CONTROLS ON NATURAL FRACTURES IN THE UPPER LEXINGTON LIMESTONE AND POINT PLEASANT FORMATION: CENTRAL-OHIO

Scott William Huck

A Thesis

Submitted to the Graduate College of Bowling Green

State University in partial fulfillment of

the requirements of the degree of

` MASTER OF SCIENCE

August 2013

Committee:

Dr. James Evans, Advisor

Dr. Charles Onasch

Dr. John Farver ii

ABSTRACT

James Evans, Advisor

This paper focuses on the occurrence of natural fractures in the Point Pleasant Formation in

central Ohio. Research was done on the Cheveron #1A Prudential core (#3410120196) at the Ohio

Geological Survey H.R. Collins Core Laboratory. Core descriptions and photographs were taken along

with the depth, width, length and orientation of healed natural fractures in the Point Pleasant Formation.

Samples taken from core were used for thin section and lithofacies analysis, SEM, and

cathodoluminescence. Geochemical data was also compiled from previous work and used in this thesis.

The Logana Member is interpreted to be a mid ramp carbonate depositional environment and is characterized by stacked sequences of tempestite (storm) deposits. These tempestite deposits are characterized by undulate bedded skeletal packstones (Lithofacies Cpu) along with wackestones

(Lithofacies Cwu) and mudstones (Lithofacies Cmu). The beds give evidence for both deposition above storm weather wave base (SWWB) and above fair weather wave base (FWWB). The base of the Logana

Member is dominated by tempestites above SWWB while the upper portion the Logana Member is dominated by amalgamated tempestite deposits which are interpreted as evidence for deposition above

FWWB.

The undifferentiated Lexington Limestone is characterized by thicker beds of tempestite deposits and includes thick successions of undulate bedded, skeletal grainstones and packstones along with undulate bedded wackestones and mudstones. These are also evidence for deposition above SWWB. The upper portion of the undifferentiated Lexington Limestone transitions from a mid ramp carbonate depositional environment to a outer ramp environment, and a condensed section (Lithofacies Cppu) marks this transition into deeper water. The condensed section is characterized by phosphate nodules in undulate bedded skeletal packstone. Above this condensed section the undifferentiated Lexington Limestone transitions to hemipelagic rhythmites (HCr). These rhythmites consist of thinly bedded wackestones and iii

mudstones with quartz silt. The quartz silt in the hemipelagic rhythmites is mostly likely derived from

eolian processes.

Lithofacies of the Point Pleasant Formation are a mixed siliciclastic and carbonate depositional system. The Point Pleasant Formation then transitions upward to mostly mixed siliciclastic and carbonate hemipelagic rhythmites, which are interpreted as evidence for deeper water conditions (below storm wave base). The upper portion of the Point Pleasant Formation transitions from hemipelagic rhythmites to carbonate tempestites. The carbonate tempestites are characterized by undulate bedded skeletal packstones (Lithofacies Cpu) interbedded with undulate bedded wackestones (Cwu) and mudstones

(Lithofacies Cmu). At the top of the Point Pleasant Formation, a condensed section marks the transition to the overlying Utica Shale. The condensed section is characterized by phosphate nodules in an undulate bedded skeletal packstone (Lithofacies Cppu). This is interpreted for a rapid transition to deeper water and sets the stage for the deposition for the Utica Shale.

There are a total of 64 healed natural calcite fractures in the core, 63 of these healed natural fractures are occurring in the Logana Member and the undifferentiated Lexington Limestone. A single calcite healed natural fracture is occurring in the Point Pleasant Formation in a skeletal packstone bed.

Cathodoluminescence has identified at least three separate cementing events in the natural fractures in the

Logana Member and undifferentiated Lexington Limestone. In the undifferentiated Lexington Limestone, the natural healed natural fractures originate in granular intervals (carbonate packstones and wackestones) and terminate upwards in cohesive intervals (carbonate mudstones). Calcite crystal growth is interpreted as slow growing in a fluid filled fracture. Differences in cathodoluminescence colors are attributed to differences in fluid composition which indicates multiple pulses of fluid movement in the natural fractures.

Differences in natural fracture length between the Logana Member and the undifferentiated

Lexington Limestone are dependent on bed thickness. Lithofacies of the upper Logana Member are very iv

homogeneous compared to the heterogeneous undifferentiated Lexington Limestone. The heterogeneous undifferentiated Lexington Limestone is responsible for shorter natural fracture lengths that terminate in some wackestones and mudstones while initiating in the granular lithofacies such as the grainstones and packstones. Refraction of the natural fractures is due to differences in stress orientations that occurred during the slow propagation and opening of the natural fractures.

Natural fractures are important reservoir characteristics and should be evaluated before hydrocarbon exploration should begin. The natural fractures in the upper Lexington Limestone and Point

Pleasant Formation occurred before early oil generation and migration. One of the natural fractures in the

Logana Shale Member contains a bitumen bleb in the middle of calcite cement. The amount of natural fracturing is extensive in the upper Lexington Limestone but not the Point Pleasant Formation.

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DEDICATION

This thesis is dedicated to my mother and father who have taught me the importance of hard work and provided me with the means to a higher education. Without them I could not of made it to where I am today.

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ACKNOWLEDGEMENTS

I would like to begin my acknowledgments by thanking my father, Rodney, and mother, Kim and my three older sisters, Kelly, Traci, and Terri for all their encouragement and support throughout my grad school career.

I would also like to thank my advisor, Dr. James Evans, for all of his support, guidance, and assistance throughout this thesis and his willingness to help me every step of the way. I would also like to thank the rest of my committee members, Dr. Charles Onasch and Dr. John Farver for their willingness to join me in this research and assist me in writing this thesis. Also, thanks to faculty in the Biological

Science Department at Bowling Green State University, including Dr. Carol Heckman and Dr. Marilyn

Cayer, for their support and guidance using the scanning electron microscope in my research.

I would also like to thank The Ohio Geological Survey, especially Greg Schumacher, for access to cores and samples used in this research.

I would like to thank EQT Production including, Joe Morris, Chris Willan, Phil Morath, and Scott

McCallum for their internship opportunity, which provided me with extensive background knowledge of the Point Pleasant Formation and support on this thesis.

vii

TABLE OF CONTENTS

INTRODUCTION………………………………………………………………………….. 1

Importance of Black Shales……………………………………………………….. 1

Origin of Black Shales……………………………………………………………. 2

Diagenesis of Black Shales………………………………………………………… 4

Role of Fractures in Diagenesis……………………………………………………. 9

Petroleum Geology of Black Shales…………………………...…………………… 10

Purpose and Goals………………………………………………………………….. 12

GEOLOGIC BACKGROUND……………………………………………………………... 14

Late Ordovician Taconic Orogeny…………………………………………………. 14

Regional Stratigraphy………………………………………………………………. 17

Mt. Simon Sandstone……………………………………………………… 17

Conasauga Formation……………………………………………………… 17

Kerbel Formation………………………………………………………. 19

Knox Group………………………………………………………………. 20

Wells Creek Formation…………………………………………………… 21

Black River Group……………………………………………………….. 21

Trenton Formation……………………………………………………….. 22 viii

Lexington Limestone…………………………………………………….. 23

Point Pleasant Formation………………………………………………… 24

Utica Shale……………………………………………………………….. 25

METHODS…………………………………………………………………………………. 28

Core Descriptions………………………………………………………………….. 28

Scanning Electron Microscopy……………………………………………………. 30

Total Organic Carbon……………………………………………………………… 30

Cathodoluminescence………………………………………………………………. 31

RESULTS…………………………………………………………………………………… 33

Lithology…………………………………………………………………………… 33

Lithofacies Analysis………………………………………………………………... 37

Lithofacies and Interpretations……………………………………………………… 37

Undulate Bedded Skeletal Packstones (Cpu)………………………………. 37

Undulate Bedded Carbonaceous Wackestone (Cwu)…………………….. 37

Undulate Bedded Carbonaceous Mudstone (Cmu)……………………….. 41

Black Carbonaceous Carbonate Mudstone: (Cmub)……………………... 41

Swaly Laminated Carbonaceous wackestone (Cws)……………………..... 43

Brown Mottled Wackestone Facies (Cwm)……………………………….. 43 ix

Undulate Bedded Grainstones (Cgu)……………………………………... 44

Heterolithic Rhythmite Facies( Cmw)……………………………………. 44

Packstone with Phosphorite Nodules (Cppu)……………………………. 48

Planar Laminated Carbonaceous Clay Shale (Scp)……………………… 49

Lithofacies Associations…………………………………………………………… 55

Tempestite Association……………………………………………………. 55

Amalgamated Tempestite Association…………………………………….. 59

Hemipelagic Rhythmite Facies Association………………………………. 61

Condensed Section Facies Associations…………………………………… 62

Depositional Environments………………………………………………………… 66

Diagenesis…………………………………………………………………………. 68

Rock Matrix………………………………………………………………. 68

Total Organic Carbon………………………………………………………………. 77

Natural Fracture Descriptions………………………………………………………. 77

Paragenesis………………………………………………………………………….. 88

DISCUSSION……………………………………………………………………………….. 94

Depositional Environments and Lithofacies………………………………………… 94

Stratigraphic Trends………………………………………………………………… 96 x

Causes for Sea Level Change……………………………………………………….. 99

TOC………………………………………………………………………………….. 99

Diagenesis……………………………………………………………………………. 100

Early Diagenesis……………………………………………………………………… 100

Middle Diagenesis……………………………………………………………………. 101

Late Diagenesis………………………………………………………………………. 102

Paragenesis……………………………………………………………………………. 103

Petroleum Geology…………………………………………………………………… 106

SUMMARY & CONCLUSIONS……………………………………………………………… 108

WORK CITED……………………………………………………………………………….. 111

APPENDIX A: GEOPHYSICAL LOG……………………………………………………….. 117

APPENDIX B: NATURAL FRACTURES………………………………………………….. 119

APPENDIX C: TOC RESULTS……………………………………………………………. 121

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LIST OF FIGURES PAGE

Figure 1. Restricted circulation model for accumulation of organic matter………………………………..3

Figure 2. Open ocean model for accumulation of organic matter………………………………………….5

Figure 3. Continental shelf model for accumulation of organic matter…………………………………….6

Figure 4. Diagenetic zones in comparison to depth and temperature………………………………………7

Figure 5. Paleogeography of North America during the Taconic Orogeny……………………………….15

Figure 6. The Utica-Point Pleasant Sub-Basin in Ohio during the Late Ordovician………………...... 16

Figure 7. Stratigraphic Column and Ages of Rock Formations of Eastern Ohio to Southwest Ohio…...... 18

Figure 8. The extent of the Logana Member of the Lexington Limestone in Ohio……………..………...26

Figure 9. Core box 132 of the Prudential #1-A………………………………………………………..…..29

Figure 10a. Core photo of undulated skeletal packstones and carbonaceous wackestones…...... 35

Figure 10b. Core photo lithofacies, carbonaceous wackestone, black carbonaceous mudstones, and swaly carbonaceous wackestone……………………………………………………………...………35

Figure 10c. Core photo lithofacies, rhythmite facies of carbonaceous calcareous wackestones, black carbonaceous calcareous mudstone, carbonaceous calcareous mudstone………...………………………35

Figure 11a. Thin section photograph of undulate bedded packestone…………………………………….36

Figure 11b. Thin section photograph of carbonaceous wackestone………………………………………36

Figure 12a. Core photo of undulate bedded packestones of the Logana Member………………………...38

Figure 12b. Core photo of black carbonaceous mudstone and swaly laminated wackestone………………………………………………………………………………………………...38

Figure 12c. Core photo of undulate bedded carbonaceous mudstone, wackestone and black mudstones..38

Figure 13. Carbonate ramp to basin model………………………………………………………………..40

Figure 14. Thin section photograph of black carbonaceous mudstones………………………….……….42

Figure 15a. Core photograph of undulate bedded grainstones of the undifferentiated Lexington Limestone………………………………………………………………………………………………….45

Figure 15b. Core photograph of hemipelagic rhythmites of the undifferentiated Lexington Limestone…45

Figure 16a. Thin section photograph of undulate bedded grainstones of the undifferentiated Lexington Limestone………………………………………………………………………………………………….46 xii

Figure 17. Thin section photograph of hemipelagic rhythimites of the Point Pleasant Formation……….46

Figure 18. Core photograph of undulate bedded grainstones, wackestones, and mudstones of the undifferentiated Lexington Limestone…………………………………………………………………….47

Figure 19. Core photograph of the Point Pleasant/Utica contact………………………………………….50

Figure 20. Thin section photograph of the undulate bedded packestone and phosphorite facies…………51

Figure 21. SEM image of phosphorite and pyrite nodules of the undulate bedded packestone and phosphorite facies of the Point Pleasant Formation…………….…………………………………………52

Figure 22. EDS scan of nodule A in the undulate bedded packestone and phosphorite facies of the

Point Pleasant Formation………………………………………………………………………………….53

Figure 23. EDS scan of nodule B in the undulate bedded packestone and phosphorite facies of the

Point Pleasant Formation………………………………………………………………………………….54

Figure 24. Carbonate tempestite stratigraphic column modified from Zhidong (1998)…………………..56

Figure 25. Stratigraphic column of the contact between the Curdsville Member and Logana Member of the Lexington Limestone………………………………………………………..57

Figure 26. Core photograph of swaly laminated wackestones of the Logana Member…………………..60

Figure 27. Stratigraphic column showing the condensed section of the undifferentiated Lexington Limestone………………………………………………………………………………………………….64

Figure 28. Stratigraphic column showing the condensed section and contact between the

Point Pleasant Formation and Utica Shale………………………………………………………………...65

Figure 29. Carbonate ramp to basin model with lithofacies associations…………………………………67

Figure 30. Photomicrograph of blocky calcite cement in natural fracture………………………………...71

Figure 31. Photomicrograph of blocky calcite cement in natural fracture………………………………...72

Figure 32. Photomicrograph of mosaic calcite cement in natural fracture………………………………..73

Figure 33. Photomicrograph of mosaic calcite cement in ostrocode shell………………………………...74

Figure 34. Photomicrograph of silica diagenesis………………………………………………………….75

Figure 35. Cathodoluminescence of natural fracture containing pyrite…………………………………...76

Figure 36. Stratigraphic column showing single fracture in the Point Pleasant Formation……………….79

Figure 37. Core photograph of single fracture occurring in the Point Pleasant Formation……………….80 xiii

Figure 38. Stratigraphic column showing fractures in the undifferentiated Lexington Limestone……….81

Figure 39. Stratigraphic column showing refracting natural fractures in the undifferentiated Lexington Limestone…………………………………………………………………….82

Figure 40. Cathodoluminescence image of a natural fracture in the undifferentiated

Lexington Limestone……………………………………………………………………………………...84

Figure 41. Cathodoluminescence of a calcite healed natural fracture in the Logana Member……………85

Figure 42. Cathodoluminescence of a calcite healed natural fracture in the Logana Member……………86

Figure 43. Core photograph of a calcite healed natural fracture containing a bitmumen bleb……………87

xiv

LIST OF TABLES PAGE

TABLE I: LITHOFACIES CODES AND DESCRIPTIONS………………………………… 39

TABLE II: LITHOFACIES ASSOCIATIONS………………………………………………. 58

TABLE III: TABLE OF CEMENTS………………………………………………………… 70

TABLE IV: TABLE OF PARAGENESIS………….………………………………………... 89

1

INTRODUCTION

Importance of Black Shales

Black shales have long been a topic of great importance due to their hydrocarbon source rock

potential (Werne et al., 2002). Along with coal, black shales are the most significant fossil fuel resource

due to their high concentrations of organic material. They are one of the most easily recoverable fossil

fuel resources that are high in organic carbon in the Earth’s crust (Tourtelot, 1979). Black shales are

known to produce high amounts of oil and natural gas. For example, the Marcellus Shale, a Middle

Devonian organic-rich black shale, is estimated to have 1,307 trillion cubic feet (tcf) of recoverable

natural gas (Boyce and Carr, 2009). Until recent technological advances, black shales were thought of as

a source rock for oil and natural gas. Today, black shales are also viewed as possible hydrocarbon

reservoirs (Tourtelot, 1979).

Potential oil generation is dependent on the chemical nature of the source organic material,

thermal history and necessary conditions to preserve the original organic matter (Dickhout et al., 1983).

The thermal maturity of these black shales is important, in order to have chemically altered the original organic material into oil and/or natural gas. The shale must be thermally mature in order to have any economic value. Thermal maturity implies that the kerogen in the shale must has been heated to

paleotemperatures between 80oC and 100oC to produce oil and gas. At higher temperatures, such as 120-

160oC, oil is no longer stable and only hydrocarbon gases will remain (Dickhout et al., 1983).

Black shales are also known for their metal deposits. Some individual beds in black shales are

known to have and abundance in certain elements such as Fe and Mn, and these beds can have more than

hundred times more than the average crustal abundance in metallic elements (Krauskopf, 1955).

2

Origin of Black Shale

Black shale has been formed all around the world and in different depositional environments and conditions, therefore, it is not the type of depositional environment that creates black shale but the types of source materials and rate of deposition that are important. The amount of oxygen and hydrogen sulfide in the environment are important chemical factors when allowing for organic matter to be preserved, and the rate of organic matter being deposited can adjust the chemical parameters of an environment and create a “preservation friendly” environment for organics (Tourtelot, 1979). Approximately 80% of the

Earth’s petroleum reservoirs have originated from oxygen- deficient or oxygen-depleted environments at the sediment-water contact (Tyson and Pearson, 1991). Examples of different depositional environments with different chemical parameters that yield black shale accumulation are shown in Figures 1, 2, and 3.

Didyk et al. (1978) describe these depositional environments in terms of amount of dissolved oxygen concentration, hydrogen sulfide concentration, and sedimentation rates. Tourtelot (1979), states that the rate of organic matter production is a critical controlling condition. This is because the decomposition of abundant organic matter consumes available oxygen, and this oxygen deficiency aids in the preservation of organic matter.

The first model described by Tourtelot is the restricted circulation model (Figure 1), which attributes the organic matter accumulation rate to the lack of oxygen present because of the lack of water column mixing between surface water and bottom water. After death of the organisms, the organic material sinks and any dissolved oxygen that is present is consumed by the oxidation of organic material.

This situation allows the continual accumulation of organic material to occur because any new free oxygen introduced is consumed by the continued deposition of the organic material. In the absence of available oxygen, sulfate is reduced to sulfide and HS-complexes form. Hydrogen sulfide gas generated by this mechanism is able to disperse into the overlying water column (Tourtelot, 1979). This model allows organic matter to be preserved because there is limited circulation in the water column that brings 3

Figure 1. Restricted circulation model (modified from Didyk et al., 1978, modified by Tourtelot 1979).

4

in any new free oxygen to the sea floor. A present day example of the restricted circulation model is the

Black Sea. The Black Sea is a restricted basin that has the same chemical and physical parameters as

discussed above.

The open ocean model (Figure 2), is also dependent on the amount of organic matter that is

produced in the photic zone. However, here free oxygen may still be present in the effective water

column, with the lowest amount is directly below the photic zone due to the oxidation of decaying organic

material. Refractory organic material may not be destroyed by oxidation during the settling process,

which might include attachment to fecal pellets or detrital minerals. At the bottom, organic matter that is

buried a few mm in the sediment will enter an oxygen deficient environment due to the slow diffusion of

free oxygen in non-bioturbated sediments (Tourtelot, 1979).

The continental shelf model (Figure 3) uses the same constraints, only it applies to a shallow

water depth between the photic zone and sea floor. There is limited recycling of the organic material that

is produced in the photic zone compared to the other two models because of faster settling to the sea floor.

Here, the water column is typically oxygen-rich due to storm mixing of the entire water column. On the

sea floor, organic material will rapidly accumulate in anoxic environmental conditions that are created by

bacterial decomposition of organic matter underneath the sediment surface (Tourtelot, 1979).

Diagenesis of Black Shale

Different zones of diagenesis need to be identified correctly in order to recognize the conditions after deposition. Most of the products of diagenesis will remain in the rocks and create unique characteristics that can be used to interpret the different zones of diagenesis (Tourtelot, 1979). The five zones described by Tourtelot occur in this order, sulfate reduction, fermentation, decarboxylation, hydrocarbon formation, and metamorphism (Figure 4). Figure 4 does not show a zone of oxidation in the top layer but this zone of oxidation may or may not be present depending on organic-matter concentration. Below the zone of oxidation lies the zone of sulfate reduction. The thickness and extent 5

Figure 2. Open ocean model (from Didyk et al., 1978, modified by Tourtelot, 1979).

6

Figure 3. Continental shelf model (from Didyk et al., 1978, modified by Tourtelot, 1979).

7

Figure 4. Diagenetic zones and products (from Didyk et al., 1978, modified by Tourtelot, 1979).

8

of both the zone of oxidation and sulfate reduction is dependent on the amount of diffused oxygen and

sulfate from the above water column into the sediment below. Reduction of this sulfate may allow the

formation of pyrite to occur. Any carbonate cements that form in these two zones will have a light

isotopic content (Tourtelot, 1979).

In the zone of fermentation, after sulfates have been reduced, methane and carbon-dioxide are

produced. The methane is due to the biological breakdown of carbon-dioxide by bacteria. Carbon dioxide

is also produced by certain types of bacteria due to continued decomposition of organic matter. The

methane produced in this zone will have isotopically light carbon due to fractionation by bacteria.

As burial temperature and depth increase, the zone of decarboxylation is reached. Here, organic

matter decomposes chemically rather than biologically. The carbonate minerals deposited here would

most likely be iron carbonates such as siderite, with the possibility of a mixed composition minerals due

to the fact that higher temperatures present in this zone could begin the breakdown of iron compounds

that survived the lower temperatures of the earlier diagenetic zones. The remaining organic matter left in

the rock is of a different composition from when it first entered the zone of decarboxylation (Tourtelot,

1979).

The transition from the lower part of the zone of decarboxylation to the upper part of the zone of

hydrocarbon formation is broad. For example, the pyrocatalytic formation of methane can begin at

temperatures of around 50o C, but large amounts are not generated until temperatures, higher than 80oC, are reached. The beginning of the zone of hydrocarbon formation will vary depending on the temperature, amount of overburden, and also rock composition. The composition of the organic material, vitrinite reflectance, and the color alteration index of conodonts can be used to identify thermal maturity and maximum burial temperatures. Water that is expelled in this zone can be the driving factor for the migration of hydrocarbon (Tourtelot, 1979).

9

Role of Fractures in Diagenesis

Natural fractures play an important role in hydrocarbon migration during diagenesis (Olson et al.,

2002). These natural fractures are formed due to the relation between the amount of stress, fluid pressure, and brittleness of the rock (Pollard & Aydin, 1988).

Local stress on rocks, such as those that create smaller features such as joints, are not identical to regional stresses but are compatible. The presence of fractures can be due to the flexing of rocks during subsidence or uplift, and does not have to be related to faulting or folding. Jointing is more prevalent in thin and weakly bedded rocks, and often has a direct association with folding (Towse, 1980).

During diagenesis, whether or not these fractures remained open or be sealed will be due to the

presence or absence of supersaturated pore fluids (Laubach et al., 2004). There can be uncertainty about

determining the timing of fracture opening and fracture sealing in sedimentary rocks (Laubach, 2003).

The timing of the fracture opening is especially difficult to determine. The sealing of fractures is also

uncertain but may be bounded by the morphology and age relationships of infilling crystals in the fracture

fill (Ramsay & Huber, 1983; Urai et al., 1991; Bons & Jessell, 1997; Bons, 2000; Hilgers et al., 2001).

For example, the growths of fibrous crystals in fractures may reveal a precipitation rate that is about the

same as the rate of fracture opening, while crystals that are elongated and blocky may indicate that the

crystal growth rate was slower than the opening rate of the fracture opening (Nollet et al., 2005).

There are two basic types or modes of natural fractures, shear (Mode II & III) and extensional

(Mode I). Shear fractures occur when the shear component generated by differential stress overcomes the shear strength of the rock. Shear mode fractures often develop in rocks that have been folded or faulted, and can also occur when rock grains are crushed during compaction by increased burial or lateral shortening. These shear fractures can be recognized by the movement of markers on opposite sides of the fracture, parallel to the fracture (Billingsely & Kuuskraa, 2006). Extensional fractures occur in a rock when there is a low amount of shear stress applied to a rock and the movement of the walls of the fracture 10

is normal to the fracture. Extensional fractures are caused due to stretching of the rock and have a much

longer length than width of the fracture (Billingsely & Kuuskraa, 2006).

Petroleum Geology of Black Shales

The unconventional Devonian shale oil and gas reservoirs in the eastern United States might be the most productive but underdeveloped oil and gas plays in the world (Pezzeta, 1979). Because these unconventional reservoirs have very low permeabilities, both natural and induced (hydrofractured) fractures are important for fluid movement throughout the reservoir. In the absence of induced fractures, collected field data indicate that production from the Devonian shales is generally related to the presence and spacing of natural fractures (Shumaker, 1976). Thus these Devonian shale reservoirs can be described as a low storage, high-flow capacity, naturally fractured petroleum system fed by a high storage, low-flow capacity rock matrix (Kucuk, 1980). For example, gas transportation in the Marcellus Shale is hypothesized to flow in a porous fracture system into which the surrounding matrix rock deliver their gas contents (Kucuk, 1980).

With the technological advances in horizontal drilling and natural fracturing, many source rocks are now viewed as the reservoir rock. This unconventional style of drilling and reservoir classification has allowed exploration and production companies to target hydrocarbons before they have migrated to reservoir rocks. Natural fractures are also important when analyzing the source rock to determine the amount of hydrocarbon migration. While source rocks with some natural fracturing can increase the hydraulic fracturing stimulation of a well, a high amount of natural fractures can indicate that hydrocarbons have already migrated away from the source rock.

Petroleum engineers rely on extensional fractures in order to economically produce from the

Barnett Shale in West Texas. These open-mode, natural fractures act as conduits for fluid flow during hydraulic fracturing which ultimately increase the treatment zone (Gale et al., 2007). Smaller sealed fractures can also be reactivated during induced fracturing treatments. The hydraulic fracturing process 11

uses water and chemicals that are pumped at extremely high pressures to fracture the rock and expand the treatment zone. Once the pressure of the fluid overcomes the strength of the rock and the stress normal to the potential fracture, fractures develop. Propant (sand) is then pumped down the well and into the induced fractures in the formation, to allow the fractures to remain open which allows hydrocarbons to flow into the well. These fractures are formed under conditions that have low in situ stresses and high injection rates and pressures (Jeffrey et al., 2010). Hydraulic fracturing in formations with natural sealed fractures cause a fracture network that forms due to shear and tensile failure. The orientation of these natural fractures allows the induced fractures to be aligned normal to the direction of the least principal compressive principal in situ stress (Jeffrey et al., 2010). Induced fracturing in rock units which already have natural fractures present in the reservoir causes widespread shear fractures (Palmer et al., 2009).

The Utica Shale has great economic value due to its thermal maturity allowing it to generate oil and natural gas. The oil and gas potential of the Utica Shale, as a source rock, is estimated around 600 million barrels of oil equivalent (MBBOE) and over 1.5 tcf of natural gas that have been produced from

Upper Cambrian, Ordovician, and Lower Silurian reservoirs (Ryder et al., 1998). Models produced by

Rowan (2006), have placed the unit in the oil-generation window during the Late Devonian to Late

Pennsylvanian and in the gas-generation window during the Middle Mississippian to Early Permian.

Migration of the oil and natural gas began shortly after hydrocarbon generation, and migrated vertically and laterally to the northwest, in the updip direction. Migration of the hydrocarbons continued until Late

Paleozoic uplift and erosion. Suggested pathways of migration include along parallel bedding porosity zones, along the Knox unconformity, and through tectonically-induced natural fractures. Overall, natural fractures in the Cambrian and Ordovician rocks supported the migration of oil from basinal, downdip locations into underlying reservoirs. Deeply buried oil that was initially trapped and converted into natural gas could be responsible for secondary hydrocarbon migration (Ryder, 2008).

12

Purpose and Goals

The focus of this research is to relate the occurrence of natural fractures to the presence of different lithologies. Available Utica Shale cores in Ohio do not have any natural fractures present but an available upper Lexington Limestone and Point Pleasant Formation core taken from Marion County, Ohio is abundant with natural fractures. Currently, in eastern Ohio, an oil and gas play has developed targeting the zones in the Upper Lexington Limestone (Logana Member and the Lexington Limestone undifferentiated) and the Point Pleasant Formation. Although this focus has shifted from the Utica Shale, it is still a part of the Utica-Lower Paleozoic Total Petroleum System (TPS) as described by Milici et al.,

2003.

The major elements of a petroleum system include defining the source rock, reservoir rock, seal rock, and overburden rock. The upper Lexington Limestone, Point Pleasant Formation, and Utica Shale in this case are all source rocks. The most important processes, when evaluating a petroleum system, is analyzing the trap formation and the generation, accumulation, and migration of hydrocarbons (Magoon

& Beaumont, 1999). One of the ways that hydrocarbons can migrate is via natural fractures. Analyzing the importance of natural fractures within a petroleum system is crucial in determining the migration pathways and also the efficiency of the seal or trap. The amount and extent of a natural fractured reservoir is important when analyzing how much hydrocarbons are present. Too many natural fractures may cause hydrocarbons to migrate away from the reservoir. The right amount can aid in production by allowing hydrocarbons to flow into the wellbore and increase production.

Knowing the depositional environment and diagenetic history of the upper Lexington Limestone

(Logana Member and undifferentiated Lexington Limestone) and Point Pleasant Formation in central

Ohio will give insight to the fracturing history of the limestone and shale and explain local variations in

TOC and/or CaCO3 content of the reservoir. In most cases, the diagenetic history of the host rock and the opening and sealing of fractures are commonly linked together (Laubach, 2003). Knowing when and how 13

post-depositional events occurred will give insight for other black shales as well and will help in the

exploration of not only the Upper Ordovician reservoirs but other unconventional reservoirs. The micro- fracture history and fracture geometry of the upper part of the Lexington Limestone and the Point

Pleasant Formation are important for application of hydraulic fracturing methods and will aid in hydrocarbon extraction.

14

GEOLOGIC BACKGROUND

The Late Ordovician Taconic Orogeny

During the Late Ordovician, the Iapetus Ocean was closing due to the continuing or impending

collision of North American and Europe. Subduction formed a chain of volcanic island arcs and an

uplifted subduction complex east of present day Ohio (Hansen, 1997). This period of mountain building

and island arc formation is known as the Taconic Orogeny, which geographically stretched from present day Newfoundland to Alabama (Figure 5). During the Ordovician, the region that is now Ohio and

Pennsylvania was located in warm, southern tropical latitudes and was covered by a shallow sea (Figure

6). Shallow-water carbonate platforms (Trenton Formation) surrounded the deeper-water shale basin

(Utica Shale and Point Pleasant Formation) and continued to keep pace with rising sea level. The Trenton platform, stretching from New York and Ontario to Michigan and Indiana consisted of clean carbonates, while argillaceous carbonates were found along the Lexington platform, stretching from southern Ohio down through Kentucky and into Tennessee (Patchen et al., 2006).

Middle to Upper Ordovician siliciclastic rocks in Ohio formed from sediment that was eroded from the newly formed Taconic highlands to the east. Above Lower Ordovician carbonates, the “Knox

Unconformity” is a record of the regional uplift of the former marine basin and subsequent creation of the

Taconic Foredeep along the western side of the Taconic Highlands. During the maximum part (Late

Ordovician) of the Taconic Orogeny, eroding sediments were deposited westward of the mountain belt to form complex delta systems. These deltas provided mud and other fine-grained sediments that were deposited in the shallow seas of Ohio and neighboring states. This delta complex, known as the

Queenston Delta, was responsible for many Ordovician shale beds in the eastern portion of Ohio. Due to subduction during the Taconic Orogeny, there were several volcanic eruptions which led to two widespread bentonite beds, the Deicke and Millbrig bentonite. These eruptions produced up to 5,000 times more volcanic ash than the 1980 eruption of Mt. St. Helens (Hansen, 1997).

15

Figure 5. Paleogeography of North America during the Ordovician, showing the Taconic Orogeny (modified from Blakely, 2011). Study area indicated.

16

Figure 6. Utica Shale/Point Pleasant Formation Sub-Basin (modified from Patchen et al., 2006). Study area indicated.

17

Regional Stratigraphy

Mt. Simon Sandstone

The Mt. Simon Sandstone represents the basal unit for the Appalachian Basin and is Cambrian in age (Figure 7). This sandstone is uniformly widespread on the eastern edges of the Michigan and Illinois

Basins (Michigan, Indiana, western Kentucky, and western Ohio) but pinches out laterally in central

Ohio. Moving eastward in Ohio this sandstone unit is thin and pinches out locally. The sandstone is around 92 m thick in western Ohio and gradually thins as it moves towards the east and is about 61 m thick in northeast Ohio (Patchen et al., 2006).

The Mt. Simon Sandstone is described as a white, pink or purple, fine to coarse-grained,

moderately to well-sorted quartz arenite that can be arkosic in some areas. Red, green, black, sandy silty

shale can also occur in the Mt. Simon Sandstone and thin beds of tight silica-cemented arenite are found

locally (Patchen et al., 2006).

The Mount Simon Sandstone is interpreted as a transgressive barrier sequence that migrated

across a basal estuarine/ lagoonal sequence in Ohio (Saeed and Evans, 2012). The earliest preserved

sediments were mostly from eroded Precambrian craton that was exposed, this eroded sediment was

reworked by transgressive waters and formed the Mt. Simon Sandstone and large amounts were deposited

over the Appalachian Basin (Patchen et al., 2006).

Conasauga Formation

The Conasauga Formation is late Cambrian in age and is 37 m thick in northern Ohio. The

Conasauga Formation is described as a mixed siliciclastic-carbonate sequence with tidal sedimentary

structures that are related to a shallow marine sedimentary environment (Banjade, 2010). Siliciclastic

lithofacies of the Conasauga Formation include, massive, planar laminated, cross-bedded, and hummocky

stratified sandstone with burrows; massive and planar-laminated siltstone; massive mudstone; heterolithic 18

Knox Unconformity

Figure 7. Stratigraphic column of Ohio modified from Patchen et al., 2006. 19

sandstone and silty mudstone with tidal rhythmites showing double mud drapes. Some of the carbonate

lithofacies include dolomitized carbonate rocks that at one time were massive, oolitic, intraclastic, and/or

fossiliferous limestones (Banjade, 2010).

The lower portion of the Conasauga Formation is composed of coarsening-upward and

thickening-upward, massive bedded, to planar laminated sandstone beds that are interbedded with

sandstone and silty mudstone having heterolithic tidal features such as flaser-bedding and wavy bedding.

The middle portion of the Conasauga Formation is characterized by dolomitized limestone with minor

sandstone and siltstone beds. The upper portion of the Conasauga Formation is a coarsening-upward

sequence of massive, planar laminated, cross-bedded, hummocky stratified sandstone consisting of very

fine sandstone and small interbedded beds of mudstone (Banjade, 2010).

The sandstone beds of the lower part of the Conasauga Formation are interpreted as part of a bay-

head delta system and were re-worked by tempestites. Other deposits include siliciclastic tidalites that

were deposited in subtidal to sand flat environments and carbonates deposited offshore. The middle

portion of the Conasauga Formation is characterized by an irregular gamma-ray response and is interpreted to be part of a lagoonal or estuarine environment. The upper part of the Conasauga Formation is characterized by fine-grained sandstone and thin interbedded layers of mudstone and is interpreted to be part of a barrier and/or shoreface deposit.

Kerbel Formation

The Kerbel Formation is Upper Cambrian in age and is lateral facies equivalent to the upper

portion of the Conasauga Formation in northern Ohio. The Kerbel Formation overlies the middle portion

of the Conasauga Formation in north-central Ohio and pinches out in eastern Ohio as it becomes

dolomitic (Ryder et al., 2008). The Kerbel Formation measures 23 m thick in north-central Ohio and is

described as a siliciclastic depositional unit with a coarsening-upward trend with fine-grained sandstone

at the base, moving upward into coarse-grained, massive, parallel laminated and cross-bedded sandstone. 20

Carbonate intraclasts consisting of micrite, peloids, ooids, and bioclasts are interpreted as wave-rewashed

lagoonal/estuarine deposits from the Conasauga Formation (Banjade, 2010).

The coarsening-upward sequence of the Kerbel Formation is interpreted as a landward migration

of the strandplain (barrier island and associated environments) during sea level rise (Banjade, 2010). The

sequences of the Conasauga Formation and the Kerbel Formation thus characterize the Late Cambrian

marine transgression. This is represented by the landward movement of a barrier island above a wave

reworked platform of an earlier esturary, lagoonal, and tidal flat environments (Banjade, 2010).

Knox Group

The Knox Group is deposited stratigraphically above the Consauga Formation and below the

Knox Unconformity (Ryder, 1997). The overlying Knox Unconformity marks the boundary between the

Knox Group and the overlying Wells Creek Formation (Hansen, 1997). The Knox Group occurs along a

322 kilometer strip running from south-central to northeastern Ohio, and the unit is over 396 m thick in

southern Ohio (Riley et al., 2002). The Knox Group is a mixed carbonate-siliclastic sequence that

includes the Copper Ridge Dolomite, the Rose Run Sandstone, and the Beekmantown Dolomite (Hansen,

1997).

The Copper Ridge Dolomite is mostly dolomite but includes up to four sandstone intervals. The

dolostones contain ooids, thrombolitic algal mounds, cryptalgal laminae, and rip-up clasts (Riley et al.,

1993). The Rose Run Sandstone is composed of interbedded sandstone, shale and dolostone. The

sandstones are mostly cross-bedded or flaser-bedded, quartz or feldspathic arenites. The Beekmantown

Dolomite is described as gray to brown, fine to medium crystalline, mottled, non-permeable dolomite that can locally contain algal stromatolites (Riley, 1995).

In Ohio, the Knox Group was deposited during the Late Cambrian to Early Ordovician. The unit is interpreted as a tidal flat environment on an extensive continental shelf with widespread carbonate buildups. The Rose Run Sandstone is interpreted as a barrier island sequence. More recent work on the 21

Rose Run Sandstone in Ohio interpreted the Rose Run Sandstone as a shallowing-upward sequence of carbonates and sandstones which, based on their sedimentary structures, represented subtidal and tidal flat environments with subtidal channels and migrating bar/dunes (Chuks, 2008). A later sea regression exposed the Knox Group and extensive erosion occurred recording the Knox Unconformity.

Production from Knox Group reservoirs has been calculated to over 62.9 MBBO of oil and 347

bcf of gas from around 1840 producing wells. Unexplored Knox Group reservoirs are estimated to still

have about 800 bcf of gas or 133 MMBOE (Riley et al., 2002). Primary reservoirs are in the Rose Run

Sandstone where laterally discontinuous sand bodies are localized (Mihir Shah, in progress) or in the

Beekmantown Dolomite where paleogeomorphic traps formed by erosional relief over unconformities

that were buried by younger strata that have vuggy porosity zones (Riley et al., 2002).

Wells Creek Formation

The Middle Ordovician Wells Creek Formation is deposited above the Knox Group and below

the Black River Group. The Wells Creek Formation is composed of shale, siltstone, sandstone, and

dolomite. The thickness of the Well Creek Formation is varied due to the nature of the erosional surface

on the Knox Group (Hansen, 1997). The average thickness of the Wells Creek Formation is around 6 m,

but it can range from zero on the topographic highs of the Knox Group to 18 m thick where

paleotopographic low spots are located (Hansen, 1997). The Wells Creek Formation serves as a seal for

the Knox Group oil and gas reservoirs (Wickstrom et al., 2011).

Black River Group

The Black River Group is Middle to Upper Ordovician in age and lies above the Wells Creek

Formation and below the Trenton Formation. The Black River Group consists of fine-grained dark-tan

limestone and is over 91 m thick in northwest Ohio and thickens to over 152 m in eastern Ohio (Hansen,

1997). It has also been described has light to medium brown/grey burrow-mottled, stylolitic mudstone by

Patchen et al. (2006). Fossils are abundant in the unit. Chert beds and rip up clasts occur locally but are 22

not widespread. The gamma-ray log response of the Black River Group is low also indicating that the rock is a clean carbonate (Patchen et al., 2006).

Lithofacies analysis of the Black River Group suggest the unit was deposited across a low-relief carbonate ramp and is composed of shallow subtidal to peritidal carbonates (Patchen et al., 2006). During the deposition of the Black River Group, the architecture of the carbonate ramp changed from passive/extensional regine to a compressive regime due to the colliding Taconic highlands to the east. The architecture slowly evolved into an extensive shallow-water carbonate ramp that dipped east/southeast from southeast Michigan/northwest Ohio across the Appalachian basin and into the Rome Trough, as epeiric seas covered the Appalachian Basin (Patchen et al., 2006).

Trenton Formation

The Trenton Formation is Middle Ordovician in age and lies above the Black River Group and below the Utica Shale in northwest Ohio. It is over 91 m thick in northwestern Ohio and thins to over 12 m thick in west-central Ohio. The Trenton Formation is correlative to the Lexington Limestone of

Kentucky. The Trenton Formation is described as a light gray, crystalline, highly fossiliferous, skeletal grainstone. The Trenton Formation accumulated on the edges of the Appalachian Basin as low relief carbonate buildups or platforms. The Trenton platform was located in what is today southeast Michigan and Indiana and extended across Lake Erie into New York. This carbonate platform is divided stratigraphically, such that limestones located in northern and eastern Ohio are called the Trenton

Formation and while limestones located in central and southern Ohio are referred to as the Lexington

Limestone.

The Lexington Limestone is considered time equivalent to the Trenton Formation, and the sharp contact between Trenton Formation and Utica Shale in northwest Ohio is correlated to the gradational contact between the Lexington Limestone and the Point Pleasant Formation in southeastern Ohio

(Patchen et al., 2006). South of the Trenton Platform moving into the Utica-Point Pleasant Basin, the 23

Trenton Limestone thins as the Point Pleasant Formation and Utica Shale thickens. Here, the Point

Pleasant Formation and Utica Shale were deposited coeval with Trenton Formation limestones on the

platform to the north (Patchen et al., 2006).

Carbonate buildups kept pace with marine transgressions and allowed for a shallow sea to cover a

major portion of Ohio. The two carbonate platforms restricted water movement within the basin and

allowed for the deposition of dark brown and black carbonaceous Point Pleasant Formation and Utica

Shale to be deposited (Patchen et al., 2006).

Lexington Limestone

The Middle Ordovician Lexington Limestone lies above the Black River Group in southern Ohio.

The Lexington Limestone is subdivided into members, in ascending order, the Curdsville Member,

Logana Member, and undifferentiated Lexington Limestone. The name Lexington Limestone was first

used by J. L. Campbell (1898) to describe 45 m of gray thin-bedded limestone near the city of Lexington,

Kentucky.

The Curdsville Member is composed of medium gray to brownish gray, medium to fine

crystalline wackestone-grainstone. The Cursdville Member is a relatively cleaner carbonate than the

Logana Member and undifferentiated Lexington Limestone, and has a lower gamma-ray response, and is marked by a gradational contact with the overlying Logana Member (Patchen et al., 2006). In Kentucky, the Curdsville Member is described as a bioclastic calcarenite that is cross-bedded in part and also phosphatic in part (Black et al., 1965).

Core descriptions and geophysical well log data indicate that the Logana Member and undifferentiated Lexington Limestone are higher in shale content than the correlative Trenton Formation or the Curdsville Member (Patchen et al., 2006).The Logana Member is an olive gray to black, calcareous, medium-to thin-bedded fossiliferous (brachiopods) shale with thin beds of coarse-to finely- crystalline, argillaceous, fossiliferous, olive grey limestone (Black et al., 1965). The Logana Member can 24

be correlated from place to place using its distinct higher gamma-ray and neutron-density log signature

and can be traced throughout much of Ohio (Figure 8). This member is viewed as a probable petroleum

source rock in Ohio wells. The Logana Member stretches acrossed most of Ohio in the sub-basin region and is mostly present were the overlying Point Pleasant Formation is found (Patchen et al., 2006). The undifferentiated Lexington Limestone is around 33 m thick throughout most of Ohio and thickens in western Kentucky and southern West Virginia to around 122 m at locations where the Point Pleasant

Formation and Utica Shale are absent (Patchen et al., 2006).

The peak of the Taconic Orogeny resulted in uplifted platforms and interplatform basins. The contact between the Black River Group and the Curdsville Member has been interpreted as a type 1 transgressive systems tract (TST) (Patchen et al., 2006). As clean carbonates continued to build up on the platforms, the basin began to fill with black shale of the Logana Member and marked a shift from a sloping carbonate ramp to a new geography of basins surrounded by carbonate platforms. Pope and Read

(1997) interpret the contact between the Curdsville Member and the Logana Member as a maximum flooding surface (MFS). As carbonate ramps continued to vertically aggrade, the sub-basin was filled with interbedded black shale and limestone of the Point Pleasant Formation and ultimately black shale of the

Utica Shale. This interval of black shale deposition continued until the rate of sea level rise outpaced the rate of carbonate platform buildup (Patchen et al., 2006).

Point Pleasant Formation

The Late Ordovician Point Pleasant Formation is above the Lexington Limestone. The Point

Pleasant Formation has been valued as a building stone, and there are numerous quarries along the Ohio

River. The Point Pleasant Formation consists of interbedded limestone and shale (Hansen, 1997). The relationship of the Point Pleasant Formation and the Trenton Formation is not clear, due to the lack of biostratigraphic markers. In northwest Ohio, the Point Pleasant Formation is believed to have a 25

gradational lateral contact with the Trenton Limestone on the carbonate platform but in southwest Ohio,

the Point Pleasant Formation overlies the Lexington Limestone.

As suggested above, the facies relationship between the Trenton Formation or Lexington

Limestone and the Point Pleasant Formation is complex. In some locations, the base of the Point Pleasant

Formation can be marked by an intercalated contact of thin limestone and shale beds that overlie the

Trenton Limestone. The Point Pleasant Formation extends along a line that stretches from the border

between Indiana and Kentucky northwest into northern Ohio and east into northern Pennsylvania and

eastern New York. North of this line the Trenton Formation is marked by an abrupt vertical contact with

the Utica Shale (Patchen et al., 2006).

Utica Shale

Above the Point Pleasant Formation, there are several shale-rich formations (including the Utica

Shale), with various stratigraphic names in eastern and western Ohio. Collectively, these shale units were

deposited over a large area of Laurentia. The Utica Shale is described as a massive, fossiliferous, organic-

rich, thermally mature, black to gray-black shale (Hill et al., 2002). The deposition of the Utica Shale took place in a subsiding trough that was oriented north-south. As this trough began to fill with sediments eroded from the highlands to the east, the locus of black shale deposition moved westward. Five different shale facies are recognized in the Utica Shale. Each facies records the periodic movement of the shale depocenter westward, because the shale onlaps an unconformity or a condensed bed at the top of an underlying carbonate bed (Hill et al., 2002). The average thickness of the Utica Shale in eastern Ohio is between 60 to 77 m (Ryder, 2008).

This shale is considered to be the petroleum source rock for Cambrian through Lower Devonian reservoirs. This organic matter is interpreted as the residual organic matter preserved in the shale after most was converted to oil and gas during the Late Devonian through the Early Permian (Rowan, 2006).

26

Figure 8. Extent of Logana Member of the Lexington Limestone, correlated by core and geophysical logs

(modified from Patchen et al., 2005). Study area indicated.

27

The Utica Shale typically contains 1% TOC, and ranges from 2-3% TOC in eastern Ohio, northern West

Virginia, and southwest Pennsylvania (Ryder, 2008).

28

METHODS

Core Descriptions

Data collection was done at the Ohio Geological Survey, Horace R. Collins Core Lab Facility

located in Columbus, Ohio. Data collection consisted of a description of the core Chevron Prudential 1-A

(Core API number 34101201960000), located in Big Island Township, Marion County, Ohio (Figure 6).

This well was cored by Chevron U.S.A Inc. on February 26, 1991.

This collected core interval ranges from the Knox Dolomite to the Lockport Group, for a total

length of 671.7 m. This study described the section from the top of the Curdsville Member (Lexington

Limestone) to the top of the Utica Shale, in other words from -452 m to -347 m below the surface. The gamma ray, resistivity, and neutron-density logs of core 3372 can be found in Appendix A. Core preservation is very good, with only short intervals of core missing. Prior to this study, other researchers had sampled the core. A total of 64 samples were previously taken and prepared for geochemical analysis

(including TMax, HI, PI, OI, and TOC) by Shell Global, Dolan Integration Group, Weatherford Inc.,

GeoMark Petroleum Sevices Div., and Geochem and Petrophysical Consulting LLC.

A full description of the core from the top of the Curdsville Member to the top of the Utica Shale was done using a handlens and recorded on 1-meter logging sheets with 10 centimeter subdivisions

(Appendix C). Descriptions of the core included grain size, color, mineralogy, and sedimentary structures

based on a centimeter scale. Separate descriptions of natural healed fractures of the core were also taken.

Natural healed fracture descriptions included core depth, fracture length and width, fracture in-fill composition and fracture orientation (Appendix D). Photographs were also taken of the core from the top of the Curdsville Member to the top of the Point Pleasant Formation, (an example is shown in Figure 9).

29

Figure 9. Core box 132 of the Prudential 1-A core in Marion County, Ohio, sharing the stratigraphic interval from -443.8m (lower left) to -441m (upper right) in the undifferentiated Lexington Limestone. 30

Sampling from the core depended on the location and density of healed fractures. A total of 21 samples were taken from the core to evaluate microstructures and cements in the fracture fills. The majority of the samples were taken from areas that have a high density of natural fractures. Additional data was obtained during the summer of 2012 when I interned at EQT Corporation in Pittsburgh,

Pennsylvania. I was assigned to work in the Point Pleasant Formation with Mr. Chris Willan and Scott

McCallum (two of the company’s exploration geologists). Data made available by EQT Corporation included thermal maturity data and TOC data.

Scanning Electron Microscopy

This study uses Scanning Electron Microscopy (SEM) to view different mineral surfaces and visible pore spaces over the stratigraphic interval from the Logana Shale through the Point Pleasant

Formation. Sample preparation included collecting six rock samples from the core at the H.R. Collins

Core Laboratory, cutting these into smaller samples around 1.5 cubic cm in size, and then coating them with gold/platinum using the Hummer VI-A sputter coater in order to allow the sample to be conductive.

Samples were placed inside a vacuum chamber on the Hummer VI-A, flushed with argon 2-3 times for

10-15 seconds each time, and then coated with gold-platinum.

Scanning Electron Microscopy was done in the Department of Biological Science at Bowling

Green State University using a Hitachi S-2700 scanning electron microscope. Samples were viewed in a vacuum and under a high electron beam of 20 KV. Determining the area and texture of the mineral surfaces allowed identification of the size, shape, and distribution of porosity on a microscale level in the mudrocks.

Total Organic Carbon (TOC)

A total of 64 samples were previously analyzed for total organic carbon (TOC) from this core by

Weatherford International, GeoMark Petroleum Services Division, and Geochem and Petrophysical

Consulting, LLC. A total of 28 TOC samples were prepared by Weatherford by using a source rock 31

analyzer, instrument model (SRA-TPH/TOC). The values of TOC were found by burning core samples in

an oxygen-rich atmosphere and measuring the amounts of carbon dioxide that were produced

(Weatherford International, 2013).

A total of six samples were prepared by GeoMark Research, Ltd. by using HCl to decarbonize the rock sample. Each sample was treated with HCl for at least two hours and then flushed by water filtration.

Once the acid is removed, the sample is placed into a LECO crucible and dried in low (<110o) temperature oven for two hours. Once samples have been dried, they were weighted to calculate % carbonate in the sample. Using the LECO C230, carbon analyzer, which had been calibrated with samples with known carbon contents, the samples were heated to 1200oC in an oxygen abundant atmosphere.

During the heating process, carbon monoxide and carbon dioxide were produced and (with the use of a

catalyst) carbon monoxide was converted into carbon dioxide. The amount of carbon dioxide was

subsequently measured by an infrared (IR) cell. Finally, the TOC amount was determined by comparing

the amount of carbon dioxide produced with the known (standard) carbon dioxide. Random and/or

selected reruns were done twice to verify that the standard deviation for TOC amounts in samples was

<10% from the known value (GeoMark Research Ltd., 2013). A detailed graph of TOC changes was

made and analyzed for TOC changes throughout the core (Appendix D).

Cathodoluminescence

Cathodoluminescence is the process in which the primary excitation of electrons in specimen is caused by a directed beam of electrons from a cathode (Marshall, 1988). Cathodoluminescence is a visible radiation occurring within the wavelengths of 400 and 700 nm and energies between 1.77 and 3.10 electron volts (Marshall, 1988). The cathodoluminescence (CL) spectrum may also be influenced by different structural imperfections in crystal structure such as defects, radiation and shock damage and impurities (Boggs et al., 2001). 32

Cathodoluminescence allowed for the identification of micro-fractures in the shale as well as the

healed natural fractures. It also allowed for distinguishing the events of precipitating minerals in the

natural fractures and if fractures experienced multiple stages of fracture fill. The intensity of its

luminescence is a characteristic of its chemical composition or impurities of the sample. Because the

healed natural fractures examined in this study are mostly filled by calcite, the main element that acts as

an activator during excitation is Mn2+ as a substitute for Ca2+. Elements that act as quenchers (or those that dull the amount of energy given off) are Fe2+, Co2+, and Ni2+ (Marshall, 1988).

Cathodoluminescence was done at Bowling Green State University Laboratories under the

supervision of Dr. Charles Onasch. A total of nine samples were used for cathodoluminescence. Each

sample was prepared as an unpolished thin section without a cover slip. The laboratory equipment

consisted of a Technosyn Cold Cathodoluminescence Systems, and the images were viewed through a

Nikon camera system via light microscope at low magnification. Digital pictures were taken with

Optronics camera mounted to the Nikon light microscope. Cathodoluminescence was done on samples

12SH002-12SH008, 12SH011, and 12SH015.

The method involved viewing three samples at a time under high vacuum conditions. Samples

were placed in the vacuum chamber and a high vacuum environment was established. An electron beam

was directed towards the surface of the thin sections. A total of at least three pictures were taken for each

sample. The three pictures consisted of a polarized digital image, a plane light digital image, and a

cathodoluminescenced image.

33

RESULTS

Lithology

Core lithologic descriptions are based on hand samples, thin sections, and core photographs from

the Chevron Prudential #1-A core, of Marion County, Ohio. In addition, geophysical logsof the Prudential

#1-A were used to evaluate core stratigraphy (Figure A1 and Figure A2). Geophysical logs include

gamma ray, resistivity, and neutron-density logs.

The lithology of the studied interval in this core consists of thin to medium (1 centimeter to 0.5

meter) beds of carbonate packstone and grainstone, interbedded with medium to thick (10 centimeter to 1

meter) beds of carbonate mudstone and wackestone. The lithology varies slightly within the different

formations discussed in this paper, specifically the Logana Member (Lexington Limestone),

undifferentiated Lexington Limestone, and the Point Pleasant Formation. The Logana Member and the

undifferentiated Lexington Limestone are both limestone units while the Point Pleasant Formation is a

mixed siliciclastic/carbonate unit. Thin section analysis shows an abundance of bioclasts in these units

including fragments of brachiopods, bryozoans, crinoids, and straight-shelled nautiloids. Other observed

features include phosphorite nodules, chert nodules, and blebs of organic matter.

The Logana Member is a limestone consisting of undulate bedded packstones interbedded with undulated bedded wackestones and mudstones that are interpreted as carbonate tempestites (later section)

(Figure 10A, 11A, 11B). The gradational contact between the underlying Curdsville Member and overlying Logana Member is marked by a series of skeletal packstone beds, and the unit fines upward into swaly laminated wackestones with interruptions of centimeter-scale skeletal packstones. The Logana

Member also has numerous vertical fractures healed by calcite cementation (later section).

The undifferentiated Lexington Limestone is marked by thicker (10 centimeter to 0.5 meter) beds of packstone and grainstone (Figure 10B) composed of bioclasts of bivalves, crinoids, and bryozoans. The transition from the Logana Member to the undifferentiated Lexington Limestone is marked by a transition 34

from carbonate marl to a skeletal packstone bed. The undifferentiated unit has abundant carbonaceous

mudstone and wackstone interbedded with thicker beds of skeletal packstones. The undifferentiated

Lexington Limestone then grades upward into rhythmites consisting of carbonaceous mudstone and

wackestone couplets.

The Point Pleasant Formation mixed siliciclastic and carbonate unit and a fining-upward

sequence that is marked by hemipelagic rhythmites (Figure 10C) consisting of carbonaceous mudstones and wackestones at the base. The Point Pleasant Formation then grades upwards into light gray shale beds with interbedded skeletal wackstone and packstone. At the top of the Point Pleasant Formation, there is a condensed section consisting of phosphorite nodules and shale hash (later section). Such features typically form on the outer shelf during an interval of sea level rise, due to sediment starvation, and this marks the transition to the deposition of the overlying dark gray shales of the Utica Shale.

35

A B C

Figure 10. A: Logana Shale Member showing undulated bedded packstones, interpreted as carbonate tempestites (storm deposits), interbedded with carbonate mudstones. B: undifferentiated Lexington

Limestone showing thicker beds of undulated bedded packstones interpreted as carbonate tempestites interbedded with carbonate mudstones and wacketones. C: Point Pleasant Formation showing rhythmites of mudstones and wackestones. 36

A b

0.1 cm

AB

s

0.1 cm Figure 11. A: Thin-section photograph of Logana Member showing undulate bedded skeletal packstone with brachiopod fossils (b) over a carbonaceous wackestone. B: Logana Member of carbonaceous wackestones with siliciclastic silt (s), interpreted as eolian silt. 37

Lithofacies Analysis

Lithofacies and Interpretations

Undulate Bedded Skeletal Packstones (Cpu):

This lithofacies consists of undulate bedded or hummocky stratified, medium-grained, normally

graded, gray-white, bivalve packstone with interbedded lenses of black carbonaceous mudstone. The

skeletal packstones range from 1 to 2 cm in thickness and are underlain and overlain by carbonaceous

mudstone or wackestone (Figure 12A) (A complete list of lithofacies can be found in Table 1).

This skeletal packstone is interpreted to be part of a storm deposit including lithofacies Cwu and

Cmu. Storm events were responsible for erosion and bringing in large amounts of shell debris. The

undulated contact between the mudstones or wackestones and the overlying packstones is interpreted a sa

scoured surface. Comparisons to modern deposits suggests the tempestites were deposited above storm

weather wave base (SWWB). According to Burchette and Wright (1992) tempestite assemblages like

these can be found on mid-ramp environments near carbonate platforms (Figure 13).

Undulate Bedded Carbonaceous Wackestone (Cwu):

Lithofacies Cwu consists of undulate bedded to planar laminated, very fine grained, carbonaceous

wackestone with intermittent bivalve shells (figure 12A and 12B). The facies ranges from 3-10 cm in thickness and are part of a tempestite deposit. Here, they are interbedded with carbonaceous mudstones

(Cmu) and/or packstones (Cpu) (Figure 12 A&B).

This facies is interpreted as an inner ramp facies that is repeatedly exposed to the introduction of platform debris including bivalves and carbonate silt. Due to the repetitive scouring of the surface and overlain by skeletal packstones, it is interpreted that the carbonaceous carbonate wackestones were

38

A B B C C

A B

Figure 12. A: Core photographs of Prudential #1-A showing undulate bedded skeletal packstones (lithofacies Cpu) interbedded with undulate bedded carbonaceous wackestones (lithofacies Cwu), and undulate bedded carbonaceous mudstones (lithofacies Cmu). B: core photograph of black carbonaceous mudstone (Cmub), and swaly carbonaceous wackestone (lithofacies Cws).C: Core photograph of undulate bedded carbonaceous mudstone (lithofacies Cmu) interbedded with undulate bedded carbonaceous wackestones (lithofacies Cwu), and undulate bedded black carbonaceous mudstone (lithofacies Cmub). 39

Table 1. Lithofacies Codes and Descriptions

Lithofacies Code Lithology Sedimentary Interpretation Structures Cmu Very fine-grained Planar laminated; Mid ramp; above carbonaceous carbonate undulate bedding SWWB mudstone Cmub Dark black, very fine- Undulate bedding Mid ramp; above grained carbonaceous SWWB carbonate mudstone Cwu Fine to medium-grained Planar laminated; Mid ramp; above carbonate wackestone undulate bedding SWWB

Cws Very fine-grained Swaly laminated Inner ramp; above carbonaceous carbonate FWWB wackestone Cwm Fine to medium grained, Mottled, bioturbated Inner ramp, above carbonaceous carbonate FWWB wackestone HCr Hemipelagic Planar laminated; Outer ramp; interbedded carbonate hemipelagic rhythmite mud and carbonate wackestone Cpu Medium-grained Normal graded; Mid ramp; above skeletal carbonate undulate bedding SWWB packstone Cgmu Medium to coarse Massive; undulated Inner ramp; above grained bioclastic FWWB grainstone Cppu Medium-grained Undulate bedding Outer ramp; skeletal packstone and condensed section well-rounded coarse- phosphorite nodules Scp Light grey clay shale Planar laminated Outer ramp

Note: SWWB= Storm-weather wave base

FWWB= Fair-weather wave base 40

Figure 13. Inner ramp to basin environments and their associations with MSL (mean sea level), FWWB

(fair weather wave base), SWWB (storm wave base) and PC (pycnocline), modified from Burchette and

Wright, 1992. 41

deposited above SWWB on the inner ramp environment of a carbonate platform (Burchette and Wright,

1992).

Carbonaceous Mudstone (Cmu):

This lithofacies consists of undulate bedded to planar laminated, very fine-grained, dark brown to dark gray, carbonaceous mudstone with sparsely intermittent brachiopod shells and small amounts of fine quartz silt (Figure 14). Individual occurrences range between 1 and 5 cm in thickness and are usually overlain by undulate bedded skeletal packstone or undulate bedded carbonaceous wackestone (Figure

12C). The carbonaceous mudstones are part of the tempestite deposits occurring throughout the core.

Where skeletal packstones are present above, there is an additional undulated surface on the mudstone.

Where carbonaceous calcareous wackestones are present the bedding is planar laminated.

This facies is interpreted as being part of a tempestite sequence. As such, it was deposited above storm wave base in a mid-ramp environment following the scouring of the surface and deposition of undulate bedded skeletal packstone. The carbonaceous mudstones range from 2-7% in organic matter

(Weatherford International, GeoMark Research, & Petrophysical Consulting LLC). The mudstones in the

Prudential #1-A do not show signs of bioturbation, and therefore suggesting that they were deposited in an anoxic environment (Burchette & Wright, 1992).

Black Carbonaceous Carbonate Mudstone: (Cmub):

Lithofacies Cmub consists of undulated bedded to planar laminated, black, carbonaceous mudstones (Figure 12C). The black carbonaceous mudstone is remarkably darker in color than other mudstones in the Logana Member, range from 0.5 to 2 cm in thickness, and are interbedded with the swaly carbonaceous wackestones lithofacies (Cws).

Most carbonate mud is produced in the shall0w subtidal “carbonate factory” due to the breakdown of bioclasts into mud-sized materials. The mud can subsequently be redistributed by waves,

42

o

0.1 cm

Figure 14. Photomicrograph of Logana Member lithofacies (Cmub) at -440.7 m, showing undulate bedding black carbonaceous mudstone with high amounts of organic matter (o).

43

currents, and tides (Morse, 2003). These carbonate muds are interpreted as mid-ramp deposits because of

their association with many tempestite beds. Badenas and Aurell, (2000), have related carbonate muds of

the Kimmeridgian rocks in Spain with scoured surfaces to have been deposited on the carbonate mid-

ramp environment.

Swaly Carbonaceous wackestone (Cws):

Lithofacies Cws consists of swaly bedded, fine-grained, light to dark brown, carbonaceous,

wackestones with lenses of dark brown to black carbonaceous carbonate mudstone. The few fossils found

in the Cws lithofacies include sparse bivalve shells. This facies ranges between 5 and 10 cm thick, and

commonly overlies carbonaceous wackestones (Figure 12B).

The lithofacies Cws is interpreted to be deposited above FWWB in an inner ramp environment.

(Aigner, 1984; Burchette, 1987, Faulker, 1988). The swaly bedded lithofacies Cws is interpreted to represent continual reworking by wave action and typically such deposits are found in lower shoreface

environments.

Brown Mottled Wackestone Facies (Cwm):

Lithofacies Cwm (Figure 12A) consists of brown to light gray, fine-grained, bioclastic

wackestone interbedded with lenticular beds of fine-grained light grey wackestone. Lithofacies Cwm

commonly occurs in the Logana Member of the Lexington Limestone. Occurences range from around 10

to 15 cm in thickness. Lithofacies Cwm commonly overlies the skeletal packstone facies (lithofacies

Cpu), or underlies the swaly carbonaceous wackestone beds (lithofacies Cws), and/or overlies the undulated skeletal packstone beds (Cpu).

The Cwm lithofacies represents a lower shoreface environment that has been subject to bioturbation which created the mottling. Initially, the sediment was deposited as a carbonaceous wackestone, after deposition this lithofacies was subject to bioturbation and became lithofacies Cwm. 44

This is interpreted to have most likely occurred above FWWB in a near shore environment, allowing for organisms to burrow and disrupt the sediment (Burchette and Wright, 1992).

Undulate Bedded Grainstones Facies (Cgu):

Lithofacies Cgu are undulate bedded, medium-grained, light grey, grainstones consisting of bioclasts of bivalve, crinoids, and bryozoans. The undulate bedded grainstone facies range from around 7 to 35 cm in thickness, overlies the carbonaceous mudstone facies (lithofacies Cmu) and underlie either the wackestone (lithofacies Cwu) and/or mudstone facies (lithofacies Cmu) (Figure 15A, 16).

The undulate bedded grainstone facies represents a shallow shelf environment that was

commonly reworked by wave action where carbonaceous mud was winnowed from the denser skeletal

debris creating the thicker successions of grainstones. Carbonate platforms provided fossil debris which

was transported via storm activitiy. Large amounts of the fossil debris suggest that the facies was more

proximal to the carbonate platforms compared to the thinner undulated skeletal packstone facies or

carbonate calcisiltite facies. According to Dattilo et al., (2012), the undulate bedded grainstones could be formed due to winnowing of carbonate mud by wave action in a mid ramp environment that was subject to shallow wave action (above FWWB).

Heterolithic Rhythmite Facies( HCr):

The heterolithic carbonaceous rhythmite facies range from 1 millimeter to 10 centimeter in thickness and are found at the top of the undifferentied Lexington Limestone. The individual lithologies that make up the carbonaceous rhythmites (lithofacies HCr) are generally 1 millimeter thick but can reach up to 4 cm thick. Lithofacies HCr are generally composed of fine grained, light grey carbonaceous wackestone and dark brown carbonaceous mudstone. Carbonaceous rhythmites overlie the undulate bedded grainstones (lithofacies Cgu) and underly the undulate bedded wackestones (lithofacies (Cwu)

(Figure 15B, 17, 18). 45

A BB

Figure 15. A: Core photograph of the undifferentiated Lexington Limestone showing undulate bedded grainstones (lithofacies Cgu) overtop carbonaceous undulated bedded mudstone facies (lithofacies Cmu). B: Core photographs of the Point Pleasant Formation showing hemipelagic rhythmite facies (lithofacies HCr).

46

b

0.1 cm Figure16. Photomicrograph of the undifferentiated Lexington Limestone (-441.9 m) showing undulate bedded grainstone (lithofacies Cgu) composed of bioclasts (brachiopod shell b).

Cwu s

Cmu 0.1 cm Figure 17. Photomicrograph of the Point Pleasant Formation (-435 m) showing hemipelagic rhythmites (lithofacies HCr) composed of couplets of carbonaceous wackestones and carbonaceous mudstones. 47

Figure18. Core photo from the Lexington Limestone undifferentiated showing interbedded carbonate grainstones (lithofacies Cgu), packstones (lithofacies Cpu), wackestones (lithofacies Cwu), and mudstones (lithofacies Cmu). 48

Lithofacies HCr is interpreted to represent deposition in an outer ramp environment. The repeating, undisrupted, planar layers of carbonate mudstones and wackestones suggest that they were deposited below SWWB. The carbonaceous wackestones are composed of silt-sized carbonate debris that is interpreted to be derived from plankton blooms or long-distance transport from the sub-tidal carbonate factory. The carbonaceous carbonate mud is interpreted to have been deposited during calm waters allowing for carbonate mud to fall out of susupension (Burchette and Wright, 1992). Such deposits are called hemipelagic.

Packstone with Phosphorite Nodules Facies (Cppu):

Lithofacies (Cppu) consists of undulate bedded, mottled, medium-grained, light grey to brown, skeletal packstones with coarse-grained, well-rounded phosphorite nodules (Figure 19, 20, 21). The phosphorite nodule and shell hash facies occurs twice in the core. One is near the top of the undifferentiated Lexington Limestone (-439.7 m). The other is at the Point Pleasant Formation and Utica

Shale contact (-410.4 m). An EDAX scan has been done on these nodules to determine their chemical composition which was found to be both pyrite and phosphorite (Figures 22 and 23).

In the undifferentiated Lexington Limestone, the packstone with phosphorite nodules (lithofacies

Cppu) shares a sharp contact with the underlying and overlying carbonate grainstone facies with a 0.5 centimeter thick layer of pyrite directly above in a carbonate grainstone bed in the undifferentiated

Lexington Limestone. Above the lithofacies Cppu, the lithology changes from undulate bedded packstones, wackestones, and mudstones to the pelagic rhythmites. This suggest a change from shallow water (above SWWB) to deep water conditions.

The packstone with phosphorite nodule (lithofacies Cppu) also occurs at the top of the Point

Pleasant Formation. It also shares a sharp contact with the overlying Utica Shale in the Point Pleasant

Formation. Here the lithofacies (Cppu) marks the transition from interbedded undulated packstones, wackestones, and mudstones of the Point Pleasant Formation to the carbonaceous shale dominant Utica 49

Shale, again indicating a change from the shallow water (above SWWB) to deep water conditions. In addition, the skeletal material may indicate erosional winnowing and the presence of an omission surface.

The lithofacies Cppu is interpreted as a condensed section that formed in the outer ramp environment. The phosphorite nodules have formed due to sediment starvation in the outer ramp setting after a rise in sea level cause a reduction in sediment supply to the outer ramp. Chemical precipitates form as a consequence of very low rates of sediment accumulation. Mottling is present due to bioturbation of the sediment starved surface. According to Haq, (1991), during a marine transgression in a carbonate environment, there is often an occurrence of a sediment starved surface resulting in a condensed section which is susceptible to concentrate and precipitate authigenic minerals.

Planar Laminated Carbonaceous Clay Shale Facies (Scp):

The planar laminated clay shale facies are present in the Point Pleasant Formation and Utica

Shale. The clay shale facies are light grey, planar laminated, and contains sparse amounts of grains of calcite and quartz. These planar laminated clay shale facies are interbedded with skeletal packestone and wackestone facies and are highly abundant throughout the upper portion of the Point Pleasant Formation and form most of the Utica Shale. They occur above the hemipelagic rhythmites (Lithofacies HCr) in the

Point Pleasant Formation and continue into the Utica Shale.

The carbonaceous clay rich shales of the Point Pleasant Formation are interpreted to be deposited on an outer ramp environment and the carbonaceous, clay-rich shale of the Utica Shale is interpreted to be deposited in a basinal environment. 50

A B

Utica Shale

Cppu Point Pleasant Formation

Cppu

Figure 19. Core photograph of the contact between the Point Pleasant Formation and the Utica Shale (-

410.4 m) with packstone and phosphorite nodule facies (lithofacies Cppu). A thin undulate bedded wackestone (lithofacies Cwu) is interbedded with the packstone and phosphorite nodules. The transition to the Utica Shale is abrupt and marked by an undulate surface transitioning up into carbonaceous shale

(Lithofacies Sc). 51

P

0.1 cm

Figure 20. Photomicrograph of Point Pleasant Formation undulated, mottled, packstone and phosphorite nodules at the contact between the Point Pleasant Formation and the Utica Shale (-410.4 m). Phosphorite nodules (P) are numerous throughout lithofacies (Cppu). The layer is interpreted as a condensed section

(or omission surface) representing very low rates of deposition.

52

A

B

Figure 21. SEM image of phosphate and pyrite nodules in the Point Pleasant Formation at -410.4 m. The nodules have two different chemical compositions, nodule A is calcium phosphate interpreted as francolite (sedimentary apatite). Nodule B is an iron sulfide, interpreted as pyrite (which may have been replaced sedimentary glauconite). The scan lines indicated can be seen in Figures 22 and 23.

53

Figure 22. EDAX chemical composition of nodule A in Point Pleasant Formation (-410.4 m). This calcium phosphate mineral is interpreted as francolite (sedimentary apatite).

54

Figure 23. EDAX scan of nodules in the lithofacies Cppu located at the top of the Point Pleasant

Formation at -410.4 m. This iron sulfide mineral is interpreted as pyrite and may be the diagenetic replacement of glauconite. 55

Lithofacies Associations

Tempestite Association

Storm deposits (tempestites) are important in identifying sediment deposition in a near shore

environment or on an open shelf environment. The abundance of tempestites in the stratigraphic record

are important because these deposits are rarely preserved in shallow water (above FWWB) due to the subsequent wave reworking and bioturbation. Carbonate tempestites found in the Cambrian Gushan

Formation in northwest China provide similar examples for the stratigraphy of carbonate storm deposits.

The carbonate tempestites usually have a scoured bases caused by wave erosion of the sea floor during a storm event.

A tempestite usually consists of a scoured surface at the base of the sequence. This surface is generally formed due to storm-generated oscillatory currents which erode the sea floor and leave the surface undulatory or convex (Zhoa et al., 2011). Above this scoured surface is the main storm deposit which can include coarse-grained deposits (rudite, floatstone, packstone, or wackestone) with hummocky stratification, or planar lamination. A typical carbonate tempestite sequence described by Zhidong (1998) can be seen in Figure 24. The stratification of these deposits is produced by fallout from suspension.

Above this layer is “waning storm deposition” which includes packstones or wackestones. These layers

generally become finer grained with weakening of storm intensity but thicker due to higher sedimentation

rates. In this layer bioturbation (including escape structures) is most likely to be present, including

Skolithos trace fossils. A typical sequence found in the Logana Member can be seen in the stratigraphic column in Figure 25. A table of lithofacies associations can be found in Table 2.

Carbonate tempestites identified by Sageman (1996) in the Western Interior Basin were characterized by sharp bases that were usually flat or had undulating relief and commonly display cut and fill features, flute marks, and tool marks. The bed tops were gradational to mudstone and limestone beds were usually massive to somewhat graded but in most instances were planar to cross-laminated. The 56

Figure 24. Carbonate tempestite stratigraphic column modified from Zhidong, (1998) showing a typical carbonate tempestite sequence with carbonate wackestones with hummocky stratification and a packestone with a scoured base. 57

Figure 25. Interpretation of the contact between the Curdsville Member and Logana Member of the

Lexington Limestone. The lower portion of the Logana Member is marked by repeated carbonate tempestites involving undulate bedded skeletal packstones and undulate bedded wackestones. 58

Table 2. Lithofacies Associations

Association Facies Interpretation Tempestite Association Cpu Deposited on a mid ramp Cwu environment above SWWB by Cmu separate storm events Amalgamated Tempestite Cws Deposited on an inner ramp Association environment above FWWB and subject to constant wave action

Pelagic Rhythmite Association HCr Deposited on an outer ramp Cppu environment below SWWB

Condensed Section Association Cppu Deposited on an outer ramp HCr environment below SWWB; evidence for sea level rise Table 2: Lithofacies Associations and their interpretations in the Logana Member, undifferentiated Lexington Limestone and Point Pleasant Formation. 59

cross-bedded units were described as gently curving low angle cross-laminated that truncated at overlying

laminae which suggest hummocky stratification (Sageman, 1996).

The tempestite lithofacies association occurs in the lower portion the Logana Member, the lower

portion of the undifferentiated Lexington Limestone, and the upper portion of the Point Pleasant

Formation. The presence of the undulate bedded skeletal packstone facies (Cpu) are usually accompanied

by the occurrence of carbonaceous wackestone (Cwu) and/or mudstone (Cmu) facies, which is an

example of a carbonate tempestite sequence found in this core. The tempestites are marked by an erosive

base marked by undulate bedding and deposition of skeletal packstones above. This then transitions into

either carbonaceous wackestones and/or mudstones with undulated bedding.

Amalgamated Tempestite Association

Because of repetitive storm events, storm beds (tempestites) can be seen stacked vertically on top of each other. When the erosion surfaces of different beds are so faint that they appear as a single unit, they are referred to as “amalgamated” (Compton, 1985). These beds are usually thicker and “cleaner” than the ordinary tempestite bed (Dattilo et al., 2008). Instead of representing a single storm event, amalgamated tempestites represent several storm events (Brandt & Elias, 1989). Tempestites found in a mid-shelf environment are usually separated by erosional bases and fine-grained material (background sediment). Tempestites found in a proximal or shoreface environment are usually amalgamated and characterized by a variety of internal laminations (Brant & Elias, 1989).

The thickness of tempestites decreases as they move into deeper water. Thicker successions of tempestite deposits can be found in more proximal facies (Aigner and Reineck, 1982; Brett et al., 1986).

Amalgamated tempestite beds are usually not separated by each event by erosional surfaces and background sediment but consist of continuous beds of complex internal laminations (Brandt & Elias,

1989). The Logana Member in the Prudential #1-A core has several stacked swaly laminated wackestones

(lithofacies Cws) (Figure 26). The swaly laminae present are several mm thick and truncate at the base of 60

Figure 26. Core photograph of the Logana Member at -447.75 m showing swaly laminated wackestones (lithofacies Cws). The swaly bedded wackestones are interpreted to be deposited in the inner ramp environment, above FWWB. 61

overlying laminae. Because the swaly bedded wackestones are not separated by finer-grained deposits,

the stacked beds of repetitive swaly stratified carbonate can continue for intervals > 30 cm thick. Zones

with over 30% swaly laminated wackestones are interpreted to be deposited in an inner ramp

environment.

The swaly bedded limestones are abundant throughout the upper Logana Member. The

amalgamated tempestite facies association is interpreted to be deposited in an inner ramp environment

above FWWB. They are evidence for a change of depositional environment from a mid shelf environment

(below FWWB) of the lower portion the Logana Member to an inner shelf environment (above FWWB).

Hemipelagic Rhythmite Facies Association

Hemipelagic sediments are referred to as being open ocean (pelagic) deposits but also receiving

sediment from adjacent shallow water environments. These sediments are typically fine-grained and well

rounded. The pelagic component includes bioclastic material from the remains of calcareous organisms

such as foraminfera, and coccoliths, and also siliceous remains of radiolarians and diatoms. These

sediments form in the photic zone of the open ocean and are deposited in deeper calmer waters as they

gradually fall out of suspension (Nichols, 2009). The continental sediment component can be derived

from eolian sediment or sediment settling from surface sediment plumes or from turbidites. Siliciclastic

quartz silt in limestones has been well documented and linked to eolian processes (Soreghan and

Soreghan, 2002; Soreghan et al., 2002). According to Soreghan (2002), strong storm events can transport

loess dust (20-70 µm) over 1,000 km (Soreghan et al, 2002).

Hemipelagic sediments can be recognized due to their composition of very fine grained material.

The typical accumulation of this material is calculated at 3-5 mm/ka-1 (Sholle et al., 1983). The rhythmitic bedding is caused by variations in terrigenous input, seasonal changes in biological production, and/or variations in dissolution rates. Millimeter thick lamination is preserved because benthic organisms are excluded due to anoxic conditions or high sedimentation rates (Scholle et al., 1983). 62

Hemipelagic rhythmites occur in the upper portion of the undifferentiated Lexington Limestone

(Figure 26) and extend into the lower portion of the Point Pleasant Formation in central Ohio. The

hemipelagic rhythmites occur above the skeletal grainstones (lithofacies Cgu) and packstone and

phosphorite nodules (lithofacies Cppu) of the undifferentiated Lexington Limestone. The hemipelagic

rhythmites are thinly bedded (0.1 to 1 cm in thickness) and are composed of two distinct lithologies,

carbonaceous wackestones and carbonaceous mudstones. The carbonaceous wackestones are planar

laminated, thinly bedded, and light gray. The mudstones are planar laminated, thinly bedded, dark gray to

black.

These deposits are interpreted to be a hemipelagic rhythmites that were deposited during

highstand systems tracts on the outer shelf in an anoxic environment. The hemipelagic rhythmites overlie

the skeletal packstone with phosphorite nodules which are interpreted to be a condensed section. The shift

of lithologies from the interbedded grainstones, packstones, wackestones, and mudstones of the undifferentiated Lexington Limestone, to a condensed section, to hemipelagic rhythmites suggest sea level rise and shift of environments from inner carbonate shelf to outer carbonate shelf.

Condensed Section Facies Associations

Condensed sections are important because they represent long periods of geological time within one condensed unit. Often, condensed sections mark significant geological events related to tectonic, paleogeographic, and sedimentation rearrangements (Loutit et al., 1988). Condensed section are thin units, often pelagic or hemipelagic sediments recording very low sedimentation rates during the fullest extent of a marine transgression. They are often concurrent with maximum flooding surfaces. Condensed sections often precipitate authigenic minerals such as glauconite, dolomite siderite, or phosphorite as well as authigenic clay minerals (Slat, 2007). They can also be characterized by abundant planktonic or benthic microfossils and wind-blown sediments. Condensed sections are important because they can tie the temporal stratigraphic framework of the distal marine successions to the proximal landward 63

successions (Loutit et al., 1988). A rapid sea level rise can bring the transformation of a once inner shelf environment to a outer shelf environment (Baraboskkin, 2009).

Condensed sections are found at two locations in the core. The first is located in the upper portion of the undifferentiated Lexington Limestone (-439.7 m). The second is located at the top of the Point

Pleasant Formation (-410.9). Both occurrences are interpreted as a shift to a deeper depositonal setting.

The condensed section in the undifferentiated Lexington Limestone marks the transition from the medium, undulate bedded grainstone (lithofacies Cgu) into a planar laminated rhythmite facies

(lithofacies Cr) of carbonaceous mudstones and wackestones (Figure 27). This condensed section

(lithofacies Cppu) is characterized by well rounded francolite nodules and shell hash matrix.

The top of the Point Pleasant Formation is also distinguished by an abrupt shift in lithofacies and depositional environments similar to that of the upper portion of the undifferentiated Lexington

Limestone. The top of the Point Pleasant Formation is marked by several carbonate tempestites overlain by a condensed section that is abruptly overlain by the Utica Shale, which is composed of planar laminated carbonaceous mudstones (Figure 28). The condensed section is characterized by francolite noduels in a shell hash matrix. Pyrite may have replaced glauconite nodules in the condensed section

(Figures 22, 23). The condensed section marks an environmental shift from mid ramp environment (as indicated by the influence of storm wave activity on their deposition) to outer ramp and basinal environments. 64

Figure 27. Inerpretation of a portion of the undifferentiated Lexington Limestone sharing the relationship of a condensed section and pelagic rhythmites. 65

Figure 28. Interpretation of a portion of the Point Pleasant Formation and overlying the Utica Shale, sharing the relationship of a condensed section marking the abrupt transition to the Utica Shale and change of depositional environments from mid ramp to basin. 66

Depositonal Environments

The Logana Member, undifferentiated Lexington Limestone and the Point Pleasant Formation

were deposited in a carbonate ramp environment. There is evidence for proximal to distal shifting of

environments in the upper Lexington Limestone and Point Pleasant Formation. The evidence for this in

the core is the shift from a mid ramp environment (Logana Member) to an inner ramp environment

(undifferentiated Lexington Limestone) to an outer to mid ramp environment (Point Pleasant Formation)

and to a basinal environment (Utica Shale) (Figure 29).

The Logana Member consists of repetitive, undulate bedded skeletal packstones, wackstones, and

mudstones. These are all interpreted to be tempestites that were deposited above storm weather wave base

(SWWB) on a mid ramp environment. Above these skeletal tempestites are amalgamated, swaly

laminated wackestones interpreted to be deposited above FWWB in an inner ramp envirnonment

(Burchette and Wright, 1992).

In the undifferentiated Lexington Limestone, thicker skeletal packstones and grainstones are present which indicates a more proximal depositional setting on an inner ramp environment. These thicker beds are attributed to winnowing of carbonate mud from the skeletal debris from storm waves which is supported by the undulated surfaces of the skeletal packstones and grainstones (Datillo et al.,

2012). The depositional environment then shifted from mid ramp to outer ramp environment and is marked by condensed section abundant in phosphate nodules (Glenn et al., 1993). Above the condensed section, the Lexington Limestone shifts to pelagic rhythmite facies of carbonate mudstones and wackestones (Burchette & Wright, 1992).

The base of the Point Pleasant Formation is marked by pelagic rhythmites of carbonate mudstones and wackestones indicating an outer ramp environment below storm wave base. The upper portion of the Point Pleasant Formation is characterized by few thin skeletal packstones and wackestone that are interpreted to be a mid ramp environment just above storm wave base (Burchette & Wright,1992). 67

Figure 29. Carbonate ramp with lithofacies associations and their interpreted depositional environments.

Note: FWWB= Fair Weather Wave Base; SWWB= Storm Weather Wave Base

68

The transition from Point Pleasant Formation to the Utica Shale is abrupt and is marked by a

condensed section that is characterized by bioturbated skeletal packstone with a high amount of phosphate

nodules (Glenn et al., 1992). Above this condensed section are the mudstones of the Utica Shale which

are interpreted to be deposited in a basinal environment.

Diagenesis

The Prudential #1-A core has undergone diagenetic processes that have modified its original

lithology characteristics. The natural fractures that occur in this core are just one of the examples of

diagenetic effects. Others included calcite cementation, pyrite precipitation, and mudstone compaction.

The initial pore water chemistry of the sediments in the upper Lexington Limestone and Point Pleasant

Formation were important for setting the stage for diagenesis.

Rock Matrix

The composition of the rock matrix in the Upper Lexington Limestone and Point Pleasant

Formation are greatly influenced by deposition of skeletal debris. Tempestites (produced by storm

activity) greatly influenced the amount of calcium carbonate introduced in to the organic-rich carbonate mud. An analysis of 14 samples taken from the Logana Member and undifferentiated Lexington

Limestone was done by Weatherford Laboratories International. Carbonates make up an average 42.79% of the rock mineralogy, siliciclastic clays make up 26.36%, and 30.86% comes from other minerals including quartz.

Lithofacies in the upper Lexington Limestone and Point Pleasant Formation have undergone calcite cementation in the natural fractures and in the rock matrix as well (Table 3). Carbonate lithofacies

Cgu, Cpu, Cppu, Cws, have all gone cementation during diagenesis in this core. Calcite cement can be seen in thin sections to have precipitated in void spaces created by shell debris. In the natural fractures, the calcite cement begins to precipitate along the rock wall first and then infilling towards the center with each pulse of fluid migration. 69

Isopachous blocky cements found in the core are present in the natural fractures and the

condensed sections. The isopachous blocky cements (Figure 30 & 31) infill the natural fractures and are

subhedral in shape in the natural fractures and also in the condensed sections. The isopachous blocky

cements are interpreted to be formed prior to the generation and migration of hydrocarbons. Mosaic

calcite cement is defined as the last cementing event in the natural fractures because they infill around the

blocky cements (Figures 32 & 33). This cement is also interpreted as fast precipitating due to its size.

The rock matrix of the Prudential #1-A core in Marion County, Ohio has been highly cemented

with Chalcedony cement found in the carbonate grainstones of the undifferentiated Lexington Limestone.

Here chalcedony is replacing brachiopod shells found in the skeletal carbonate grainstones (Figure 34).

Chalcedony is siliceous cement and is known to replace calcium carbonate fossils in other limestone

formations without adding any siliceous cement to the rock matrix itself. Lovering and Patten (1962),

theorized that a cold, neutral solutions supersaturated with silica and without Na-ions are capable of

traveling long distances through carbonate rocks until contact with CO2 or Na-ions in which they will precipitate silica. The source of most silica in other limestones, such as the Edwards Limestone in Texas, is from deposition of spicules of siliceous sponges (Pittman, 1959).

70

Table 3. Table of Cements

Mineral Crystal Morphology Example Implication Calcite Meniscius or Pendant Not observed Freshwater Vadose Zone cements

Calcite Isopachous bladed Not observed Freshwater Phreatic Cement Zone

Calcite Isopachous Micrite Not observed Early Marine Cements Rims

Calcite Isopachous Blocky Figure 30, 31 Marine Phreatic Zone Cement

Calcite Mosaic Calcite Cement Figures 32 & 33 Burial Diagenesis Silica Fibrous Chalcedony Figure 34 Silica Inversion Pyrite Framboidal Figure 35 Sulfate Reduction Dolomite Rhombic Not observed Dolomitization

Table 3. List of all cements present in the study interval. Examples of these cements can be found in the figures listed under the column “Examples”. 71

b

Figure 30. Photomicrograph of natural fracture calcite cement in the Logana Member at -447.3 m. Isopachous blocky calcite (b) cements along fracture wall.

B 72

b B

Figure 31. Photomicrograph of calcite cement in the Point Pleasant Formation at -410.41 m. Isopachous blocky cement (b) infilled porous shell hash matrix in condensed section at the top of the Point Pleasant Formation.

73

m M

Figure 32. Photomicrograph of calcite cement in a natural fracture in the undifferentiated Lexington Limestone -440.7 m. Calcite cement is mosaic cement (m) forming in the middle of a natural fracture. 74

m

0.1 cm

Figure 33. Photomicrograph of Logana Member at -446.9 m showing mosaic calcite cement (m) fill.

75

Ch Ch

Figure 34. Photomicrograph of the undifferentiated Lexington Limestone (-441.9 m) showing the mineral chalcedony (Ch) replacing calcium carbonate of brachiopod shell in a skeletal grainstone (lithofacies

Cgu).

76

P

0.1 cm

Figure 35. Cathodoluminescence image of pyrite forming in natural fracture in undifferentiated Lexington Limestone -440.7 m. Pyrite (P) has formed due to the migration of hydrocarbons through the middle of the calcite healed natural fracture. 77

Total Organic Carbon

The Logana Member and the Lexington Limestone Undifferentiated have higher amounts of TOC than the Point Pleasant Formation in the Prudential #1-A core in Marion County, Ohio. Changes in TOC from the top of the Curdsville Member through the base of the Utica Shale can be seen in Appendix C 1.

Natural Fractures were also found in zones with higher TOC than zones with lower amounts. The average amount of TOC in the Logana Member is 4.1%. The average amount of TOC in the Lexington Limestone

Undifferentiated is 4.0% and the average for the Point Pleasant Formation is 2.0%. Moving upward through the core into the Utica Shale the amount of TOC varies between 1% and 3%.

TOC also decreases with the abrupt change from alternating layers of carbonaceous calcareous mudstones/wackestones and skeletal packstones/grainstones in the upper portion of the Lexington

Limestone Undifferentiated. Moving up section into the Point Pleasant Formation, TOC decreases as the carbonaceous calcareous mudstones and wackstones become bioturbated. Organic matter from the

Prudential #1-A consists of type I, and type II kerogen, and has entered the oil generation window.

Natural Fracture Descriptions

This study only looked at healed or partially healed fractures to avoid any fractures produced as a result of coring operations. All of the healed natural fractures in the Prudential #1-A core, from the

Logana Member of the Lexington Limestone to the base of the Utica Shale, are cemented with calcite.

Several of the natural fractures studied have been cemented with dolomite or pyrite. Several of the fractures have visible (using a handlens or light microscope) pyrite in the middle of calcite-cemented healed natural fractures and in the rock matrix adjacent to the fracture wall. All of the fractures have vertical orientation. This paper did not study any micro fractures (µm sized) that are in the study interval.

A table of the recorded natural fractures can be viewed in Table B 1.

In the Prudential #1-A core, between the depths of -453.8 to -438.8 m (below surface), there are a total of 62 healed natural fractures. This generates an average occurrence of 4.1 fractures per vertical 78

meter. There were 37 (approximately 55%) fractures occurring in the Logana Member, there were 29

(around 44%) fractures occurring in the undifferentiated Lexington Limestone, with only about 1%

(single fracture) occurring in the Point Pleasant Formation (Figure 36&37). The fractures range in width from 1 mm to 3 cm. The fracture lengths range from 2cm to 37.5cm, depending on the orientation of the fracture with respect to the long direction of the core. Some of the fractures terminate as lithologies change from skeletal grainstones to mudstones (Figure 38). When the natural fractures cross lithological boundaries, there are changes in fracture orientation (fracture refraction) (Figure 39).

Most of the natural fractures have been healed by multiple episodes of calcite cementation, based on cathodoluminescence analysis (Figure 40). The different colors given off by the calcite cements during cathodoluminescence separates each cementation event from each other. This is primarily because the calcites would have to be precipitated from different fluids with different chemical compositions. Other healed natural fractures remain partially open and are characterized by one episode of calcite cementation on that void spaces in the center of the fracture (Figure 41). Several natural fractures are healed by multiple episodes of calcite and pyrite precipitation in the center of the natural fracture. Pyrite typically precipitated in the middle of healed natural fractures (Figure 42). A single natural fracture near the base of the Logana Member contains bitumen precipitation in the middle of a natural fracture which has calcite cementation on either side (Figure 43).

The rate of calcite precipitation is dependent on the thickness of the natural fracture. Thinner natural fractures appear to have been filled by one episode of calcite cementation while wider natural fractures have been filled by two to three episodes of calcite cementation. The order of calcite cementation can be seen in Figure 40 as three different episodes of calcite cementation are occurring.

Event C1 consists of small blocky calcite crystals precipitating along the fracture wall. The crystals are euhedral and are spread out along each side of the fracture wall and are not connected. This is considered to be evidence for slow crystal growth inside a fluid filled fracture (Nollet et al., 2005). 79

Figure 36. Stratigraphic column of the Point Pleasant Formation with the occurrence of the single calcite healed natural fracture occurring in undulated skeletal packstones and wackestone beds. 80

Figure 37. Natural fracture occurring in the Point Pleasant Formation in an undulated skeletal packstone and wackstone bed; fracture ends at contact with overlying and underlying mudstones.

81

Figure 38. Stratigraphic column of the undifferentiated Lexington Limestone showing natural fractures (red lines) in the grainstones truncating at the facies contact of the carbonaceous mudstones.

82

Figure 39. Stratigraphic column showing calcite healed natural fractures refracting in mudstone layers. 83

The fracture cements have been labeled C1, C2, and C3. Event C1 is characterized by a dull

cathodoluminescence (CL) isopachous blocky calcite. This dull CL calcite is interpreted to of precipitated

from fluid with low amount of Mn2+ or higher amounts of impurities. Due to the euhedral shape of the calcite crystals in C1, it is also interpreted to of formed in a fluid filled fracture.

Event C2 consists of larger isopachous blocky calcite crystals precipitating along the fracture wall. It forms beside event C1 and cannot be distinguished as precipitating before or after. Event C2 is a light orange CL euhedral calcite crystal and is not uniformly precipitating along the fracture wall but appears as single crystals. This is evidence for slow crystallization and precipitation in a fluid fill fracture

(Nollet et al., 2005).

Event C3 consists of mosaic calcite cement that has precipitated between events C1 and C2. This can therefore be assigned as the third cementing event as it is forming in the middle of the fracture after events C1 and C2. Event C3 is a darker orange CL mosaic calcite and is evidence for fast precipitation within the natural fracture. Fluid composition is different for all three cementing events as the CL color for each event is different coloration.

In the lower portion of the Logana Member, a bitumen blep is found in a calcite healed natural fracture. In the fracture, the bitumen blep is present in the middle of the fracture with calcite forming along both sides of the fracture wall. This is evidence for hydrocarbon migration after calcite cement was precipitated.

Pyrite is also found in the natural fractures throughout the core. The pyrite precipitation in the core is attributed to a sulfate reducing environment during hydrocarbon migration through these natural fractures. Pyrite is interpreted as precipitating last because it is found in the middle of the natural fractures.

84

C3

C2 C1

0.1 cm

Figure 40. Cathodoluminescence image of undifferentiated Lexington Limestone (-439.3 m) fracture sample displaying three different slow episodes of calcite precipitation. The first two are forming along the fracture wall, represented by dull CL subhedral blocky calcite (C1) and light orange CL euhedral blocky calcite crystals (C2) Final episode of subhedral calcite precipitation is represented by dark orange CL mosaic calcite (C3).

85

C2

v

0.1 cm

Figure 41. Cathodoluminescence image of the Logana Member healed natural fracture sample at -446.9 m displaying one episode of calcite cementation (C2) identifiable by the light orange CL calcite and open void space (v) in the center of the natural fracture.

86

p

C3

C1

0.1 cm

Figure 42. Cathodoluminescence image of Logana Shale Member healed natural fracture sample at (- 447.3 m) displaying two episodes of calcite cementation and final forming pyrite in middle of healed natural fracture. First episode of calcite cement is portrayed by the dull CL calcite along the fracture wall (C1). Second episode of calcite cementation is represented by the lighter red CL calcite (C2). Final episode is pyrite framboids forming in void spaces of natural fracture and is represented by the opaque spot in the center of the photomicrograph (p).

87

b

Figure 43. Core photo of the Logana Member at -449.5 m showing calcite forming around a bitumen bleb (b). 88

Paragenesis

Paragenesis of the Logana Member, undifferentiated Lexington Limestone and Point Pleasant

Formation in the Prudential #1-A core in Marion County, Ohio was found by using core descriptions, thin

section microscopy, cathodoluminescence and EDAX for chemical compositions. A table (Table 4) was

made listing the order of events post deposition. The events, listed in Table 4, will now be described in

greater detail.

Mottling, created by bioturbation, is the first event and was created post-deposition of the

sediment. Organisms living on the sea floor burrowed and disrupted the sediment creating the mottling

characteristics. Bioturbation can be caused by gastropods and ostracodes that live in a variety of different

carbonate ramp environments including proximal inner ramp and distal outer ramp (Rogov et al., 2012).

The bioturbation in the Logana Member was most likely caused by marine organisms such as gastropods

and ostracodes, due to the fact that fossil assemblages of both are found throughout the core. The

environment for which these organisms lived can be either inner ramp or outer shelf depending on

different oxygen levels.

The physical reworking of sediment during burial is the second sequence in paragenesis. The

physical reworking of the sediments found in the core is cause by storm events during the Late

Ordovician. Repeated storm events have caused the repeated stacking of undulate bedded grainstones,

packstones, wackestones, and mudstones (Lithofacies Cgu, Cpu, Cwu, and Cmu) throughout the core.

The presence of tempestites allows for the identification of deposition above storm weather wave base

(Aigner and Reineck, 1982; Brett et al., 1986; Brandt & Elias, 1993).

Third on the paragenesis sequence is the formation of authigenic minerals. These minerals are

primarily found in the condensed sections (Lithofacies Cppu). The authigenic minerals found in the condensed sections are fracolite and glauconite nodules which are confirmed using EDAX. These 89

Table 4: Paragenesis Sequences

Sequence Process Evidence Implication 1 Bioturbation Mottling Minor, implies rapid deposition rate or anoxic bottom water 2 Physical reworking Storm Deposits Erosion and Deposition above storm weather wave base (SWWB) 3 Authigenesis Fracolite Nodules Chemical diagenesis on sea floor (condensed section) 4 Early calcite Inversion of aragonite Loss of primary cementation (rock to calcite porosity matrix) 5 Silica diagenesis Chert Nodules Early burial diagenesis

6 Pyrite replacement of Condensed sections Early burial diagenesis glauconite glauconite nodules

7 Mechanical compaction Tight packing of Loss of % porosity bioclast, virtual absence of shelter porosity 8 Fracture formation Vertical fractures Probable initiate in granular overpressuring (Fluid layers and escape through refract/terminate in consolidating units) cohesive layers 9 Burial Calcite Multiple phases of 9a slow growing calcite Cementation (fracture) calcite cementation crystal in fluid filled In fractures fracture

9a bladed calcite 9b slow growing calcite crystals in fluid filled 9b blocky calcite fracture

9c mosaic calcite 9c rapid growing calcite cement infilling fracture and around 9a and 9b cements 10 Organic matter Bitumen blebs Early burial diagenesis diagenesis 90

Sequence Process Evidence Implication 11 Oil Migration Residue on calcite and Natural fractures pyrite in fractures facilitated oil migration 12 Pyrite precipitation Lining on calcite in Burial diagenesis fractures

Table 4: Paragenesis of the Logana Member, undifferentiated Lexington Limestone (Lexington Limestone) and Point Pleasant Formation in the Prudential #1-A core in Marion County, Ohio. 91

fracolite and glauconite nodules formed on the sea floor during periods of very low sedimentation and mark the transition from an inner ramp environment to an outer ramp environment dominated by hemipelagic rhythmites. The authigenic minerals formed were precipitated at the sediment-water contact on the sea floor during periods of low deposition (Glen et al., 1993; Baraboshkin, 2009; Nichols, 2009).

Early calcite cementation is identified to be sequence event number 4 in paragenesis. The mosaic calcite cement occurring in the middle of an ostracode shell (Figure 34) is interpreted to be an early cementing event during diagenesis due to the fact that the porosity created by the ostracode shell would have been lost during compaction.

Silica diagenesis has occurred in the undifferentiated Lexington Limestone and is marked by the presence of chalcedony that has replaced brachiopod shells in a skeletal grainstone (Lithofacies Cgu). The transformation of shallow water carbonate sediments to siliceous cements has been a question. Some believe that the source of silica-rich pore fluid is from the dissolution of spicules from sponges but there has been much controversy if this is enough to precipitate silica cement (Dapples, 1959). For silicification during intermediate stages of diagenesis, the combined pressure solution of quartz in the sediment as well as the smectite to illite transformation in shales have been a possible silica source along with biogenic silica (Hesse, 1987).

Sequence #6 is the replacement of glauconite with pyrite. This is suggested from the presence of pyrite nodules in the condensed sections. Glauconite is a common authingenic mineral and the iron present in glauconite allowed for the replacement to take place in a reducing environment.

Natural fracture formation occurred after mechanical compaction of the carbonate sediments. The occurrence and propagation of natural fractures is dependent on the bed thickness and homogeneity of the lithofacies present. Natural fractures formed in the lower portion of the undifferentiated Lexington

Limestone are much shorter in comparison to those formed in the upper portion of the Logana Member.

The undifferentiated Lexington Limestone fractures are short, approximately 5-10 cm in length and 92

mostly occur in granular beds such as the skeletal grainstones and packstones (Lithofacies Cgu & Cpu)

and terminate at the contacts of wackestones and mudstones (Lithofacies Cwu & Cmu). The lower

portion of the undifferentiated Lexington Limestone is also very heterogeneous and is characterized by

thin beds of packstones, wackestones, and mudstones.

Natural fractures occurring in the upper portion of the Logana Member are much longer (30 cm)

and occur primarily in the swaly laminated wackestones. The upper portion of the Logana Member is

very homogeneous compared to the undifferentiated Lexington Limestone and is mostly composed of the

swaly laminated wackestones (Lithofacies Cws).

Next in the paragenesis sequence is the burial diagenesis of calcite cement. In the natural

fractures, there are at least three separate cementing events. These separate cementing events were based

on the cathodoluminescence colors given off by the calcite crystals. The three main cementing events are

distinguished by dull CL calcite, light orange CL calcite, and dark orange CL calcite. The first cementing

event (C1) is characterized by dull CL isopachous blocky cement. This cement is interpreted as the first cementing event because it is forming along the fracture wall. Cement C1 is interpreted to be a slow

precipitating calcite that has precipitated in a fluid-filled fracture due to the euhedral shape of the calcite crystals and crystals are not uniformly precipitating along the wall. Cementing event C2 is light orange

CL isopachous blocky calcite cement and has also precipitated along the fracture wall. Therefore, the timing of cement C1 or C2 cannot be determined in relation to each other by using cathodoluminescence alone. To date when either cementing event would of occurred would include the use of fluid inclusion methodology. Cement C2 is also euhedral in shape and is also interpreted to be slow growing in a fluid filled fracture. Cementing event C3 is the final calcite cementing event and is characterized by dark orange CL mosaic calcite cement and infills the remaining void space around events C1 and C2 in the middle of the fracture. 93

Organic fracture formation and cementation, organic matter in the carbonate rocks underwent oil generation and then migration. This is supported by the presence of bitumen in one the healed natural fractures in the Logana Member. A bitumen blep can be seen in the middle of the calcite cement in the natural fracture. No oil was present during precipitation of the calcite and is supported by the fact there is no evidence for oil inclusions in the calcite cement.

After oil generation and migration, pyrite precipitated in the middle of the partially healed natural fractures. This is due to the hydrocarbon migration through the natural fractures. The major source of sulfide to produce the pyrite precipitation is from the microbial break down of the hydrocarbon which produced hydrogen-sulfide gas (Schumacher, 1996).

94

DISCUSSION

Depositional Environments and Lithofacies

Lithofacies of the Upper Lexington Limestone and Point Pleasant Formation are interpreted to be deposited in a carbonate ramp environment in central Ohio during the late Ordovician (Figure 44). The

Logana Member and undifferentiated Lexington Limestone are characterized by undulated skeletal packstones and grainstones interbedded with undulated carbonate mudstones and wackestones. The tempestites of the Logana Member are interpreted to be deposited in an inner to mid ramp environment.

The lower portion of the Logana Member is marked by repetitive tempestites and interpreted to be deposited in an outer ramp environment above SWWB. The upper portion of the Logana Member is marked by amalgamated tempestites which are characterized by the swaly bedded wackestones

(Lithofacies Cws). This is interpreted to be deposited in an inner ramp environment above FWWB

(Brandt & Elias, 1989).

The undifferentiated Lexington Limestone is marked by thicker beds of skeletal packstones and grainstones (Lithofacies Cgu & Cpu) which are interpreted carbonate tempestites deposited in an inner ramp environment and subject to winnowing processes removing carbonate mud from skeletal packstones and grainstones (Dittela et al., 2012). These grainstones and packstones are interbedded with udulate bedded wackestones and mudstones which indicate storm erosion. The thicker tempestites beds give evidence that it was deposited in an inner ramp environment rather than an outer ramp due to the fact that most tempestites usually decrease in thickness in deeper water (Aigner and Reineck, 1982; Brett et al.,

1986). The Lexington Limestone then transitions to pelagic rhythmites interpreted to be an outer carbonate ramp environment (Burchette and Wright, 1992). This transition is marked by a condensed section above the skeletal tempestites which is characterized by a skeletal packstone and phosphate nodules (Glenn et al., 1993). The pelagic rhythmites consist of interbedded, planar laminated carbonaceous mudstones and carbonaceous wackestones deposited below SWWB.

95

Figure 44. Interpreted depositional environment for the Logana Member, undifferentiated Lexington Limestone and Point Pleasant Formation in central Ohio. 96

The Point Pleasant Formation is marked by pelagic rhythmites deposited in an outer ramp

environment moving upwards into small undulated skeletal packstones and wackestones interpreted to be

deposited in a mid ramp environment above SWWB. The transition from the Point Pleasant Formation to

the Utica Shale is abrupt and marked by a condensed section containing phosphate nodules (Glenn et al.,

1993).

Stratigraphic Trends

The upper Lexington Limestone and Point Pleasant Formation is marked by several

depositional environmental shifts from mid to inner ramp environments above FWWB to outer ramp environments below SWWB. This transition can be seen several times throughout the study interval in the core. Other authors have indicated that the early Late Ordovician was subject to sea level rise and fall due to glacioeustasy, which will be discussed later in greater detail (Elrick et al., 2013).

Below the Logana Member is the Curdsville Member which is described as over 9 m of fossiliferous crystalline Limestone (Black et al., 1965). The Curdsville Member is interpreted as part of a transgressive systems tract (TST) (Patchen et al., 2006). In the Logana Member, the shift from mid ramp

to inner ramp is seen by the change from mid ramp tempestites to inner ramp amalgamated tempestite

associations. The mid ramp tempestites are characterized by tempestites of thin, undulate bedded skeletal

packstones (Lithofacies Cpu) separated by thicker beds of undulate bedded wackestones and mudstones

(Lithofacies Cwu & Cmu). The Logana Member transitions upward to an inner ramp environment

characterized by amalgamated tempestite associations of ~30 cm swaly bedded wackestones (Lithofacies

Cws). This is viewed as a progradation of the subtidal carbonate factory. Tempestites decrease in

thickness as water depth increases away from the carbonate platform (Aigner and Reineck, 1982; Brett

et al., 1986; Brandt & Elias, 1993).

The transition into the undifferentiated Lexington Limestone marks continuing sea level rise as

the lithofacies change from swaly bedded tempestites to thicker, udulate bedded tempestites. The thicker 97

Figure 45. Stratigraphic trends of the Logana Member, undifferentiated Lexington Limestone, and Point

Pleasant Formation in central Ohio. Note: SL=change in sea level 98

undulate bedded tempesites are represented by undulate bedded skeletal grainstones and packstones

(Lithofacies Cgu & Cpu). The thickness of these beds is evidence for deposition in a proximal

environment/inner ramp due to the fact that tempestites usually decrease in thickness as water depth

increases (Aigner and Reineck, 1982; Brett et al., 1986; Brandt & Elias, 1993).

The undifferentiated Lexington Limestone then transitions from thick tempestite deposits to pelagic rhythmites, which is marked by a condensed section. The condensed section is undulate bedded and composed of phosphate nodule in a shell hash matrix (Lithofacies Cppu) and lies above the undulate bedded skeletal grainstones (Lithofacies Cpu). This condensed section was formed due to sediment starvation during sea level rise allowing for authigenic minerals to form on its surface (Glen et al., 1993;

Baraboshkin, 2009; Nichols, 2009).

Above this condensed section are planar laminated pelagic rhythmites consisting of carbonate mudstones and wackestones interpreted to be deposited on an outer shelf environment below SWWB

(Burchette and Wright, 1988; Nichols, 2009). The planar laminated pelagic rhythmites (Lithofacies HCr) are made up of repetitive layers of thin bedded wackestones and mudstones. The pelagic rhythmites are interpreted as deep water deposits and evidence for sea level rise from the inner shelf to outer shelf

(Hattin, 1987; Nichols, 2009; Burchette and Wright, 1988).

The pelagic rhythmites continue into the overlying Point Pleasant Formation and transitions to undulated skeletal tempestites in the upper portion of the Point Pleasant Formation. The skeletal tempestites are composed of undulate bedded skeletal packstones, wackestones, and mudstones

(Lithofacies Cpu, Cwu, and Cmu). This represents a shift from an outer carbonate ramp environment below SWWB to a mid ramp environment above SWWB (Burchette & Wright, 1988; Aigner and

Reineck, 1982; Brett et al., 1986; Brandt & Elias, 1993).

The top of the Point Pleasant Formation is characterized by a condensed section which marks the transition to the Utica Shale. The condensed section is composed of phosphorite nodules and pyrite which 99

may have replaced glaconite nodules during early diagenesis in the lithofacies Cppu. This condensed section is evidence for sea level rise and sediment starvation on the sea floor (Glen et al., 1993;

Baraboshkin, 2009; Nichols, 2009). The Utica Shale is marked by planar laminated, clay-rich shale interpreted to be deposited in a basinal environment.

Causes for Sea Level Change

The causes for sea level fluctuations during the Late Ordovician have been studied by many authors. Elrick et al., (2013) attributes the changes in sea level in the Lexington Limestone of Kentucky to orbital scale climatic fluctuations which resulted in the growth and melting of continental glaciers. The growth and melting of glaciers during the Late Ordovician led to sea level changes and development of widespread marine sedimentary cycles. This study tested changes in δ18O values of conodont apatite and found that δ18O changes from 0.7%-2.5% were due to changes in ice volume (Elrick et al., 2013). Other authors have studied sea level changes in the Late Ordovician and also concluded that lithological and oceanic changes are attributed to the onset of icehouse conditions which allowed for glaciers to form

(Lavoie, 1995; Pope and Read, 1997; Lavoie and Asselin, 1998). This theory of glaciers is refuted by

Herrmann and Haupt (2010) as they refer to the Late Ordovician as a transition period from hot-house to ice-house, until any glacial deposits are found.

TOC

Total organic carbon (TOC) also changes throughout the Prudential #1-A core. TOC for the

Logana Member and undifferentiated Lexington Limestone is higher than that of the Point Pleasant

Formation. This decrease in TOC begins in the undifferentiated Lexington Limestone with the shift of the skeletal packstones and grainstones to the pelagic rhythmites which is also a depositional environment shift from a mid ramp environment to an outer ramp environment. This could imply upwelling (in mid- ramp) due to more biological activity (Hiatt & Budd, 2003). TOC in the Point Pleasant Formations averages around 2% while the Logana Shale and Lexington Limestone Undifferentiated average 4% for 100

both. This is due to the overall carbonate production with respect to water depth. On a carbonate ramp environment, carbonate production decreases with increased water depth while in shallow waters carbonate production is greater (Badenas and Aurell, 2001). The organic matter produced in the shallow water carbonate ramps are dependent on these “carbonate factories” to have high production rates in order to be preserved.

The lithofacies of the upper Lexington Limestone are significantly higher in abundance of skeletal packstones and grainstones. These lithofacies are thin in the Logana Member but are thickest in the undifferentiated Lexington Limestone. The Point Pleasant Formation is marked by few beds of skeletal packstones and wackestones near the top of the Formation. The thicker packstone and grainstone beds in the undifferentiated Lexington Limestone can be attributed to more proximal environments and mud winnowing by storm waves (Dittalo et al., 2012).

Diagenesis

The upper Lexington Limestone and Point Pleasant Formation have been subject to many different diagenetic processes post burial. The order in which these events occurred can be interpreted from the different sedimentary structures, minerals present, and different cements found in the natural factures. The diagenetic processes have been separated between early, middle, and late in the following paragraphs.

Early Diagenesis

Processes that occurred during early diagenesis are bioturbation and reworking of the sediment on the sea floor. These two events are distinguished by the mottling characteristics of the wackestones that are occurring throughout the core. Burrows are present and disruption of the sediment is evidence for bioturbation. Sediment reworking is present throughout the core and is due to storm events that have scoured the sea floor and winnowed any mud from grainstones occurring in the undifferentiated

Lexington Limestone (Burchette & Wright, 1992; Dattilo et al., 2012; Sageman, 1996). 101

Calcite cementation in the rock matrix is also interpreted as an early diagenetic process. Calcite cementation can occur shortly after burial of the sediments in a marine environment. The chemical composition of the cement is related to the pore water chemistry and also the overlying sea water chemistry. The precipitation of high magnesium calcite is mostly likely due to low pore water calcium (Thorstenson and Mackenzie, 1974; Gaillard et al., 1986). In oxygen deficient environments, the chemical diagenesis of the carbonate may be related to the sulfate reduction of organic rich sediments, which can increase alkalinity which further can precipitate calcium carbonate (Berner, 1971).

The appearance of chalcedony and pyrite in the rock matrix is also interpreted to of taken place during early diagenesis. The replacement of brachiopod shells with chalcedony is most likely to siliceous material originating from the quartz silt found throughout the core but also could be from biogenic silica such as those found in sponge spicules (Dapples, 1959). Pyrite forming in the rock matrix and also the replacement of glauconite in the condensed sections in interpreted as an early diagenetic process. Pyrite is an iron mineral that is known to form shortly after burial of the sediments due to the presence of dissolved sulfide produced by microbial sulfate reduction and reaction with iron from different minerals present

(Berner, 1970, 1984). The replacement of glauconite nodules (condensed sections) with pyrite is also interpreted as an early diagenetic process (Rabenhorst et al., 1989).

Middle Diagenesis

Compaction of the carbonate sediments is interpreted to occur during middle diagenesis.

The compaction of the carbonate sediments allowed for the lithification and consolidation of the sediments allowing for brittle deformation and natural fracture formation. The calcite healed natural fractures associated with the Prudential #1-A core in Marion County, Ohio are primarily occurring in the

Logana Member and the undifferentiated Lexington Limestone in beds of undulated skeletal packstones and grainstones. In the core, 63 of the total 64 natural fractures occur in the Logana Shale and 102

undifferentiated Lexington Limestone and one calcite healed natural fracture occurs in the Point Pleasant

Formation.

The relationship of natural fractures and zones higher in carbonate material can be related to the brittleness of the rock and the tensile stress that is placed on that rock allowing it to fracture. Bed thickness was also a controlling factor on the lengths of the natural fractures in the core. In the undifferentiated Lexington Limestone, varying lithofacies and thicknesses caused natural fractures to terminate at the contact of the carbonaceous carbonate mudstones. Each of these lithofacies have different brittleness associated with them. The carbonate mudstones would be the least brittle while carbonate grainstones would be the most brittle.

The calcite cement present in the natural fractures of the Prudential #1-A core display evidence of zoning which indicates differences in fluid composition in different cementing events. Cements found show evidence of up to three cementing events in wider natural fractures while thin natural fractures show evidence of only one cementing event. The separate cementing events are distinguished through cathodoluminescence based on different crystal colors. Different colors are representative of different fluids flowing through the natural fractures with different chemical compositions.

Late Diagenesis

Geochemical data places the upper Lexington Limestone and Point Pleasant Formation in the early oil generation window. Bitumen is also found in the middle of calcite healed natural fractures in the

Logana Member (Figure 42). Hydrocarbon migration in this case allowed for reduced sulphide rich pore fluid causing the pyrite to form in the middle of the fractures and also in the surrounding rock matrix.

Moller and Friis, (1999), attributed the precipitation of pyrite and hydrocarbon flow through natural fractures in the Cambrian sandstones in Denmark.

Pyrite is also a common precipitating mineral that is present in many of the natural fractures of the upper Lexington Limestone. Pyrite usually occurs in the middle of the natural fractures surrounded by 103

calcite cement. Pyrite is also present in the rock matrix surrounding the calcite healed natural fracture.

The precipitation of pyrite if found closely associated with the natural fractures. Comparatively much less

pyrite is found away from the natural fractures but increases in close proximity to the natural fracuture

(Figure 34&41).

Paragenesis

The ordering of events both during deposition and post deposition (diagenesis) have been

interpreted and placed in the order for which they occurred. The first event to be recognized is mottling

which has been created due to bioturbation of the sediment as it was deposited. The reworking of these

sediments was most likely done by gastropods and ostrocods because the shells of these organisms are

present throughout the study interval (Burchette & Wright, 1992).

Second event is reworking of the sediments. Sediment reworking is found throughout the core

interval and is attributed to storm activity which has created undulate bedding and also winnowed some of

the mud away from the shell rich beds, i.e. grainstones and packstones (Lithofacies Cgu and Cpu). The

presence of storm events is also evidence for deposition above SWWB (Lithofacies Cgu, Cpu, Cwu,

Cmu) and above FWWB (Lithofacies Cws) (Dittalo et al., 2012, Aigner and Reineck, 1982; Brett et

al., 1986; Brandt & Elias, 1993).

The third event found in the core is the precipitation of authigenic minerals (francolite and glauconite nodules) on the surface of the sea floor. The francolite and glauconite nodules primarily appear in the condensed sections and formed at the sediment water contact (Lithofacies Cppu). Glauconite nodules are interpreted to have been replaced by pyrite later in diagenesis. Condensed sections are very important because they are evidence for rapid sea level rise and sediment starvation (Loutit et al.,1988;

Vail et al., 1984; Slat, 2007; Glenn et al., 1993; Baraboshkin, 2009). This interpretation is also supported by chemical composition analysis of the nodules present as well as the lithofacies change from a mid ramp environment to an outer ramp environment at both occurrences in the core. 104

Fourth event to occur in the core is the precipitation of early calcite cement. This cement is seen infilling void spaces created by shell debris such as the gastropod and ostracode shells. This is interpreted as an early diagenetic process because the porosity created by the ostracode shell would have been lost during compaction of the sediment. The precipitation of early calcite cement is a common marine process and can take place by internal redistribution of unstable bioclastic material that is dissolved by bacterial oxidation of organic matter (Molenaar & Zijlstra, 1997; Thorstenson and Mackenzie, 1974; Gaillard et al.,

1986).

After the early precipitation of calcite cement, silica replacement occurred in the skeletal grainstones (Lithofacies Cgu) in the form of chalcedony. This replacement mineral most likely originated from the quartz silt found throughout the core but also could be from biogenic silica such as those found in sponge spicules (Dapples, 1959). The occurrence of pyrite in the condensed section is interpreted to have replaced glauconite during early diagenesis. The replacement of glauconite with pyrite is common occurring process and occurs in a reducing environment (Rabenhorst et al., 1989).

Next to occur in paragenesis is the compaction of the sediments. The sediments would have undergone compaction to allow brittle deformation and natural fracture formation in the rock. Stephen

Bourne (2003) relates the brittleness of layered rocks to tensile stress and failures occurring within each rock layer. Bourne states that rock layers can acquire tensile stress if it was bonded to another softer rock during compaction. During compaction, the softer rock layer would increase the layer-parallel elongation of the stiff layer higher than normal. In this case, fractures are able to open and propagate through the entire stiff layer. The propagation of a natural fracture to continue into a different lithofacies is dependent on the elastic properties of that bed (lithofacies). Beds with higher elasticity will not allow for natural fracture propagation to occur.

The natural fractures that are occurring in the Logana Member and undifferentiated Lexington

Limestone are dependent on bed thickness and different elastic properties between rock layers. In the

Logana Member, the natural fractures are much more extensive and longer than compared to those in the 105

undifferentiated Lexington Limestone. The differences in lithofacies thickness and homogeneity are also

a key difference in between the two. The undifferentiated Lexington Limestone is very heterogeneous and

has much shorter bed thicknesses compared to the upper portion of the Logana Member. The fractures

forming in the undifferentiated Lexington Limestone are much shorter and some terminate or change

orientation in the mudstone and some wackestone layers.

The changing of orientations of the natural fractures occurring throughout the core is evidence for

slow natural fracture propagation. This is also evidence for changing orientation of principle stresses and

differences in rock brittleness. Alternating layers of grainstones, wackestones, and mudstones increases

the heterogeneity of the rock and the stresses that created each natural fracture.

Cementation of the natural fractures happened shortly after the natural fractures were formed.

Using cathodoluminescence, at least three different calcite cementation events can be seen in the natural

fractures. Cathodoluminescence of the calcite crystals can identify differences in their chemical

compositions based on the cathodoluminescence color that is given off. Lighter colors are interpreted for

calcite crystals with higher amounts of Mn2+ while calcite crystals with darker or duller colors have lower

amounts of Mn2+ or higher amounts of impurities.

The first cementation event (C1) is characterized by dull CL isopachous blocky calcite crystals.

These crystals are recognized as first forming because they occur along the fracture wall. The isopachous

blocky cements are subhedral and are not continuous along the fracture wall which gives evidence for slow crystal growth.

The second cementing event (C2) overlaps with (C1) due to the fact that C2 has formed in areas along the fracture wall that C1 has not. Therefore, we cannot recognize which cements were precipitated first. C2 is light orange CL isopachous blocky euhedral calcite. This cement is also described as slow growing as the crystals are spread out along the wall and are not continuous.

The third cementing event (C3) has filled the middle portion of the natural fracture in between cementing events C1 and C2, therefore we are interpreting this as the last cementing event. The cement 106

C3 is described as dark red CL mosaic calcite. The mosaic calcite in interpreted as fast crystal growth in the natural fractures due to their size and the fact that they are completely infilling around the cementing events C1 and C2. These cements are viewed as precipitates of separate diagenetic fluids rather than precipitating from sea water due to the fact that their crystal form is more equant rather than “needle like” like most sea water precipitated calcite (Lahann, 1978).

Oil generation and migration has occurred in the Logana Member, undifferentiated Lexington

Limestone and Point Pleasant Formation in central Ohio. Evidence for oil migration can be seen in the natural fracture occurring in the Logana Member where a bitumen bleb is present in an open fracture between calcite that has precipitated on host rock wall. This is evidence for oil generation and migration after the formation and cementation of the natural fractures. According to Rowan, (2006) oil generation of the Utica Shale occurred sometime during the late Devonian. Since the Logana Member is considered part of the same petroleum system, oil generation most likely occurred during the late Devonian as well.

Post oil generation and migration, pyrite has precipitated in the middle of the partially open natural fractures due to the sulfate reduction caused by the migration of hydrocarbons. Pyrite has also formed in the rock matrix surrounding the natural fractures as well. According to Oehler and Sternberg,

(1984) pyrite can be precipitated in a reducing environment when there is an abundance of sulfur and iron. The major source of sulfur in a petroleum reservoir is the hydrogen sulfide gas that is produced from the hydrocarbons itself from anaerobic bacteria activity (Schumacher, 1996). Iron can either be sourced from either the iron oxide coatings in sandstone, chlorite leached from clay, rock inclusions or deeper meteoric waters (Oehler and Sternberg, 1984).

Petroleum Geology

The lithofacies present in the Prudential #1-A core give insights to how they control natural fracture formation during diagenesis. The undifferentiated Lexington Limestone is characterized by much shorter fractures than the Logana Member and is controlled by the higher degree of heterogeneity and shorter bed thickness. The variable fracture orientations suggest that stresses σ2=σ3 and differences in 107

elastic properties of the multiple lithofacies present is responsible for changes in fracture orientations.

This is also evidence for slow growing fractures that refract when the orientation of the principle stresses

change. The Logana Member is characterized by longer more extensive fractures and is very

homogeneous compared to the undifferentiated Lexington Limestone.

These lithofacies, although present in central Ohio, need to be further investigated in eastern Ohio where the heart of the oil and gas play is occurring. Once data is available, the spatial relationship of the lithofacies occurring in the upper Lexington Limestone and Point Pleasant Formation can be interpreted throughout central and eastern Ohio. The effects of heterogeneity on natural fracture propagation and hydraulic fracing are important when deciding where to drill and what target depth to pick. Dunphy and

Campagna (2011) confirm this theory for hydraulic fracing in a well-connected natural fracture network.

The results, confirmed by microseismic surveys, showed that hydraulic fractures created in a well connect natural system were much more extensive and complex compared to less extensive, linear fractures created in a poorly connected system.

Natural fractures are important in a hydrocarbon reservoir to allow pathways for hydrocarbons to flow into the wellbore and increase production in a vertical or horizontal well. With today’s technology such as hydraulic fracing, producers can increase the production potential of a well by stimulating and reopening the calcite healed natural fractures. Knowing the timing of hydrocarbon generation, natural

fracturing and cementation is important when exploring for oil and gas reservoirs. Knowing the order of

events will allow exploration geologist to gain further insights if hydrocarbons have migrated and to what

extent has migration affected the economic quality of a potential reservoir. 108

SUMMARY AND CONCLUSIONS

The lithofacies present in the upper Lexington Limestone and Point Pleasant Formation in the

Prudential #1-A core in Marion County, Ohio give insights to how carbonate reservoirs are subject to natural fracturing and fluid migration. The lithofacies present in the upper Lexington Limestone and Point

Pleasant Formation were deposited on a carbonate ramp environment that was subject to storm wave action. The skeletal tempestites found in the core are indicators of deposition above storm wave base allowing for skeletal debris to be deposited throughout the upper Lexington Limestone and Point Pleasant

Formation in central Ohio. The presence of condensed sections and a shift of depositional environment in the undifferentiated Lexington Limestone and Point Pleasant Formation, indicate sea level rise and fall in the Point Pleasant sub-basin during the late Ordovician.

The lower portion of the Logana Member is composed of stacked tempestite sequences of centimeter scale undulated skeletal packstones, wackestones, and mudstones. The undulated skeletal packstones are primarily composed of disarticulated brachiopod shells, while the carbonaceous wackestones are composed of calcite silt and carbonaceous mud. The upper portion of the Logana

Member transitions up into swaly carbonaceous carbonate wackestones and is interpreted to be deposited on a mid ramp environment above FWWB.

The undifferentiated Lexington Limestone is characterized by thicker beds of undulate bedded skeletal tempestites composed of undulate bedded grainstones and packstones interbedded with carbonaceous wackestones and mudstones interpreted to be deposited above SWWB. The thicker beds of skeletal grainstones and packstones are interpreted to be deposited in an inner ramp environment and subject to mud winnowing by storm waves. Near the top of the undifferentiated Lexington Limestone a phosphaic skeletal packstone bed is present which is interpreted to be a condensed section during sea level rise, prior to depositon of hemipelagic rhythmites of carbonaceous mudstone and wackestone. 109

The Point Pleasant Formation continues with hemipelagic rhythmites and transitions into light gray mudstones with few interruptions of undulated skeletal packstones deposited above SWWB on a mid ramp environment. The transition to the Utica Shale from the Point Pleasant Formation is marked by a undulate bedded phosphaic skeletal packstone interpreted to be a condensed section during another sea level rise and deposition of the clay-rich Utica Shale.

The calcite healed natural fractures in the core primarily occur in the Logana Member and undifferentiated Lexington Limestone. A total of 63 of the 64 calcite healed natural fractures occurring in the Logana Member and undifferentiated Lexington Limestone while a single fracture occurs in the Point

Pleasant Formation. Differences between the upper Lexington Limestone and Point Pleasant Formation can be seen lithologicaly. The Logana Member and undifferentiated Lexington Limestone both have a higher amount of packstones and grainstones compared to the Point Pleasant Formation. The calcite healed natural fracture occurring in the Point Pleasant Formation is occurring in an undulated skeletal packstone bed.

Fracture length is dependent on bed thickness and degree of heterogeneity of the Logana Member and undifferentiated Lexington Limestone. The undifferentiated Lexington Limestone is very heterogeneous consisting of medium size beds of grainstones, packstones, wackestones, and mudstones, therefore, the length of natural fractures occurring here are much shorter than the natural fractures occurring in the homogeneous upper Logana Member.

Timing of the fractures occurred after compaction during diagenesis. These fractures were slow growing due to the fact that they are changing orientation and terminating at the contacts between undulate bedded grainstones/packstones and the undulate bedded wackestones/mudstones. Different natural fracture orientations suggest that stress orientations σ2 = σ3.

Multiple calcite cementing events have healed the natural fractures in the core. Three calcite cementing events were observed with chemical compositions ranging from high Mn2+ to lower Mn2+ 110

based on their CL colors. Different crystal sizes and CL colors is interpreted as separate crystal growth during separate pulses of fluid flow with different chemical composition. Zoning in euhedral calcite crystals is also evidence for slow crystal growth in a fluid filled fracture with changing chemical composition. The first two cementing events were slow growing in a fluid filled fracture which is recognized due to their euhedral structure and spaces between each crystal. The last cementing event, represented by mosaic calcite cement, is interpreted to be fast and infilling much of the remaining void space in the natural fractures. Pyrite is interpreted as the last cementing event in the natural fractures (post oil migration) as it is infilling void space around the last calcite cementing event.

The natural fracturing of reservoirs is important for exploration companies seeking to enhance the delivery of hydrocarbons to the wellbore. This thesis describes how natural fractures can be related to different lithofacies that are occurring in a carbonate reservoir in central Ohio and how extensive the natural fractures can be due to the heterogeneity of the reservoir. The idea of natural fractures occurring in zones of higher carbonate can be used in carbonate reservoirs throughout the world.

111

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APPENDIX A

GEOPHYSICAL LOG OF CHEVRON PRUDENTIAL 1-A

Figure A1. Chevron, Prudential #1-A well log indicating depth of study interval with gamma ray, resistivity, and neutron density curves. 118

GEOPHYSICAL LOG OF CHEVRON PRUDENTIAL 1-A

Figure A2. Chevron, Prudential #1-A well log with study intervals including stratigraphic names.

119

APPENDIX B

PRUDENTIAL #1-A NATURAL FRACTURES

Fracture No. Depth (ft) Length Aperature Orientation (cm) (mm) 1 1489 11 2 68 2 1487 14 1 81 3 1483' 11" 14 1 87 LOGANA MEMBER 4 1476' 10" 11.2 <1 88 5 1474' 10' 6.5 2 73 6 1474' 6" 9.5 2.5 48 7 1474' 5.5" 11.5 2 55 8 1474' 5 30 1 75 8a 1474' 4.5" 5 <1 58 8b 1474' 3.75" 9 <1 64 9 1473'10 6.5 2 88 10 1474 22 1 85 11 1473'3" 57 10 86 11a 1472' 8.5" 6.5 2 70 13 1472'10" 7.5 1.5 80 14 1472'6.5" 6.5 1 77 15 1471'9.5" 5 3 74 16 1470' 7" 18 1 84 16a 1470'7' 9 1 59 16b 1470'3" 4 1.5 50 17 1469'8.5" 22 <1 80 18 1469'8" 2 1 Na 19 1469'2" 6 2 83 20 1469' 30 1 75 21 1468'3.5" 9 ~1 na 22 1467'9.25" 9 3 50 23 1467'7" 9 3 66 24 1467'6" 33 1 80 25 1467 4 1 84 26 1466'11.5" 20 <1 66 27 1466' 9" 25 <1 83 28 1465'9.25" 29 1465'8" 120

30 1465'2" 17 <1 86 31 1463'2" 26 2 87 32 1462"4.75" 26 <1 83 33 1462'3.5" 13 <1 85 34 1457' 4" 20 <1 83 35 1457'2" 23 1 86 36 1456'2" 14 2 65 LEXINGTON LIMESTONE UNDIFFERENTIATED 37 1454' 2" 8 5 52 38 1455'6.5" 8 1 47 39 1454'2" 6 1 82 40 1453'11" 6.5 <1 85 42a 1453'8.5" 18 <1 85 42 1453'4.5" 37.5 2 84 42B 1452'7" 6.5 1 83 44 1452'4" 9 1.5 86 45 1451"10.75" 14 2 86 46 1450' 11" 8.5 na 45 47 1450'5.5" 3.5 na na 48 1449'6" 22.5 1 85 49 1449'4.75" 7 <1 na 50 1448'7.5" 11 2 60 51 1448'3" 2 1 60 52 1447'7" 8.5 1 88 53 1446'9" 6 1.5 65 54 1446'5" 12 1 75 55 1446'5" 14 <1 82 56 1445'1" 22.5 <1 87 57 1443'7" 13 2 80 58 1442'4" 13 2 60 59 1442 24 1 86 59a 1441'10.5" 11 1 80 59b 1441'9" 7 <1 75 60 1441'3.5" 17.5 <1 75 61 1441 11 1 82 62 1440'2.75" 13 <1 55 63 1439'10.5" 17.5 <1 65/84 POINT PLEASANT FORMATION 64 1403 9 1 85 Table 1: Natural fractures from the Logana Member to the Point Pleasant Formation. 121

APPENDIX C Total Organic Carbon % 0 1 2 3 4 5 6 7 8 1218 1273 1313 Utica Shale 1352 1377 1397 Point Pleasant Formation 1414 1425 1428.3 1431.3 1433.6 1435 1436.8 Lexington Limestone Und. 1437.4 D 1442 E 1446.8 P 1450 T 1451 H 1456

1459 1462 1466.2 1469 1469 1472.6 Logana Shale Member 1474.6 1476 1477 1478.6 1480.6 1481.6 1486.5 Curdsville Member

Figure C1. TOC differences in throughout the upper Lexington Limestone and Point Pleasant Formation.