase. Distal debris apron
sediments consisted largely of unstratified-to-thickly-bedded successions of color-mottled-to- dark grey mudstone, skeletal wackestone and floatstones (lithofacies Mm, Sfm and Sw) all of which lacked sedimentary structures or bedforms and the majority was interpreted to be 104 deposited from normal marine background sedimentation. One interval consisted of a 0.5 thick interval of intraclastic floatstone conglomerate (Ifb) which is interpreted to be a debrite, suggesting debris flow-turbidite deposits (FA4) interfinger with subtidal back-reef sediments
(FA3) deposited by normal background sedimentation. The uppermost occurrence of distal debris apron deposits consists of mudstone which is completely devoid of fossils and is interpreted to represent the deepest/most distal segment of the debris apron, where benthic biota was less abundant. Diagenetic processes (micritization, dolomitization, dissolution etc.) may have also destroyed fossil evidence in this environment.
Role of Storm-influence on Debris Apron Development
Off-bank transportation of bioclastic detritus from reef areas into leeward slopes via storm-wave resuspension is commonly observed in modern carbonate reef systems (Playton et al., 2010; Hine et al.,1981; Jordan, 1973). Due to poor core coverage within leeward slopes of
Guelph Formation reefs, in addition to the priority generating an agreeable reef growth model, discussions on the role of storm-wave influence in Guelph Formation pinnacle debris apron development and their effect on reservoir quality has been briefly mentioned-to-absent in many pivotal studies of Michigan Basin pinnacle reef systems (Rine, 2016, 2017; Balogh, 1981; Huh,
1973; Gill, 1973).
Historically, the observation and description of well-graded bioclastic deposits resembling the tempestite successions of the 9-33 well has been varied across the literature
(Rine, 2016, 2017; Qualmen 2009; Ritter 2008; Balogh, 1981; Huh, 1973; Gill, 1973). Rine
(2016) noted the occurrence of skeletal wackestones and mudstones occurring across the proximal debris apron, interpreting these to be deposits shed from up-dip carbonate factory settings and being transported via wave processes; however, no observations of grading or grain- 105 supported textures were observed. Both studies by Balogh (1981) and Gill (1973) described the occurrence of bioclastic wackestones within reef apron flanks, however, they saw no prominent graded successions and generalized these deposits as a detritus facies shed from the reef during high-energy conditions. Studies conducted by Qualmen (2009) and Ritter (2008) noted the occurrence of a skeletal detritus grainstone and wackestone facies deposited in various zones across the reef environment (in accordance with depositional models developed prior to Rine,
2016). Qualmen (2009) discussed the importance of differentiating well-sorted skeletal grainstones from other detrital facies due to their prominent wave-influence represented by sorting of bioclasts. The most notable description of reef apron deposits that resemble the tempestite successions observed in this study was by Huh (1973) within the northern trend of
Guelph Formation pinnacle reefs. In the study, the author described a skeletal reef-detritus wackestone facies, observing interbedded successions of bioclastic sediment and carbonate mudstone, interpreting these deposits to be sorted by wave processes. The absence of core descriptions resembling the tempestite successions of this study from studies characterizing reefs of the southern trend may suggest preservation of such beds is confined to the taller reef pinnacles of the northern slope of the Michigan basin, that may feature more accommodation space within the leeward slope.
Within the cored interval of the proximal debris apron, deposits interpreted to be tempestites occupy 5 m of the total 12 m (40’) section of core recovered. While the proximal debris apron is most likely larger than interval captured during coring operations, the sheer volume of sediment transported directly by wave-resuspension processes is interpreted to suggest that off-bank transport of bioclastic sediment by storm-waves is the primary sedimentation mechanism for delivering bioclastic sediment from the reef bank to the uppermost segments of 106
the slope apron system. The observation of storm-wave resuspension being the primary sedimentary control on near-reef slope sedimentation has been reported in the modern leeward slope apron environments of the Bahamas by Hine et al. (1981). In their investigation of off-bank sediment transport mechanisms within leeward bank margins, the authors found that normal wave, tidal, and bottom-current conditions do not yield enough energy for critical threshold velocities to be sufficiently exceeded in order to transport sediment coarser than mud. Instead, fair weather conditions act as a prominent winnowing mechanism rather than a transport mechanism (Hine et al., 1981). While comparisons between the dynamics of Silurian pinnacle reef leeward margins to modern reef margins is limited, the volumetric prominence of storm- derived deposits observed within the Guelph Formation proximal debris apron of this study is in good general agreement with the findings by Hine et al. (1981) that off-bank transport by storm waves is the primary mechanism for transporting sediment off the pinnacle reef crest into adjacent slope environments.
Role of Sediment Gravity Flows on Debris Apron Development
In the depositional slope apron model by Mullins and Cook (1986) developed from ancient and modern carbonate aprons systems (Mulder et al., 2017; Tournadour et. al., 2015), sediment gravity flows arising from slope destabilization were found to be the primary mechanism for transporting bioclastic sediment downslope. Within this study of a debris apron environment located on the leeward side of a pinnacle reef, debrites (lithofacies Ifb) occupied
50% of the proximal debris apron succession and were observed to extend in occurrence downslope into distal apron segments.
The occurrence of various intraclastic floatstone conglomerate facies has been described in both northern and southern Guelph Formation pinnacle reef trends (Rine, 2016, 2017; Gill, 107
1973; Huh, 1973). However, due to the important differences between windward and leeward
slope development and the types of sediment gravity flows which occur in both, proper
identification of which margin is which is of extreme importance. Studies prior to that of Rine
(2016) generally did not differentiate leeward versus windward margins because they interpreted
a generally similar development of flanking detritus material on both margins. However,
descriptions and photographs of intraclastic deposits described in windward margins by Rine
(2016) do not resemble those seen in the leeward debris apron of the 9-33 well, as they consisted
of intraclasts interpreted to be directly sourced from lithified segments of the reef zone, bioherm
and peritidal cap.
The only description of intraclastic conglomerates which compare with the debrite
deposits seen within the debris apron of the 9-33 well are those described by Gill (1973) and Huh
(1973). Gill (1973) described the occurrence of a skeletal lithoclast facies described as a poorly sorted, clast-to-mud-supported fabric consisting of fragmented bioclastic material with dark
micritic envelopes and brown-to-grey, subangular-to-subrounded, 1-20 mm-sized intraclasts of mudstone. Gill (1973) described these deposits as being 3-7.6 m (10-25 ft) thick and were rarely continuously observed in a vertical core succession over 18m (60 ft) thick, which Gill (1973) interpreted as being suggestive of episodic deposition. Gill (1973) interpreted this lithoclastic deposit to be of a high energy origin, depositing along or capping the bioherm growth stage, described in the studies reef growth model. While the mudstone fragments are smaller than those observed in the 9-33 well debrite deposits, the facies Gill (1973) describes bares a strong resemblance to the debrites observed in the 9-33 well, but possibly having undergone a higher degree of wave-reworking (as seen in lithofacies Rcg). Huh (1973) described the occurrence of a skeletal lithoclast facies that consisted of fossils and arenite mudstones which displayed poor-to- 108
moderate sorting of bioclasts and lithoclasts. Huh (1973) interpreted this deposit to be of a high-
energy origin, deposited along the reef flank and to have undergone some degree of wave-action.
Descriptions made by studies by Gill (1973) and Huh (1973) are interpreted to possibly be of a
similar origin to the debrites described in the 9-33 well (Lithofacies Ifb, Rcg), as these did exhibit cross-lamination of bioclasts within the uppermost segment of a debrite interval, suggesting wave-action was present.
Turbidite deposits are commonly associated with debris flows and are a major element of
the debris apron model developed by Mullins and Cook (1986). Turbidite beds were rarely
observed within the 9-33 core, only occurring in a few intervals of the proximal debris apron, as interpreted Bouma Ta and Tde divisions overlying distally sourced debrites (peloidal rudstone facies Rcg). Turbidity flows are known to develop overriding debris flows along the mid-to- distal segments of run-out zones (Fig.5) (Mullins and Cook, 1986). Due to the volumetric prevalence of proximal debrites (mud-supported intraclastic conglomerate floatstone, facies Ifb) combined with the rare occurrence of turbidites within the recovered section of the proximal debris apron, it is interpreted that this recovered zone, on average, is correlative to upper segments of debris flow run-out zones. This suggests that most of sediment gravity flow deposits are sourced from the uppermost segments of the proximal debris apron, with sediment likely to be remobilized from the tempestite deposits. This interpretation is corroborated by the prominent
occurrence of pale-grey mudstone intraclasts that strongly resemble the mudstone interbeds
(facies Mpg) that are abundant in tempestite succession. If this interpretation is accurate, turbidite deposits can be expected to occur more frequently along the mid-to-distal leeward slope segments. However, the mid-slope segment was largely not represented in the 9-33 well core due
to poor core recovery (~25 m gap in recovered core). At the bottommost segment, a 0.5 m thick 109
intraclastic conglomerate is observed interpreted to suggest debrite-turbidite deposits extend from proximal-to-distal slope segments of the debris apron.
At the base of the distal slope apron succession, just below autochthonous sediments, a
unique 0.5 m thick interval of intraclastic floatstone conglomerate (facies Ifb) was observed featuring a boulder-sized intraclast and was completely devoid of fossil content, seen seemingly floating in a non-fossiliferous matrix. Interpretations for this unique lithology are limited,
however many studies have reported the potential for occurrence of megabeccias or olistostromes, correlating to submarine slides and slump events in distal-to-basinal segments of the slope systems (Reijmer, et al., 2015; Mullins and Cook, 1986). These lithologies featured the largest-sized intraclasts observed in the debris apron, far larger than the cobble-sized intraclasts observed in the proximal debrites located up-slope. While no conclusions can be made regarding this unique lithology, it can be suggested that this may be correlated to slide or slump events occurring in the mid-to-distal slope settings of the apron system.
As reported in other studies of carbonate debris aprons (Tournadour et al., 2015), slope destabilization events create small gullies, ravines, chutes, and scars across the debris apron system which act as sediment by-pass zones and preferential escarpment surfaces for subsequent destabilization events, channelizing sediment gravity flows down slope. It should be inferred that
debrites observed within the leeward slope of Guelph Formation pinnacle reefs may be
channelized deposits to some degree and not laterally continuous sheet deposits across the entire
debris slope. Several mechanisms could trigger the subaqueous sediment gravity flows witnessed
with the leeward debris apron environment of the Guelph Formation pinnacle reefs. Tournadour
et al. (2015) summarized possible sediment gravity flow trigger mechanisms within a modern
debris apron environment as being earthquakes, diapirism, large storms, high sedimentation 110
rates, or increased pore pressures. The Michigan Basin is thought to have been relatively
tectonically stable during the Silurian period (Catacosinos et al., 1990) and according to the most
recent pinnacle reef growth model by Rine (2016), evaporitic deposition did not ensue until after
deposition of the stromatolitic cap in the uppermost intervals of the Guelph Formation. This
study found that storm wave-resuspension is the primary sediment transport mechanism for
delivering sediment into the proximal leeward debris apron environment. This conclusion
coupled with the observations of 8 m (26 ft) thick successions of coarse-grained tempestites in
the 9-33 core, suggests that accumulation of high volumes of bioclastic sediment during storm
events into gullies, ravines, chutes, etc. may cause exceedances in repose angles, potentially
triggering sediment gravity flows.
Comparison to Mullins and Cook (1986) Slope Apron Model
This study hypothesized that the debris apron of the Guelph Formation pinnacle reefs
developed similarly to the slope apron model developed by Mullins and Cook (1986). The debris
apron model developed by Mullins and Cook (1986) was divided into upper slope, inner apron,
outer apron and basin sub-environments. The upper slope sub-environment described by Mullins
and Cook (1986) was only present in base-of-slope aprons where a zone of sediment by-passing occurred between the reef bank and downslope environments. No evidence of sediment by- passing was observed in the debris apron environment of the 9-33 well, but sediment by-passing zones may be present, however the base-of-slope apron model will not be discussed. Inner
(proximal) slope apron successions were described by Mullins and Cook (1986) as mainly consisting of mud-supported proximal debrites interbedded with proximal turbidites with thick
Ta divisions with evidence for broad shallow channels infilled by coarse sediment. The distal
outer apron was described as turbidite dominant with clast-supported reworked conglomerates 111
interpreted to be distal debrites, while basal plain successions were comprised of distal turbidites
interbedded with pelagic sediment and oozes.
The debris apron environment recorded in the 9-33 well core was found to be a 52 m
(170 ft) thick package of muddy reef-derived detritus that accumulated leeward of the pinnacle reef crest, coeval to reef development. Off-bank transport of reef detritus was found to be predominately through storm-wave resuspension, redepositing sediment into slope margins apron areas above wave-base. This sediment was then remobilized by sediment gravity flows, distributing reef detritus downslope into more distal areas. Due to problems in core recovery, the
indistinct geophysical log signatures of the debris apron environment, and a general lack of well density and other core data within the leeward profile of the Dover 33 field, examination of the spatial variations in key deposits witnessed within this environment, or any comment on architecture is unfortunately not feasible. However, some general comparisons may be made between dominant trends observed in the debris apron of the 9-33 well and the slope apron model of Mullins and Cook (1986).
Approximately half of the cored section of the proximal debris apron consisted of mud- supported conglomerates, grain-supported peloidal rudstones, convoluted mudstones and skeletal grainstone interbeds interpreted to be proximal-to-distally sourced debrites and turbidite Ta and
Tde divisions. Debrites showed evidence of wave-influence through the occurrence of bioclastic
cross-laminations on top of one debrite deposit (lithofacies Rcb), and generally marked the
transition from proximal to distal segments (separated at wave base). The remaining half of the
proximal segment of the debris apron was dominated by normally graded, well-bedded to
amalgamated bioclastic deposits, interpreted to be tempestites. The prominence of subaqueous
debrites was a key factor in the debris apron model of Mullins and Cook (1986) and is generally 112
in good agreement with the debris apron successions observed within the 9-33 well. However,
the occurrence of predictable successions of storm deposits with predictable lateral successions
and characteristics has not been noted previously.
Sequence Stratigraphic Framework of the 9-33 Well
Due to the missing intervals of core recovered from the 9-33 well, indistinct geophysical
log signatures, and absence of other available cores within the study field, the generation of a
detailed-sequence stratigraphic model for Guelph Formation leeward debris apron development
was not accomplished. Sequence stratigraphic interpretation is further complicated by the laterally deviated nature of the case-study well, yielding the absence of a truly vertical sequence
to evaluate. Therefore the deepening-upward trends between facies associations of reef stage deposits most likely do not represent time-transgressive progradational or aggradational behavior of the leeward debris apron and associated pinnacle reef, but are more likely to represent the assemblage of laterally adjacent environments present moving down the leeward slope from the shallow water depths of the carbonate factory into deeper inter-reef areas. That being said, a generalized sequence stratigraphic framework will be created from the assemblage of depositional environments observed within 9-33 well core.
Recovered core from the leeward profile of a Guelph Formation pinnacle reef was found to be comprised of two main depositional stages separated by an unconformity: an active reef growth and debris apron accumulation stage (stage 1), and a peritidal stage (stage 2). Stage one sedimentary successions were observed to exhibit a general deepening-upward trend, transitioning from the shallow depths of the reef zone down the leeward slope into the distal slope apron setting in inter-reef areas. Stage 1 deposits of the leeward slope apron are interpreted to be deposited contemporaneously with pinnacle reef growth and most likely accumulating 113
during TST-EHST conditions. Within the leeward slope apron environment, slope destabilization events are interpreted to be a major control (small-scale to large-scale collapse events) on the amount of the accommodation space available, and act as an agent for potential lower-to-toe of slope progradation from sediment gravity flows transported down-slope. Rates of off-bank transport are interpreted to be a large control on the amount of sediment supply available within the slope apron system, with increases in sediment supply potentially occurring during high- stand bioclastic shedding or periods of increased storm activity. Theoretically, when sediment supply outpaces available accommodation space, collapse events of various frequencies will occur, resulting in downslope transport and backfilling of the slope apron (Playton et al., 2010).
Within the 9-33 well core, Guelph Formation peritidal deposits are observed to unconformably overly reef stage deposits, marking an end to the generalized deepening-upward trend and shifting the stratal record into a series of shallowing-upwards trends. The interval separating these two depositional trends is a 2.5 cm thick, dark grey to black, non-fossiliferous mudstone located at core depth of 1693 m (measured depth) (Fig.29). The lowermost contact of this mudstone is a dark, wavy (red line, Fig.29) contact interpreted to be a sequence boundary, marking the end of HST deposition in the Guelph Formation. The 2.5 cm thick interval of dark vuggy mudstone overlying the sequence boundary is interpreted to represent LST deposition along the leeward debris apron slope. This LST mudstone is interpreted to most likely be part of a larger sequence of LST deposits, that are probably more prominent further downslope moving into inter-reef areas. Following deposition of the LST mudstone along the leeward debris apron, a period of subaerial exposure is interpreted to have ensued, as evidenced by development of vuggy porosity within the LST mudstone (Fig.29). It is assumed that if the leeward debris apron was subaerially exposed, then the pinnacle reef itself was also exposed. 114
Overlying the dark LST vuggy mudstone is a wavy erosive boundary interpreted to be a
transgressive surface of erosion, representing onset of TST deposition of the overlying Guelph
Formation peritidal deposits. A general shallowing-upward peritidal succession of laminated mudstones interbedded with increasingly desiccated microbial mats was then observed for the remainder of Guelph Formation deposition. The Guelph Formation-lower Ruff Formation
contact was observed to be unconformable and interpreted to be a second TSE, separating
supratidal wrinkled microbial mats of the Guelph Formation from overlying intertidal domal
microbial mats of the Lower Ruff Formation. Development of micro-karst and vuggy porosity
just below the boundary is interpreted to indicate Guelph Formation peritidal deposits were subaerially exposed. This second TSE located between the two formations is interpreted to mark the onset of a successive TST parasequence, moving into the lower Ruff Formation.
The observation of an intraformational unconformity within Guelph Formation sediments of the 9-33 well core generally supports the Guelph Formation pinnacle reef models of Huh
(1973) and others (Sears and Lucia, 1979; Mesolella, 1975; Shaver, 1974) and disputes the models by Rine (2016, 2017) and Gill (1975) that argue conformable deposition throughout the
Guelph Formation.
Controls on Reservoir Quality and Development A comprehensive reservoir characterization effort was made to resolve petrophysical profiles and controls on reservoir quality across the depositional environments of the Guelph
Formation, located leeward of the pinnacle reef crest. Reservoir characterization analysis included: 1) thin section analysis to identify pore types present in addition to general diagenetic alterations; 2) MICP analysis to classify capillary behavior of Guelph Formation sediments and gain insight into pore-throat networks of sampled lithologies; and 3) core plug analysis was 115
conducted to examined the distribution of porosity and permeability values of sampled
lithologies and assess petrophysical predictability using porosity-permeability transforms.
Thin section analysis revealed a variety of pore type assemblages and diagenetic
alteration was present across the leeward debris apron environment of the Guelph Formation.
Primary depositional processes were responsible for generating pore types including
interparticle, fenestrae, moldic, and growth-framework porosity. Diagenetic alterations resulted
in generation of vug, intercrystalline, channel and fracture porosity. Both primary and secondary pores were observed to show evidence of enhancement via dissolution and subsequent partial-to-
full occlusion by dolomitic burial cements, generating intercrystalline porosity.
Stylolitization was witnessed across many bioclastic fabrics and to a lesser degree in mudstones, with stylolitization most prominent in the proximal debris apron and reef environments, where stylobreccoid fabrics were observed. Dissolution was observed to be most prevalent within the reef and proximal debris apron associations, where preferential dissolution along stylolite sutures was often observed creating large, complex networks of microchannels and diffusive microporosity from suture zones. Most stylolite-centered channels were fully open; however, some channels were partially-to-fully occluded with bright white dolomitic burial cements, suggesting a continued interplay between stylolitization, dissolution, and cementation during intermediate-to-late stages of burial. Burial cements were present however was largely observed to be in early-to-mid stages of pore occlusion. A perplexing recrystallized isopachous cement was observed to line many vuggy pores and was interpreted to possibly be a remnant of
RFC cement. This remnant-recrystallized cement was present from intertidal to reef zone successions, however, was most commonly observed in the proximal debris apron. 116
These observations suggest three generalized stages of diagenetic alterations occurred which significantly altered the pore systems of the Guelph Formation lithologies in the 9-33 core including: 1) fabric selective dissolution of fossils and various allochems; 2) Non-fabric dissolution generating vugs, microporosity, and existing porosity enhancement and 3) pressure solution, continued dissolution, and burial cementation resulting in creation of stylolites, dissolution along stylolite suture zones, and early-to-mid stage occlusion of porosity by burial cements.
MICP experiments conducted on samples from the leeward debris apron environment were used to characterize pore-throat distributions and classify general capillary behavior.
Overall, Guelph Formation deposits exhibited bimodal pore-throat aperture size distributions and three distinct types of capillary-pressure behavior. Samples collected from the reef zone exhibited the most complex pore throat aperture sized distributions, as they were multimodal.
Type I and type II capillary classes were the most common of all lithologies sampled from intertidal, distal debris apron, proximal debris apron and reef environments. The capillary behavior and diverse pore-throat size distributions of these fabrics are interpreted to reflect the presence of multimodal pore systems consisting of mud-matrix pores (interparticle) along with the presence of secondary pore systems such as vugs, fossil molds, fenestrae, and channel pores.
Type I capillary behaviors were most often observed in skeletal grainstones and mudstones of the proximal debris apron. These lithologies featured strongly unimodal, macro-sized pore-throat distributions interpreted to be related to the presence of well-sorted grain-dominated fabrics.
Measurements of porosity and permeability from core plugs obtained from the 9-33 well core was subjected to exploratory analysis (descriptive statistics) and power functions were fit to petrophysical populations from each environment to model permeability as a function of total 117
porosity. Application of power functions were found to exhibit mixed success in successfully
modeling petrophysical variance, generating R2 values of 0.81, 0.72, 0.48, 0.54, and 0.65 from
the reef association, proximal debris apron, distal debris apron, intertidal association, and Guelph
Formation (collectively), respectively.
Tempestite successions from the proximal debris apron were found to exhibit good
petrophysical character and exhibit a higher degree of petrophysical predictability interpreted
through the generated of porosity-permeability transforms. A power function was fitted to core
plug porosity and permeability data, yielding a R2 value of 0.80, indicating the model fit the population well. Furthermore, tempestite deposits featured the highest measured values of permeability (up to 20 mD) and porosity values reaching up to 13%. Tempestite successions within the proximal debris apron were comprised of well-sorted bioclastic grainstone, rudstone and mudstone, interpreted to be deposited through storm-wave resuspension processes. Units interpreted to be located proximal to the FWWB were winnowed by fairweather wave-action, resulting a concentration of bioclasts and removal of mud content. Tempestite successions located more distal from the FWWB consisted of interbedded, normally graded, well-sorted grainstone and mudstone units. Tempestites deposits featured the best data clustering highest petrophysical predictability of any deposits analyzed in the 9-33 well core. This is interpreted to be due to the high volume of bioclastic content, which was subsequentially dissolved generating higher values of porosity and the well-sorted nature of the lithologies resulting in grain- supported fabrics. These attributes are primary in nature resulting from deposition by wave- resuspension processes.
Debrite deposits also exhibited notable petrophysical character. These deposits consisted of mud-supported intraclastic floatstone conglomerate and grain-supported peloidal rudstone. 118
Debrite deposits exhibited permeability values as high as 4 mD, however generally exhibited values between 0.01-1 mD, and porosity values between 4-11% porosity. However, these
deposits exhibited a lower degree of petrophysical predictability, with the fitted power function
generating a R2 value of 0.57, indicating the function did not describe the population well. The petrophysical character and lower predictability between porosity-permeability values observed
in debrite deposits is interpreted to be related to their high volumes of bioclastic content and
dissolved allochems, generating good porosity shows, however the prevalence of poorly sorted
textures lowers permeability and decreases the ability to predict permeability as a function of porosity.
Depositional signatures within the leeward debris apron environment of the Guelph
Formation are generally observed to be second-order factors affecting reservoir quality, largely
being overprinted by diagenetic processes including dolomitization, recrystallization, dissolution,
stylolitization and fracturing. Diagenetic overprints are interpreted to generally lead to an overall
enhancement of porosity but subsequently decrease the correlation between porosity and
permeability. However, primary fabrics generated by storm-wave resuspension (and fairweather wave-action) are shown in this study to exhibit a first-order control on the reservoir quality of geologic units within the leeward debris apron environment. The high-degree of clast sorting generated by wave-action is shown by MICP data to generate class I and III capillary behaviors with unimodal pore-throat distributions. The combination of unimodal pore-throat distributions along with intergranular/intercrystalline porosities is interpreted to be the leading factor in generating the predictable reservoir quality within these deposits.
119
Future Work
This study builds on the pivotal work of many previous publications in attempting to
resolve the depositional history, architecture, and reservoir quality of Guelph Formation pinnacle
reefs and their associated deposits. Substantial progress has been made by previous authors in
resolving reef growth models of the Guelph Formation (Rine, 2016, 2017; Mesolella,1974; Huh
1973; Gill, 1973), however questions regarding the development of the leeward debris apron and
windward talus remain. This study has shown that the leeward debris apron developed similarly
to the debris apron systems described by Mullins and Cook (1986), however a detailed
understanding of the depositional architecture and reflectance to reef growth cycles remains elusive. This study brings awareness to the fact that the leeward debris apron exhibits a complex depositional history and exhibits a semi-predictable depositional model with sedimentation controlled by wave-resuspension and downslope transport processes. Results of this investigation instigate additional research questions such as: How do changes in reef height affect the thickness of tempestite successions? Were sediment gravity flows deposited as lenticular sheets draping the debris apron or in broad-lenticular channels? With improvements in geochronologic control, can bioclastic shedding recorded in marginal environments of Guelph Formation reefs be correlated to changes in paleoclimate, sea-level or reef growth? How did high-stand bioclastic shedding affect the debris apron development? The answer to these questions may be achieved by: 1) improvements in geochronologic dating of reef and debris apron sediments; 2) presence of an array of wells to examine debris apron architecture; 3) the ability to track key beds using geophysical logs; 3) generating porosity tomography maps from seismic or geophysical logs to examine ties between the geometry of high porosity zones and interpreted geology to identify lenticular channels, gullies or ravines within the debris apron. The author hopes that this 120 investigation will spawn additional studies to gain a more detailed understanding of the marginal environments of Guelph Formation reefs and that data incorporated in this study will be used for subsequent studies to further scientific knowledge of these unique reef systems.
121
CHAPTER VI: SUMMARY & CONCLUSIONS
Analysis of a 70m (230 ft) thick succession of core obtained from the 9-33 well captured
a 69 m thick section of the Guelph Formation and a 2.4 m thick succession of the lower Ruff
Formation. The Guelph Formation was completely dolomitized as was the succession recovered
from the Ruff Formation.
Core analysis of sedimentary deposits recovered from 9-33 well core identified 16
lithofacies which were interpreted to have been deposited within five facies associations
including reef, tempestites, debris flow and turbidites, subtidal back-reef, intertidal, and supratidal environments. Reef zone conditions were interpreted from the occurrence of tabulate coral-stromatoporoid framestones (facies Fr) and muddy-bioclastic sediment which infilled reef cavities (facies Sfm and Mm). A tempestite succession was interpreted from the presence of undulate-to-flat grey mudstone (facies Mpg), undulatory normally-graded skeletal grainstone
(facies Sg), and amalgamated skeletal rudstone (facies Ram) with laterally aligned clasts. A debrite-turbidite was interpreted from the presence of thick mud-supported intraclastic floatstone conglomerates (facies Ifb) with undulatory cross-stratified tops (facies Rcb) interpreted to be proximally sourced debrites, medium bedded peloidal rudstones (facies Rcg) interpreted to be distally-sourced debrites, in addition to a sparse occurrence of interbeds of skeletal grainstone
(facies Sg) and convoluted mudstone (facies Mpg) occuring above distal debrites interpreted to be partial Bouma turbidite divisions (Ta and Tde). A subtidal back-reef origin was interpreted from the presence of massive-to-thickly bedded, mottled mudstone (facies Mm), skeletal
wackestone (facies Sw) and floatstone (facies Sfm), interpreted to be deposited below wave-base
and to be mainly autochthonous sediment. Intertidal conditions were evidenced from the 122
occurrence of mottled, massive-to-laminated mudstones (facies Ml and Mm), flat-to-wavy heterolithic cryptalgal bindstones (facies Hcr) that featured rare high-amplitude domal stromatolites, and periodic grain-rich storm deposits (facies Hcrd and Sp). Supratidal conditions
were interpreted from the occurrence of cryptalgal bindstones with wrinkled-to-serrated mat
geometries, desiccation cracks and teepee structures (facies Bcr) along with intraclastic
floatstone breccias (facies Ifcr).
Guelph Formation sedimentary successions were interpreted to represent two distinct
depositional stages: a stage of active reef growth and accumulation of the reef-derived bioclastic
debris apron (stage 1) and a peritidal stage (stage 2). These two depositional successions were
observed to be unconformable. At the base, reef zone deposits (FA5) consisting of massive
tabulate coral-stromatoporoid framestones (facies Fr) were observed to be interbedded-to-
adjacently located to skeletal floatstones (facies Sfm) and mudstones (facies Mm), interpreted to be cavity-infilling sediment. No stratigraphic trends were observed with the reef association, however growth patterns of tabulate coral colonies indicated vertical growth occurred perpendicular with respect to the core.
Reef association framestones were visibly onlapped by muddy, bioclastic sediments of the debris apron environment. The debris apron association was split into proximal and distal segments, separated at wave-base. At its lowermost depths, the proximal debris apron was
observed to consist of tempestite successions exhibiting a deepening upward trend, with deposits
showing evidence of movement from locations proximal to the FWWB into deeper-more distal depths above the SWWB, further away from the carbonate factory. Tempestite successions were
overlain by a debrite-turbidite succession with rare evidence of wave-reworking at the top of one
debrite, suggesting a location above wave base. A large (~25 m) break in the recovered core 123
occurred, followed by the appearance of a 0.5 m debrite, suggesting the debrite-turbidite
succession extends below wavebase into the distal debris apron. The debrite-turbidite association
is overlain by autochthonous deposits of the subtidal back-reef association, observed to exhibit a
sharp decrease in bioclastic content moving up-section, suggesting shifting depths into distal
slope segments.
The general deepening-upward trend observed within environments of the active reef
depositional stage of the Guelph Formation was interpreted to represent the lateral shift of
environments downslope from the active carbonate factory (reef zone-pinnacle crest), to distal
segments of the leeward slope located below wave-base. Significant volumes of the core
recovered from the leeward debris apron were comprised of tempestite and sediment gravity
flow (debrite) successions, indicating that off-bank transport via storm-wave resuspension was
the primary sediment transport mechanisms responsible for feeding the development of the
proximal debris apron. Events of slope destabilization, possibly caused by high sediment fluxes
during storm events, are interpreted to be the primary sediment transport mechanism for
distributing sediment from near wave-base to more distal segments of the leeward slope, through
sediment gravity flows. These interpretations coupled with the stratigraphic successions
observed within the leeward debris apron environment of the 9-33 well suggest that the leeward
debris apron may generally be modelled according the slope apron model generated by Mullins and Cook (1986).
The accumulation of the leeward debris apron is interpreted to be coeval with pinnacle reef growth, evidenced through the onlapping relations observed within the 9-33 core. Due to the laterally deviated nature of the 9-33 well, deepening upward trends seen in the stage 1 deposits are interpreted not to be comprised of a true vertical succession, and therefore not representative 124
of time-transgressive dynamics of the slope apron and pinnacle reef system. Instead, the succession reef stage deposits represent an assemblage of laterally adjacent environments that are present leeward of the pinnacle reef crest at any given point of time. Reef stage deposits were interpreted to most likely represent TST-EHST conditions, where the debris apron formed contemporaneously with pinnacle reef growth. It is interpreted that slope destabilization events of various frequencies are the main control on the amount of accommodation space present within the debris apron, and that off-bank sediment transport via storm-wave resuspension is the primary method of sediment supply into the slope apron system.
TST-EHST reef stage deposits were observed to be capped by a sequence boundary, interpreted to mark the end of HST deposition. Immediately overlying this boundary, a 2.5 cm thick section of vuggy mudstone was observed, interpreted to represent LST deposition along the leeward slope. The LST mudstone was observed to by erosively overlain by a TSE boundary and transgressive peritidal deposits.
From its base, the peritidal depositional stage of the Guelph Formation consisted of thickly bedded mottled mudstones (facies Mm) that transitioned into laminated mudstone (facies
Ml), which then transitioned into flat-to-wavy microbial laminites and stromatolites, that were periodically interbedded with grain-rich storm deposits. These uppermost wavy microbial laminites were observed to be overlain by a thin microbial mat with wrinkled algal mat geometry and teepee structures. The peritidal succession of the Guelph Formation is interpreted to represent a shallowing-upward trend, reflecting the transition from subtidal-to-supratidal environments along the leeward slope and interpreted to represent TST deposition.
125
The contact between the lower Ruff Formation and upper Guelph Formation was observed as a sharp and erosive irregular unconformity interpreted to be second TSE boundary.
Just below the contact, a shallowing-upward package consisting of intertidal wavy microbial laminites (Hcr) was observed to become increasingly interbedded with storm-generated algal breccia floatstones and overlain by a thin wrinkled algal mat with teepee structures, marking the transition into supratidal conditions within the leeward flank. The overlying Ruff Formation microbial laminites disconformably blanketed contact topography, exhibiting low-amplitude domal algal geometries, interpreted to be suggestive of shallow subtidal-to-intertidal settings.
Extensive vuggy porosity development was observed in the uppermost Guelph Formation sediments just below the contact, which was interpreted to suggest a second period of subaerial exposure occurred along the leeward flank of the pinnacle reefs. The second TSE was interpreted to mark deposition of successive TST parasequences moving from the upper Guelph Formation into the lower Ruff Formation.
Geophysical log analysis and generation of a geophysical log model from the succession of depositional environments observed in the 9-33 core revealed that reef and leeward debris apron deposits displayed irregular to cylindrically-shaped gamma-ray patterns. The gamma-ray signatures from these environments were generally indistinct, making identification of these environments difficult using the gamma-ray log alone. Only when coupled with supplemental geophysical logs such as neutron porosity and resistivity borehole image logs can key stratigraphic successions be resolved. The tempestite and debrite successions of the proximal debris apron featured strong contrasts in measured neutron porosity which was interpreted to be related to the varying degree of sorting and bioclastic content between the two successions.
Additionally, both successions were imaged remarkably well within the RBHI log due the 126
presence of mud-supported intraclastic conglomerates and interbedded grainstone-mudstone successions, creating unique resistivity-based fabrics observed in both static and dynamic views.
Geophysical-log signatures of reef and distal debris associations were found to be indistinct in
RBHI and neutron porosity logs.
Gamma-ray profiles of peritidal stage deposits of the Guelph Formation were
characterized by a well-defined, positive funnel-shaped curve that is typical of shoaling-upward
deposits. Additionally, microbial mat successions were imaged very well by the RBHI log,
interpreted to be related to their interbedded nature and presence of stratiform vugs.
A reservoir characterization initiative was conducted to resolve controls on reservoir
quality, capillary pressure behavior and assess petrophysical predictability across the leeward
slope environment of the Guelph Formation. Core and thin section analysis found that the
leeward debris apron environment is comprised of both mud-supported and grain-supported
lithologies which have undergone extensive diagenetic alterations as a result of cementation,
dolomitization, recrystallization, dissolution, and replacement, fracturing and stylolitization.
Primary depositional processes were responsible for generating pore types including
interparticle, intergranular, fenestrae, moldic, and growth-framework porosity. Secondary pore types observed were most commonly vuggy intercrystalline porosities, however selective dissolution along stylolite swarms was observed to generate large open channels and diffusive microporosity. Dissolution was observed to be extensive throughout all depositional environments, however it was most pervasive within the proximal debris apron. Observations suggest three main diagenetic alteration stages occurred affecting the porosity of sediments recovered from the core, including: 1) fabric selective dissolution of fossils and various allochems; 2) non-fabric dissolution generating vugs, microporosity, and existing porosity 127
enhancement and 3) pressure solution, continued dissolution, and burial cementation resulting in
creation of stylolites, dissolution along stylolite suture zones, and early-to-mid stage occlusion of porosity by burial cements.
MICP analysis revealed the capillary pressure behavior of all 16 lithofacies encountered within the leeward profile of Guelph Formation reefs can be generalized by three classes of
capillary behavior. Pore-throat size distributions obtained from sampled lithofacies generally
consisted of mainly bimodal pore-throat aperture size populations that spanned from micro-to-
macro pore throat aperture sizes, however in all zones with the exception for the mud-rich distal
debris apron, the bulk majority of mercury that was injected into the sample was through macro-
sized pore-throat apertures, suggesting macro-sized pore throat apertures dominate the pore-
throat networks in many samples. Samples from the tempestite successions exhibited the most
desirable capillary pressure behavior, where the presence of well-sorted grainstones and
mudstone yielded simple, predictable class I capillary pressure behaviors with unimodal, macro-
sized pore-throat distributions.
Analysis of measured porosity and permeability values revealed porosity varies from 1-
15% and permeability ranges from 0.001-20 mD across the Guelph Formation. Power functions were determined to be the best fit for modeling permeability as a function of total porosity, generating a R2 value of 0.65 for the Guelph Formation collectively, 0.81 within the reef environment, 0.72 within the proximal debris apron, 0.48 within the distal debris apron, and 0.54 in the intertidal environment. Furthermore, samples taken from the tempestite succession were
found to exhibit the highest reservoir quality by exhibiting some the highest measured values in
permeability and porosity in addition to a good degree of petrophysical predictability with the
fitted power function generating a R2 value of 0.8. These desirable attributes are interpreted to be 128 manifestation of the high volume of bioclastic content present within the lithologies, which was subsequentially dissolved, generating higher values of porosity, and good-sorting of bioclasts which resulted in grain-supported fabrics. These attributes are primary in nature resulting from deposition by wave-resuspension processes.
Generally, petrophysical characterization of lithofacies sampled from the leeward profile of the Guelph Formation saw primary depositional signatures to be a second-order factor affecting reservoir quality, largely being overprinted by diagenetic processes including dolomitization, recrystallization, dissolution, stylolitization and fracturing. Diagenetic overprints are interpreted to generally lead to an overall enhancement of porosity but subsequently decreasing the correlation between porosity and permeability. However, primary fabrics generated by storm-wave resuspension are shown in this study to exhibit a first-order control on the reservoir quality of lithologies, spawning from the high degree of sorting that is generated from this sediment transport mechanism. The large thickness of tempestite successions, coupled with their onlapping relationship with the pinnacle reef and lateral encasement within mud-rich fabrics suggests a potential for utilization within EOR strategies.
129
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Talling, P.J., Masson, D.G., Sumner, E.J., Malgesini, G., 2012, Subaqueous sediment gravity flows: Depositional processes and deposit types, Sedimentology, V.59, p.1937-2003.
Tournadour, E., Mulder, T., Borgomano, J., Hanquiez, V., Ducassu, E., Gillet, H., 2015, Origin and architecture of a mass transport complex on the Northwest Slope of Little Bahama Bank 139
(Bahamas): Relations between off-bank transport, bottom current sedimentation and submarine landslides, Sedimentary Geology, V.317, p.9-26.
Trout, J.L, 2012, Faunal Distribution and Relative Abundance in a Silurian (Wenlock) Pinnacle reef complex-Ray Reef, Macomb County, Michigan [a master’s thesis]: Western Michigan
University, 300 p.
Van der Voo, R., 1988, Paleozoic paleogeography of North America, Gondwana, and intervening displaced terranes: comparison of paleomagnetism with paleoclimatology and biogeographical patters, Geological Society of America Bulletin, V.100, p.311-324.
Walker, R. G., ed., 1984, Facies models, 2nd ed.: Geoscience Canada Reprint Series 1, 317 p.
Webb, P.A., 2001, An introduction to the physical characterization of materials by mercury intrusion porosimetry with emphasis on reduction and presentation of experimental data,
Micrometrics Instruments Corp, Norcross, Georgia, p.1-23.
Yang, S., Fundamentals of petrophysics, Springer Mineralogy 2017 Beijing, China, 502 p.
Zhou, Z., Willems, H., Li, Y., Luo, H., 2011, A well-preserved carbonate tempestite sequence from the Cambrian Gushan Formation, eastern North China Craton, Paleoworld, v.20, p.1-7.
140
APPENDIX A: TABLES Table 1. Lithofacies from the Guelph Formation (Rine, 2016) Depositional Depositional Lithofacies Lithologic Attributes Facies Environment Mottled gray and white; micritic mud Below the Crinoidal mudstone matrix; crinoid ossicles; abundant SWWB stromatactis Bioherm Mottled gray and white; micritic mud Skeletal matrix; crinoid ossicles, bryozoans, SWWB - wackestone tabulate corals, rugose corals; abundant FWWB stromatactis Bioherm Crystalline Devoid of fossils; composed of dolomite Above the toe/cap dolomite rhombs; intercrystalline porosity FWWB Coral Frame-building organisms (tabulate Above the Stromatoporoid corals, stromatoporoids) FWWB boundstone Reef Core Intra-reef faunal assemblages (bryozoans, Skeletal SWWB- brachiopods, crinoids, rugose corals);50- Wackestone FWWB 75% micritic matrix Hemispheroid Above Dark brown; laterally linked hemispheroid Stromatolitic FWWB to stromatolites Stromatolitic Bindstone Intertidal Cap Skeletal fragments composed of reef core Skeletal Above the organisms (tabulate corals, brachiopods, Wackestone FWWB bryozoans); abundant cements Minor skeletal fragments; abundance of SWWB- Skeletal Mudstone drusy calcite, spar cement lining vuggy FWWB Proximal Reef porosity Apron Skeletal Transported fossils (tabulate corals, Above the Wackestone brachiopods, bryozoans, crinoids) FWWB Minor skeletal fragments; abundance of Distal Reef Below the Skeletal Mudstone drusy calcite spar cement lining vuggy Apron SWWB porosity Coarse lithoclastic Large, dark gray angular clasts in light Below the Reef Rubble conglomerate gray micritic mud matrix; crinoids present SWWB Conglomerate Skeletal lithoclastic Abundant tabulate corals, brachiopods Below the conglomerate crinoids; moldic porosity SWWB Stromatolite Stromatolitic Clasts composed of stromatolites; poorly Below the Rubble lithoclastic sorted SWWB conglomerate Conglomerate Distal Reef Minor skeletal fragments; abundance of Below the Skeletal mudstone Rubble drusy calcite spar lining vuggy porosity SWWB Modified from Rine (2016) 141
Table 2. Lithofacies of the Lower Salina Group (Rine 2016) Depositional Depositional Lithofacies Lithologic Attributes Facies Environment White to gray; tan, crinkly Crinkly laminated cyanobacterial mats; stromatolitic Supratidal; restricted devoid of mud Cyanobacterial bindstone Mats White to gray; broken Stromatolitic fragments of crinkly Supratidal; restricted conglomerate stromatolitic bindstone Mottled dark brown micrite Thrombolitic Thrombolitic with light tan sparry cement; Subtidal to intertidal; bindstone bindstone irregular patchy mesoclots restricted to normal
Abundance of spherical, Oncolitic Oncolitic cyanobacterial mat encrusted Intertidal; restricted to packstone packstone clasts normal Wavy parallel laminations of Laminated Peloidal spherical to elliptical peloidal Subtidal to intertidal; peloidal wackestone grains restricted to normal wackestone
Brown to gray carbonate mudstone laminae with black Thinly-laminated Below the SWWB; Poker chips carbonaceous films; “poker- mudstone restricted (lower, middle, chips” parting along upper) laminations Micro-laminated Dolomitic mud; carbonaceous SWWB-FWB; mudstone mat chips; massively bedded restricted Massively bedded crystalline Crystalline Above the FWWB-to- A-1 Packstone dolomite composed of dolomite Dolomite subtidal; restricted
Nodular anhydrite in a micro- Rabbit Ear Nodular laminated carbonate mudstone; Intertidal- Supratidal; Anhydrite anhydrite white to bluish nodules restricted flattened parallel to bedding Mottled tan, tan gray, to dark Upper A-1 Micro-laminated Above the FWWB-to- brown; vertical fractures lined Mudstone mudstone Subtidal; restricted with halite; lime mud Modified from Rine (2016)
142
Table 3. Sequence Stratigraphy of the Guelph Formation-Lower Salina Pinnacle Reef Complex (according to Rine, 2016) Stage Summary System R.S.L. (m) R.S.L. (m) Geologic Tract Reef Crest Flank unit 1 Initiation of Bioherm; evolution and growth of TST- Guelph skeletal reef; coeval 30 to 10 30 to 100 EHST Formation deposition of reef talus, apron and reef rubble 2 Deposition of Stromatolitic Guelph Cap; slowing of relative sea- LHST 10 to 0 100 Formation level rise 3 Deposition of A-0 Carbonate and A-1 Evaporite; 3rd order Salina Group: SL fall; Complete exposure FSST- 20 to Exposed A-0 Carbonate; of reef complex; decoupling LST Exposed A-1 Evaporite of basin from normal marine water 4 10 to 70; 70 Deposition of lower A-1 to exposed carbonate and Rabbit Ear Salina Group: TST- Exposed (Model A) anhydrite; re-establishment Ruff HST; (Model A); Or of cyanobacterial organisms Formation HST- Or 10 to 100; in flank position; relative SL Rabbit Ear LST 10 (Model B) 100 to rise and freshening of basin Anhydrites exposed waters (Model B) 5 Deposition of Upper A-1 Carbonate; First (model A) or second (model B) time Salina Group: sea-level reached reef TST- Exposed to 3 10 to 100 Ruff complex crest since end of HST Formation stage 2; re-establishment of Cyanobacterial organisms on reef crest 6 Deposition of A-2 Evaporite; second 3rd order SL fall; complete exposure of reef LST- 20 to Salina Group: exposed complex; decoupling of basin TST exposed A-2 Evaporite from normal marine ocean waters 7 Deposition of A-2 Anhydrite Salina Group: and A-2 carbonate; Reef HST 3 to 10 3 to 10 A-2 Anhydrite, complex physically encased A-2 Carbonate Modified from Rine (2016)
143
Table 4. Capillary Pressure Type-Curve Classification System
Classification Criteria Quick Look Class Attributes Pc Curve Pore Anticipated Pc Curve PTSD Class Pc50 value Throat Pore System Reservoir Kinks Sorting Sizes Quality Moderate Class I <100 psi No Good Unimodal Good to Large
Poor to Bi-to- Class II N.C. Yes Varied Complex Moderate multimodal
Small to Class III >100 psi No Good Unimodal Poor Moderate
Abbreviations: Pc= capillary pressure; Pc50 = Pressure of median saturation; PTSD = Pore throat Size Distribution; N.C.= Not considered.
144
Table 5. Lithofacies of the 9-33 Well Core
Code Lith. Sedimentary Structures & Features Fossil Content Interpretation M Mudstone Massive-medium bedded; mottled Not Observed Wide Distribution Ml Mudstone Laminated (planar, wavy); hopper crystals Not Observed Intertidal mudstone Thinly bedded; undulate-to-flat contacts; Mpg Mudstone Not Observed Tempestite/Turbidite rare convoluted bedding Subtidal-Intertidal Sw Wackestone Massively bedded; mottled Bi, Bry Wackestone Wavy bedded (thin, medium); erosive; Subtidal-Intertidal Sp packstone Not Observed clast orientation storm deposit Thinly bedded; normally graded; Sg Grainstone Brach, Skl Deb Tempestite/Turbidite undulatory-to-flat contacts; burrows Penta Brach, Subtidal skeletal Sfm Floatstone Medium bedded; mottled Rug Cor, Bry, floatstone Cri, Ech, Intraclastic Thinly bedded; pebble-sized breccia; Crypt Algal Intertidal-Supratidal Ifcr Floatstone imbrication Lam Storm deposit bedded (very thickly, medium); sorted Intraclastic Cri, Brach, Ifb (poor, well); Inverse Grading; Debrite Floatstone Bry, Tab Cor rounded-angular, intraclasts bedded (thin-medium); broken spherical Rcg Rudstone Brach Debrite coated-grains (cobble-pebble); Peloids Brach, Ech, Massive; crude grading; amalgamated Ram Rudstone Cri, Tab Cor, Tempestite beds; heavily cemented; aligned clasts Bry Medium bedded; bioclastic cross- Frag Brach, Rcb Rudstone stratification; well sorted; undulatory Debrite Bry, Skl Deb bedding Subtidal-Intertidal Heterolithic Mudstone/bindstone; laminated to bedded Crypt Algal Hcr microbial laminite (planar, wavy); clotted mud textures Lam; Strom
medium bedded; disturbed cryptalgal Hcrd Heterolithic Dist Crypt Subtidal-Intertidal mats/clotted mudstones; clast orientation Algal Lam storm deposit burrows Crypt Algal Upper Intertidal- Bedded (thin, medium); teepee-structures; Bcr Bindstone Lam Supratidal microbial wrinkled-serrated lam; desiccation cracks laminite Strom; Tab Reef Core Fr Framestone Massive; branching and laminar growth Cor Framestone Abbreviations: Tab cor = tabulate coral; Brach=brachiopod; Cri=crinoid; Skl Deb=skeletal debris; Rug Cor= rugose coral; Bry=bryozoan; Ech=echinoderm; Algal Lam= Cryptalgal laminations; Strom=Stromatoporoids; 145
Table 6. Facies Associations of the 9-33 Well
Facies Water Code Facies Organization Association Depth Succession of cryptalgal bindstone interbedded with storm lags; 0.3m-1.5m thick; shallowing-upward trend Above FA1 Supratidal Bcr, Ifcr with increasing desiccation; overlies intertidal FWWB sediments (FA2) Ml, Succession of massive-to-laminated mudstone, Mm, cryptalgal laminites/stromatolites interbedded with storm deposits; 0.6-7m thick; shallowing upward trend; Above FA2 Intertidal Hcr, FWWB Hcrd, overlies distal debris apron deposits (FA3) and underlies
Sp supratidal deposits (FA1) Massive-to-thickly bedded succession of mottled, mudstones, skeletal wackestones and floatstones; Mm, interpreted to be mainly autochthonous; drastic Below Subtidal FA Sw, volumetric drop in bioclastic material vertically, 6m-? Wave 3 back-reef Sfm, thick; deepening upward trend with increasing distance base from carbonate factory; overlies debrite succession
(FA4) and underlies intertidal deposits (FA2) Massive-to-medium bedded succession of intraclastic floatstone conglomerate interbedded with cross- Ifb, stratified bioclastic rudstone, peloidal rudstone, FWWB Rcg, Debrite/ normally graded skeletal grainstone and grey mudstone to FA4 Rcb, Turbidite with rare convoluted bedding; Proximal and distal below Sg, debrites with Bouma Ta and Tde division turbidites; 0.6 wavebase Mpg (?)-7.9m (?) m thick; overlies tempestite succession FA5 and underlies subtidal back reef deposits FA3 Medium-to-thickly bedded succession of normally graded skeletal grainstone, non-fossiliferous mudstone Sg, SWWB and extensively cemented, amalgamated bioclastic FA5 Tempestite Mpg, to rudstone with laterally aligned clasts; 0.45-4.7 m thick; Ram FWWB overlies reef association (FA6) and underlies debrite succession (FA4) Delicate “branching” colonial tabulate coral and coalesced laminar stromatoporoid framestones and Skeletal Fr, Sfu, Above FA6 internal sediment (floatstones, and mudstones); 7.9m-? Reef Mm thick; Located at bottom of cored interval, overlain by FWWB
tempestite deposits (FA4) Notes: FWWB= Fair weather wave base; SWWB= storm weather wave base; ? = unknown
146
Table 7. Distribution of pore types observed within thin sections of the Guelph Formation
Sample Pore Types Observed Deposit Facies Depth Facies (Choquette and Pray 1970; Type (Ft.MD) Association Lonoy, 2006)
5535.5 Hcr Microbial laminite Intertidal FE, BP, BC 5540.5 Disturbed microbial Hcrd Intertidal FE, BP, BC laminite 5550.5 Mm Mottled mudstone Intertidal VUG, BP, BC 5559.5 Mm Mottled mudstone Distal Apron VUG, BP, BC 5599.5 Sw Skeletal Wackestone Distal Apron BC, BP, VUG, MM 5694.5 Intraclastic Ifb Proximal Apron BC, MM, VUG, BP floatstone 5718.55 Skeletal grainstone Sg BC, MM, VUG, BP * Proximal Apron
5721.45 Sg Skeletal grainstone Proximal Apron CH, BC, BP, VUG, M 5726.45 Skeletal Sg/Mpg Proximal Apron BC, CH, VUG, BP, grainstone/mudstone 5727.5 Mpg Mudstone Proximal Apron MM, BC, VUG, BP 5734.5 Fr Framestone Reef VUG, BC, MM, 5738.5 Sfm Skeletal Floatstone Reef MM, BP, BC, VUG 5752.5 Fr Framestone Reef BP, VUG, BC 5758.5 Sw Skeletal Wackestone Reef BC, BP, VUG Abbreviations: FE=fenestral; BP=interparticle; BC=Intercrystalline; MM=mudstone microporosity; CH=Channel
147
Table 8. Guelph Formation Core Plug Analysis Summary
Statistic Permeability to Air (mD) Porosity (%)
Mean 2.77 7.14
Median 0.23 6.51
Standard Deviation 5.89 2.93
Population Variance 34.68 8.56
Range 0.001-27.62 1.51-14.16
Skewness 2.74 0.37
N (individual core plugs) 80.00 80.00
P Value (95% Confidence) 1.31 0.65
148
Table 9. Reef Association (FA5) Core Plug Analysis Summary
Statistic Permeability to Air (mD) Porosity (%)
Mean 7.20 8.36
Median 1.63 7.64
Standard Deviation 9.88 3.32
Population Variance 97.56 11.04
Range <0.01-27.62 2.75-14.16
Skewness 1.24 0.01
N (individual core plugs) 15.00 15.00
P Value (95% Confidence) 5.47 1.84
149
Table 10. Proximal Debris Facies Association (FA4) Core Plug Analysis Summary Statistic Permeability to Air (mD) Porosity (%)
Mean 1.48 6.73
Median 0.11 6.20
Standard Deviation 3.98 2.66
Population Variance 15.85 7.06
Range <0.01-20.45 2.49-13.31
Skewness 3.95 0.62
N (individual core plugs) 35.00 35.00
P Value (95% Confidence) 1.37 0.91
150
Table 11. Distal Debris Facies Association (FA3) Core Plug Analysis Summary
Statistic Permeability to Air (mD) Porosity (%)
Mean 1.10 6
Median 0.62 5
Standard Deviation 1.79 3
Population Variance 3.21 9
Range <0.01-5.03 1-11
Skewness 2.29 0.46
N (individual core plugs) 7.00 7
P Value (95% Confidence) 1.66 2
151
Table 12. Intertidal Facies Association Core Plug Analysis Summary
Statistic Permeability to Air (mD) Porosity (%)
Mean 2.27 7.25
Median 0.19 6.81
Standard Deviation 4.40 2.85
Population Variance 19.33 8.13
Range <0.01-15.71 3.16-9.66
Skewness 2.21 0.3
N (individual core plugs) 24.00 24.00
P Value (95% Confidence) 1.86 1.20
152
APPENDIX B: FIGURES
Figure 1. Map showing the relative location of the 9-33 well in reference to the pinnacle reef field. The blue line shows the trajectory of the laterally deviated well (provided by Battelle, 2020).
153
Figure 2. Generalized shallowing-upward successions of Proterozoic, Paleozoic, and Mesozoic-
Cenozoic-aged peritidal carbonate environments (labelled left to right) (from Pratt,2010).
154
Figure 3. Stratigraphic successions as a function of distance from platform margin, and concept models of (A) carbonate debris aprons, and (B) base-of-slope apron environments (modified from Mullins and Cook, 1986).
155
Figure 4. Vertical successions and sub-environments of slope apron systems described by
Mullins and Cook (1986).
156
Figure 5. Conceptual model of the development of multicomponent sediment gravity flows consisting of basal debris flows and overriding turbidity currents as the sediment gravity flow moves down slope. Debris flows are first observed as mud-supported conglomerates, which then transition into increasingly clast-supported textures while overriding turbidity flows deposit increasingly preserved turbidite successions on top of the basal debrites (From Mullins and
Cook, 1986).
157
Figure 6. Concept model for medium-grained, medium sized calciclastic submarine fans. The model illustrates: (1) upper slope cut by erosive tributary gullies; (2) depositional channel-levee system with braided axis; (3) depositional lobes/sheets; and (4) fan fringe environments (from
Payros and Pujalte, 2007).
158
Figure 7. Associations, sedimentary structures and paleontology of tempestite deposits as a function of distance from source (from Perez-Lopez and Perez-Valera, 2011)
159
Figure 8. Idealized Bouma Sequence (from Shanmugam, 1996)
160
Figure 9. Idealized Miescher sequence, describing the ideal carbonate turbidite succession (from
Flugel, 2004).
161
Figure 10. Paleogeographic map of the Michigan Basin showing stable structural features surrounding the basin, along with interpreted oceanic inlet locations and relative bathymetry. The distribution of generalized environments includes: (1) carbonate platform (light blue), (2) platform slope (medium blue), where pinnacle reef development ensued, and (3) basin depositional center (dark blue). The red box denotes the relative location of this study (Modified from Rine, 2016). 162
Figure 11. Chronostratigraphic chart showing Silurian-aged lithostratigraphic units of the
Michigan Basin. Unit nomenclature corresponds to informal unit names used by industry. Red arrows denote lithostratigraphic units characterized within this study (Modified from Rine,
2016).
163
Figure 12. Depositional Model of the Niagara-Lower Salina pinnacle reef complex, with associated depositional assemblages distributed across the reef complex, as a function of paleo-wind direction and preceding topography (Modified from Rine, 2016). The red ellipse denotes the approximate location of the cored interval from the 9-33 well examined in this study (Battelle, 2016).
164
Figure 13. Gamma-ray geophysical log signatures of coarsening upwards (C-U) and fining upwards (F-U) in rock fabrics (from Walker, 1984).
165
Figure 14. Idealized shapes of gamma-ray curves that may be used for interpretations of depositional environments within sedimentary successions (From Walker, 1984).
166
Figure 15. Six idealized capillary drainage (Pc) curves for samples exhibiting unimodal pore
systems. Pc curve obtained from a sample with: (a) well-sorted pore throat distributions and
large-sized pore throats, (b) well-sorted pore throat distributions and moderate-sized pore throats,
(c) well-sorted pore throat distributions and small-sized pore throats, (d) poor-sorted pore throat distributions with varied pore throat sizes, (e) poor-sorted pore throat distributions with varied pore throat sizes (skewed to smaller sizes), (f) very poorly-sorted pore throat distributions with varied pore throat sizes (from Yang, 2017).
167
Figure 16. Characteristic pictures of various mudstone lithofacies including (A) massive, mottled, dolomitic mudstone facies (Mm); (B) planar-to-wavy laminated, dolomitic mudstone facies (Ml); and (C) thin-to-medium bedded, undulatory-to-flat, pale grey, dolomitic mudstone facies (Mpg). The leftmost scale denotes inches, while the rightmost scale denotes decimal feet.
168
Figure 17. Microscopic image showing the recrystallized isopachous blocky cement lining a vug, thought to potentially represent remnant RFC cement. Image was taken from sample at 1743.8 m
(5721 ft) under cross-polarized light at 25x magnification.
169
Figure 18. Characteristic pictures of skeletal grain-rich lithofacies including: (A) massive, mottled, dolomitic skeletal wackestone facies (facies Sw); (B) bedded, dolomitic skeletal packstone facies (facies Sp); (C) undulatory, thinly bedded, normally graded, dolomitic skeletal grainstone facies (facies Sg); and finally (D) bedded, mottled dolomitic skeletal floatstone (facies
Sfm). Note the speckled appearance of grains showing preferential grain orientation. The leftmost scale denotes inches and the right most scale denotes decimal feet. 170
Figure 19. Characteristic pictures of intraclastic floatstone lithofacies Ifcr and Ifb. Lithofacies
Ifcr is observed as observed as a thinly bedded, imbricated, dolomitic, pebble-breccia floatstone
(A). Lithofacies Ifb is observed to exhibit three intraclastic textures including: (B) texture 1; (C) texture 2; and finally (D) texture 3. The leftmost scale denotes inches while the rightmost scale denotes decimal feet. 171
Figure 20. Characteristic pictures of rudstone lithofacies including: (A) thin-to-medium bedded,
peloidal-to-cortoidic dolomitic rudstone (lithofacies Rcg); (B) massive, crudely-to-normally
graded, amalgamated dolomitic rudstone with laterally aligned-bioclasts (facies Ram); and finally (C) Medium bedded, undulatory, cross-stratified, dolomitic skeletal rudstone (facies Rcb).
The leftmost scale denotes inches while the rightmost scale denotes decimal feet.
172
Figure 21. Characteristic pictures of cryptalgal bindstone lithofacies including facies Hcr, which
occurs with three types of textures, displayed in three photos (A-C). Some intervals of this lithofacies feature hemispherical stromatolites (A) and shorter domal stromatolites (photo B).
Photo D shows facies Hcrd, observed as a bedded, heterolithic unit with brecciated to discontinuous cryptalgal laminites, clotted mudstone and skeletal floatstone. Note the speckled appearance of the matrix, showing preferential grain alignment. The scale on the left denotes inches, while the rightmost scale denotes decimal feet. 173
Figure 22. Microscopic image of interpreted cryptalgal laminations (denoted by red arrow) taken at 25x magnification under plane-polarized light. This image was taken from a thin section sampled at 1687 m (5535 ft).
174
Figure 23. Microscopic image of interpreted clotted microbial mudstone fabrics taken at 25x magnification under plane-polarized light. This image was taken from a thin section sampled at
1688.6 m (5540 ft).
175
Figure 24. Characteristic photos of dolomitic cryptalgal bindstone facies (Bcr) and massive, dolomitic tabulate coral-stromatoporoid framestone facies (Fr). Cryptalgal bindstone facies Bcr occurs with two textures: (A) Tan/white and black bindstone with wrinkled to serrated algal growth geometries and (B) brown and grey wrinkled cryptalgal bindstone. Facies Fr occurs with two types of textures including: (C) laminar anastomosing stromatoporoid framestone; and (D) tabulate coral framestone. The scale to the left denotes inches while the rightmost scale denotes decimal feet. 176
Figure 25. Legend of symbology for stratigraphic columns used to described lithologic successions observed from the 9-33 core.
177
Figure 26. Composite stratigraphic column describing grain size, sedimentary structures, and fossils observed in the 9-33 core (Key to symbols given in Figure 22).
178
Figure 27. Interpretation of core 9-33, showing depositional successions, facies associations and stratigraphic trends. Blue triangles represent deepening upward trends, while yellow triangles represent shallowing upward trends. Please refer to Figure 26 for color code for lithofacies.
179
Figure 28. Depositional model constructed for the Guelph Formation reef depositional stage associations observed within the 9-33 core. Reefal deposits (FA6) form rigid wave-resistant topography, while leeward-reef slope deposits accumulate along the leeward margin. Proximal debris apron successions consist of tempestites, debris flows and turbidites. Distal debris apron deposits occur below wave-base, consisting of debris flows interbedded with autochthonous skeletal floatstones and wackestones, that then transition into non-fossiliferous mudstone moving further downslope.
180
Figure 29. (A) original and (B) annotated core photographs of the interpreted sequence boundary
(SB) and overlying unconformable surface interpreted to be a transgressive surface of erosion
(TSE) observed at 1693 m (5554.5 feet) in the 9-33 well core, between reef stage and peritidal stage deposits of the Guelph Formation.
181
Figure 30. (A) original and (B) annotated core photos of the Ruff Formation and Guelph
Formation contacts. This photo shows the subtidal-to-intertidal sediments of the Ruff Formation, unconformably overlying the erosional contact of the supratidal Guelph Formation succession.
This surface is interpreted as a transgressive surface of erosion (TSE). The leftmost scale denotes inches and the right most scale denotes decimal feet. 182
Figure 31. Geophysical well log profiles of the 9-33 core, displaying gamma-ray (green), neutron porosity (black/red), and bulk density logs (blue), intervals of core recovered (cross hatched), the correlation with facies associations and the contact between the Guelph and Ruff Formations
(red line). Note that porosity cutoff has been applied to the neutron porosity log, infilling it with red when the measured porosity is above 10%. 183
apron Proximal debris debris Proximal Reef Association
Figure 32. Geophysical profiles of reef zone (5760-5333 ft) sediments, including gamma-ray
(green), neutron porosity (black/red), bulk density log (blue), and resistivity image log (yellow brown). Note that porosity cutoff has been applied to the neutron porosity log, infilling it with red when the measured porosity is above 10%. 184
Figure 33. Geophysical profiles of proximal reef apron (5733-5690 ft) sediments, including gamma-ray (green), neutron porosity (black/red), bulk density log (blue), and resistivity image log (yellow brown). Note that porosity cutoff has been applied to the neutron porosity log, infilling it with red when the measured porosity is above 10%. 185
Figure 34. Geophysical profiles of peritidal successions of the Guelph Formation (5555-5333 ft) and Ruff Formation (5533-5525) including gamma-ray (green), neutron porosity (black/red), bulk density log (blue), and the resistivity image log (yellow brown). Note that porosity cutoff has been applied to the neutron porosity log, infilling it with red when the measured porosity is above 10%. 186
Figure 35. Porosity-permeability transform from core plug analysis values of the Guelph
Formation. The function fit to the data is a power function (displayed above).
187
Figure 36. Distribution of mercury-to-air capillary injection curves across sampled facies associations of the Guelph Formation.
188
Figure 37. Capillary-pressure classes present within the Guelph Formation.
189
Figure 38. Pore throat size distribution plot of the Guelph Formation. Note the heavy skew toward macro-size pore throats, which is interpreted to be mainly produced from dissolution- enhancement of pore architecture.
190
Figure 39. Porosity-permeability transform created from core plug analysis data obtained from the Reef association. A power function (displayed above) was fit to core plug data to model permeability (K) as a function of measured porosity (PHI).
191
Figure 40. Distribution of capillary pressure behaviors observed within the reef association of the
Guelph Formation. This depositional environment yielded sediment with type I and type II behaviors.
192
Figure 41. Porosity-permeability transform created from core plug analysis data obtained from the proximal debris apron environment. A power function (displayed above) was fit to core plug data to model permeability (K) as a function of measured porosity (PHI).
193
Figure 42. Distribution of capillary pressure profiles observed within the proximal debris apron association of the Guelph Formation.
194
Figure 43. Porosity-permeability transform created from core plug analysis data obtained from the distal debris apron environment. A power function (displayed above) was fit to core plug data to model permeability (K) as a function of measured porosity (PHI).
195
Figure 44. Distribution of capillary pressure profiles observed within the distal debris apron association of the Guelph Formation
196
Figure 45. Porosity-permeability transform created from core plug analysis data obtained from the intertidal association. A power function (displayed above) was fit to core plug data to model permeability (K) as a function of measured porosity (PHI).
197
Figure 46. Distribution of capillary pressure profiles observed within the intertidal association of the Guelph Formation. 198
APPENDIX C: PETROPHYSICAL DATA
Table C1. Reef Association Core Plug Analysis Data
Depth Lithofacies Lithology Facies Permeability Measured Association to Air Porosity 5733.5 Fr Framestone Reef 13.38 9.4
5734.5 Fr Framestone Reef 27.62 10.1 5735.5 Fr Framestone Reef 5.34 10.1 Bioclastic Reef 0.20 6.2 5738.5 Sfm Floatstone Bioclastic Reef 3.07 7.6 5739.5 Sw Wackestone Bioclastic Reef 1.63 14.2 5740.35 Sfm Floatstone 5741.45 Fr Framestone Reef 0.03 6.4 5743.6 Fr Framestone Reef 26.17 12.8 Mottled Reef 0.47 7.1 5746.55 Mm Mudstone Mottled Reef 10.61 11.1 5747.5 Mm Mudstone 5752.5 Fr Framestone Reef 0.01 5.5 5753.4 Fr Framestone Reef 0.00 2.7 Mottled Reef 0.01 3.4 5756.4 Mm Mudstone Mottled Reef 19.05 11.6 5758.5 Mm Mudstone 5759.4 Mm Mudstone Reef 0.40 7.1
199
Table C2. Proximal Debris Apron Core Plug Analysis Data
Facies Permeability Measured Depth Lithofacies Lithology Association to Air Porosity 5690.5 Ifb Floatstone Debrite-Turbidite 1.859 11.29 5691.5 Ifb Floatstone Debrite-Turbidite 0.005 3.77 5693.5 Ifb Floatstone Debrite-Turbidite 0.055 6.20 5694.5 Ifb Floatstone Debrite-Turbidite 0.022 5.60 5696.5 Ifb Floatstone Debrite-Turbidite 0.001 2.49 5697.5 Ifb Floatstone Debrite-Turbidite 0.002 3.71 5698.5 Ifb Floatstone Debrite-Turbidite 0.025 6.71 5699.5 Ifb Floatstone Debrite-Turbidite 0.358 6.56 5700.5 Ifb Floatstone Debrite-Turbidite 0.012 4.56 5701.5 Ifb Floatstone Debrite-Turbidite 0.021 4.60 5702.5 Ifb Floatstone Debrite-Turbidite 0.420 9.92 5703.5 Ifb Floatstone Debrite-Turbidite 0.854 9.53 5704.5 Ifb Mudstone Debrite-Turbidite 0.079 5.18 5705.5 Ifb Mudstone Debrite-Turbidite 0.048 3.28 5707.5 Ifb Floatstone Debrite-Turbidite 0.073 4.47 5708.55 Ifb Floatstone Debrite-Turbidite 0.003 5.76 5709.5 Mpg Mudstone Tempestite 0.002 3.22 5710.5 Sg Grainstone Tempestite 0.152 6.41 5711.55 Ifb Floatstone Tempestite 0.109 6.38 5713.5 Rcg Rudstone Tempestite 0.745 5.12 5716.5 Ifb Floatstone Tempestite 0.051 5.90 5718.55 Sg Grainstone Tempestite 6.100 11.33 5719.5 Sg Grainstone Tempestite 1.808 9.14 5720.5 Ram Rudstone Tempestite 0.381 6.10 5721.45 Sg Grainstone Tempestite 0.134 7.83 5722.5 Ram Rudstone Tempestite 0.525 7.51 5723.5 Ram Rudstone Tempestite 1.053 8.76 5724.5 Sg Grainstone Tempestite 11.919 13.31 5725.5 Mpg Mudstone Tempestite 2.563 11.14 5726.45 Ram Rudstone Tempestite 20.448 8.73 5727.5 Mpg Mudstone Tempestite 0.003 3.83 5728.5 Ram Rudstone Tempestite 1.229 8.84 5729.4 Ram Rudstone Tempestite 0.096 4.70 5730.5 Mpg Mudstone Tempestite 0.008 6.02 5731.5 Sg Grainstone Tempestite 0.703 7.52
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Table C3. Distal Debris Apron Association Core Plug Analysis Data.
Facies Permeability Measured Depth Lithofacies Lithology Association to Air Porosity Bioclastic Subtidal Back- 0.007 4.97 5598.4 Sfm Wackestone Reef Bioclastic Subtidal Back- 0.623 8.48 5599.5 Sfm Wackestone Reef Subtidal Back- 5.028 11.31 5600.5 Mm Mudstone Reef Subtidal Back- 0.018 6.48 5602.65 Mm Mudstone Reef Subtidal Back- 1.113 4.09 5603.45 Mm Mudstone Reef Subtidal Back- 0.002 1.51 5604.6 Mm Mudstone Reef Subtidal Back- 0.910 5.27 5609.5 Mm Wackestone Reef
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Table C4. Peritidal Core Plug Analysis Data
Facies Permeability Measured Depth Lithofacies Lithology Association to Air Porosity Cryptalgal *5525.5 Bcr Supratidal 1.140 7.85 Bindstone Cryptalgal *5526.5 Bcr Supratidal 0.251 5.76 Bindstone Cryptalgal *5528.65 Bcr Supratidal 1.415 5.96 Bindstone Cryptalgal *5529.6 Bcr Supratidal 0.110 8.04 Bindstone Cryptalgal *5530.4 Bcr Supratidal 0.383 5.95 Bindstone Cryptalgal *5531.5 Bcr Supratidal 0.436 6.84 Bindstone 5533.45 Hcr Heterolithic Intertidal 0.085 5.49 5534.5 Hcr Heterolithic Intertidal 0.013 7.09 5535.5 Hcr Heterolithic Intertidal 0.006 3.83 5536.6 Hcr Heterolithic Intertidal 0.148 6.53 5537.5 Hcr Heterolithic Intertidal 1.630 5.49 5538.5 Hcr Heterolithic Intertidal 2.419 10.72 5539.5 Hcr Heterolithic Intertidal 7.000 9.48 5540.5 Hcrd Heterolithic Intertidal 1.528 8.52 5541.5 Hcrd Heterolithic Intertidal 0.103 4.63 5542.4 Hcr Heterolithic Intertidal 0.256 5.05 5543.5 Hcrd Heterolithic Intertidal 0.574 8.71 5544.5 Sp Bioclastic Packstone Intertidal 0.029 3.16 5545.5 Sp Bioclastic Packstone Intertidal 0.074 5.76 5546.5 Hcr Heterolithic Intertidal 0.014 3.36 5548.5 Sp Skeletal Packstone Intertidal 0.279 7.52 5549.5 Sw Skeletal Wackestone Intertidal 0.093 10.47 5550.5 Mm Mudstone Intertidal 15.709 10.72 Laminated 5551.5 Ml Intertidal 13.046 12.82 Mudstone 5552.5 Mm Mudstone Intertidal 9.347 11.58 5553.5 Mm Mudstone Intertidal 1.929 8.87 Laminated 5559.5 Ml Intertidal 0.025 9.92 Mudstone 5560.5 Mm Mudstone Intertidal 0.003 4.34 5562.5 Mm Mudstone Intertidal 0.002 4.32 Notes: * Denotes sample obtained from the lower Ruff Formation
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Figure C1. Pore Throat-size frequency distribution for the reef association (FA6, Table 6).
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Figure C2. Pore throat-size frequency distribution for the proximal debris apron environment.
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Figure C3. Pore throat-size frequency distribution for the distal debris apron environment
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Figure C4. Pore throat-size frequency distribution for the intertidal facies association (FA2, Table 6).