Integrated Subsurface Study on Lithofacies and Diagenetic Controls Over Porosity Distribution in the Upper Ordovician Trenton Limestone in Northwestern Ohio

Home , Authigenesis, Black River Formation, Black River Group, Depositional environment, Knox Dolomite, Wells Creek Formation

INTEGRATED SUBSURFACE STUDY ON LITHOFACIES AND DIAGENETIC CONTROLS OVER POROSITY DISTRIBUTION IN THE UPPER ORDOVICIAN TRENTON LIMESTONE IN NORTHWESTERN OHIO

Mustafa Ahsan

A Thesis

Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

May 2019

Committee:

James Evans, Advisor

John Farver

Jeffrey Snyder ii

ABSTRACT

James Evans, Advisor

Previous studies of the Upper Ordovician Trenton Limestone in northwestern Ohio interpreted an extensive subtidal carbonate platform on the northwest margin of Taconic foreland basin. This study focused on the subsurface facies analysis and paragenesis of the Trenton

Limestone using eight different wells (3564, 20347, 2878, 2971, 2972, 2973, 3374, and 20239) in Hancock, Wood, and Wyandot counties. The wells were correlated using gamma-ray and density logs. Paragenesis was studied using petrography, SEM, EDAX, and cathodoluminescence.

A total of 51 thin sections were used to study petrography, microfacies and diagenesis.

Drill cores from two well sections (Core#3564 and Core# 3374) were used for lithofacies analysis. Cores 2878, 2971, and 2972 were used only for thin section analysis.

The Trenton Limestone mainly consists of bioclastic carbonate (mudstone, wackestone, packstone, and grainstone) with minor amounts of siliciclastic shale. The dominant lithofacies, in order of importance, are heterolithic laminated carbonate mudstone and siliciclastic shale

(lithofacies Cml) which are interpreted as tidalites, massive gray carbonate mudstone (Cmm), massive light gray carbonate mudstone with Stromatactis (Cms), massive bioclastic grainstone

(Cgm) which are interpreted as storm layers, massive grainstone with lithoclasts (Cgm) which are interpreted as reworked beachrock, skeletal packstone (Cps), massive dolomicrite (Dmm) and massive dolograinstone (Dgm), alternation of planar laminated carbonate mudstone and wackestone (Cmw), alternation of skeletal packstone and skeletal mud/wackestone (Cpm), alternation of packstone and mudstone (Cmf), and carbonate wackestone (Cws). The presence of iii

tidalites, mudcracks, lithoclasts, shell debris, and storm layers indicate that the Trenton

Limestone in Northwestern Ohio was deposited in a peritidal carbonate ramp environment.

Three lithofacies associations were the tempestite association lithofacies, back ramp lithofacies association and proximal deep ramp lithofacies association.

The sequence of diagenesis has been interpreted to be: (1) bioturbation (2) authigenesis

(glauconization and pyritization) (3) burial and moderate compaction (4) calcite cementation (5) micritization (6) silicification (7) dolomitization (8) late pyritization and, (9) dedolomitization.

Different diagenetic features in the paragenetic sequence such as dolomitization, dissolution and dedolomitization helped in porosity creation and bitumen stains in their vicinity meant petroleum migration and accumulation. This knowledge may be used in future if any exploration work is undertaken.

ACKNOWLEDGMENTS

I wish to express my gratitude to my advisor Dr. James E. Evans for his continual guidance, assistance and mentoring. I appreciate his patience, advice, and knowledge towards the completion of this project. I would also like to thank my thesis committee Dr. John Farver and

Dr. Jeffrey Snyder for their helpful suggestions and advice. I wish to thank Dr. Charles Onasch for helping with the cathodoluminescence study.

Special thanks go to Ohio Divisional of Geological Survey for granting access to the cores used in this research. I wish to thank Mr. Aaron Evelsizor and Mr. Gregory Schumacher for their help with accessing the cores at the Ohio Geological Survey, H.R. Collins Core

Laboratory in Delaware, Ohio.

I would also like to thank all the faculties, staffs and fellow student of the Department of

Geology for offering various types of help during my work on this research.

Sincere thanks are due to the Society of Petrophysicists and Well Log Analysts for funding this research project. Thanks are also due to the BGSU Department of Geology, the

BGSU Graduate Student Senate and the Geological Society of America North Central Section for providing travel funds associated with presenting this research.

TABLE OF CONTENTS

Page

INTRODUCTION ...... 1

Carbonate Depositional Environments ...... 1

Carbonate Sediments ...... 5

Common Storm Deposits ...... 5

Carbonate Minerals ...... 6

Carbonate Cements ...... 8

Carbonate Diagenesis ...... 8

Diagenetic Environments ...... 9

Dolomitization ...... 10

Importance of Carbonate Diagenesis in Petroleum Exploration ...... 13

Purpose ...... 17

GEOLOGIC BACKGROUND ...... 18

Regional Geologic Setting ...... 18

Precambrian Geology...... 18

Cambrian Transgression ...... 18

Taconic Orogeny ...... 19

Regional Stratigraphy ...... 19

Knox Group ...... 21

Wells Creek Formation ...... 24

Black River Limestone ...... 25

Trenton Limestone ...... 26 vi

Origin of Name ...... 26

Depositional Environment ...... 26

Open-shelf Facies ...... 27

Platform Facies ...... 28

Platform-margin Facies ...... 28

Petroleum Geology ...... 29

METHODS ...... 31

Core Stratigraphy ...... 31

Geophysical Logs...... 31

Petrographic Study ...... 35

Mineralogy and Constituents of Carbonate Rocks ...... 36

Cathodoluminescence ...... 41

Scanning Electron Microscopy/ Energy Dispersive X-Ray Analysis ...... 43

RESULTS ...... 45

Lithology ...... 45

Limestone ...... 45

Dolostone ...... 48

Chert ...... 49

Shale ...... 50

Bentonite ...... 50

Lithofacies and Microfacies ...... 50

Carbonate Packstone (Cpm) ...... 60

Highly Bioturbated Wackestone Lithofacies (Cws) ...... 60 vii

Massive Carbonate Grainstone Lithofacies (Cgm) ...... 60

Massive Dolo-grainstone Lithofacies (Dgm)...... 62

Gray Carbonate Mudstone Lithofacies (Cmm) ...... 62

Alternation of Calcareous Shale and Carbonate Mudstone

Lithofacies (Cml) ...... 63

Skeletal Packstone (Cps)...... 63

Carbonate Mudstone to Wackestone with Stromatactis (Cms) ...... 64

Dolomudstone Lithofacies (Dmm) ...... 64

Alternation of Planar Laminated Carbonate Mudstone and Wackestone

lithofacies (Cmw)...... 64

Alternation of Packstone and Mudstone (Cmf) ...... 71

Microfacies ...... 71

Lithofacies Associations ...... 71

Back Ramp (lagoonal) Lithofacies Association ...... 71

Tempestite Facies Association ...... 72

Amalgamated Tempestite Association ...... 73

Proximal Deep Ramp Lithofacies Associations ...... 74

Depositional Sequence ...... 74

Stratigraphy ...... 75

Diagenesis ...... 86

Burial and Moderate Compaction ...... 86

Calcite Cementation ...... 87

Micritization ...... 87 viii

Marine Hard Grounds ...... 87

Pyritization ...... 88

Silicification ...... 88

Dolomitization ...... 88

Dedolomitization ...... 91

Cathodoluminescence ...... 91

Scanning Electron Microscopy/Energy Dispersive X-Ray Analysis ...... 97

Paragenesis ...... 98

Bioturbation ...... 99

Authigenesis (Glauconization and Pyritization) ...... 99

Burial and Moderate Compaction ...... 99

Early Cements (Calcite Cementation) ...... 104

Silicification ...... 104

Dolomite Replacement and Dolomite Neomorphism ...... 104

Dedolomitization ...... 105

Porosity ...... 105

DISCUSSION ...... 111

Depositional Environment ...... 111

Stratigraphic Trends ...... 111

Depositional Architecture ...... 112

Paragenesis ...... 114

Porosity ...... 114

Timing of Petroleum Migration ...... 115 ix

SUMMARY AND CONCLUSIONS ...... 117

REFERENCES ...... 121

APPENDIX A: LITHOLOGS ...... 133

APPENDIX B: CATHODOLUMINESCENCE PHOTOMICROGRAPHS ...... 150

APPENDIX C: SEM PHOTOMICROGRAPHS ...... 155

LIST OF FIGURES

Figure Page

1 A schematic diagram showing the location of the shallow subtidal carbonate factory in

platform, shelf and ramp settings ...... 2

2 Subdivisions of a carbonate ramp environment ...... 4

3 Carbonate ramp depositional model ...... 4

4 A shallowing upward calcareous tempestite sequence ...... 7

5 Carbonate grain types and their depositional setting ...... 12

6 Types of diagenetic cements ...... 14

7 Carbonate cements in various diagenetic environments ...... 15

8 Schematic diagram of carbonate diagenesis ...... 16

9 Schematic diagram of major diagenetic environments ...... 16

10 Geographic position of the continents in Late Ordovician showing location of

North America and Ohio ...... 20

11 Ordovician Cross section of Eastern North America showing Trenton shelf and

foredeep basin ...... 20

12 Geologic cross section of the Precambrian and overlying rocks in Ohio ...... 22

13 Stratigraphic Column of Lower Paleozoic rocks in Ohio ...... 22

14 Upper Ordovician paleogeographic map of study area ...... 23

15 Location map of the study area ...... 32

16 Planar and non-planar dolomite ...... 39

17 CL zones in calcite and dolomite ...... 42

18 (A) Skeletal packstone with nodular mud in well #3564 (B) Alternating xi

mudstone and wackestone in well #3564 (C) Alternating mudstone and wackestone

(varved) in well #3564 and (D) Skeletal packstone in well # 3374 on the top

separated by abrupt contact in the middlde ...... 47

19 Photomicrograph (under plane polarized light and and cross polarized light

respectively) of blocky calcite cement (C) and replacing dolomite (D). Depth

-395.3m (Core 2878) ...... 51

20 Bioclastic packstone with micrite matrix microfacies. Crinoid and mollusk

fossils observed under plane polarized light. Depth -398.4m (Core 2878) ...... 52

21 Photomicrograph of crinoid (C) and brachiopod (B) shells in micrite matrix,

bioclastic packstone microfacies observed under plane polarized light. Depth

-396.8m (Core 2878) ...... 52

22 Bioclastic packstone microfacies with crinoid (C) and other matrix material

observed under plane polarized light. Depth -397.5m (Core 2878)...... 53

23 Bioclastic wackestone with mollusk and other shell fragments observed under

plane polarized light. Depth -396.6m (Core 2878) ...... 53

24 Bioclastic packstone microfacies showing brachiopod (Br) and other fragments

observed under plane polarized light. Depth -403.3m (Core 2878) ...... 54

25 Mudstone/wackestone microfacies showing peloidal structure (P) observed under

plane polarized light. Depth -402.4m (Core 2878) ...... 54

26 Skeletal packstone with brachiopod shells (Br) being replaced by dolomite.

Stylolitic seam containing pyrobitumen observed under plane polarized light.

Depth -431m (Core no. 2971) ...... 55

27 Chert (Ch) replacement of micritized crinoid shell observed under plane polarized light. xii

Depth -450.6m (Core no. 2972) ...... 55

28 Planar-s dolomite with vuggy porosity with relict peloidal texture observed under

plane polarized light. Depth -460.8m (Core no. 3564) ...... 56

29 Dolomite fault breccia observed under plane polarized light. Depth -460.8m (Core

no. 3564) ...... 56

30 Saddle Dolomite with wavy extinction observed under cross polarized light. Depth

-445.2m (Core no. 3564) ...... 57

31 Pseudo-punctate brachiopod shell with antitaxial and blocky dolomite cement on

the inside an isopachous fibrous cement on outside observed under plane polarized

light (left) and cross-polarized light (right). Depth -454m (Core no. 3564)...... 57

32 Planar-s dolomite with recrystallization pores hosting pyrobitumen observed

under plane polarized light. Depth -463.1m (Core no. 3564)...... 58

33 Planar-e dolomite with surrounding organic matter and argillaceous substances

observed under plane polarized light. Depth -442.4m (Core no. 3564)...... 58

34 Bentonite bed in between packstone in core no 3374 at a depth of -406.5m ...... 59

35 Lithofacies Cpm and Cws at -447.4m and 465.6m in core no. 3564 ...... 65

36 Core photograph of lithofacies Cgm at -450m in core no. 3564 ...... 66

37 Lithofacies Dgm at -404.62m in core no. 3374 and Cmm at -453.4m in

core no. 3564 ...... 66

38 Lithofacies Cps and Cmm at -454.8m in core no. 3564 ...... 67

39 Core photograph of lithofacies Cml at -447.8m in core no. 3564 ...... 67

40 Lithofacies Cms and Cmm at -464.8m in core no. 3564 ...... 68

41 Lithofacies Dmm at -470.74m in core no. 3564 ...... 69 xiii

42 Lithofacies Cmw at -464. 4m in core no. 3564 ...... 70

43 Lithofacies Cmf (Heterolithoc ripples and mud drapes) at -454.1m in

core no. 3564 ...... 70

44 Proximal tempestite sequence at -454.1m (top) in core no. 3564 ...... 77

45 Proximal tempestite sequence at approximately -452 m (top) in core no. 3564.

Scour base not present ...... 78

46 Amalgamated tempestite sequence approximately at -451.7m ...... 79

47 Contact between Utica Formation and Trenton Limestone in well #3564

at -442.5m depth ...... 82

48 Contact between Black River (bottom) and Trenton Limestone in well #3564 at

-469.95 m depth ...... 82

49 Gamma ray log correlation of the studied wells of Trenton Limestone ...... 83

50 Correlation between geophysical logs and graphic log in the top portion of well

#3564...... 84

51 Correlation between geophysical logs and graphic log in the top portion of

well #3374 ...... 85

52 Marine hard ground with characteristic cemented appearance at depth of -464.4m

(A) and -453.4m (B) respectively in core no. 3564 ...... 90

53 Karst collapse breccia at with angular chert fragments in well no 3564 at

depths of -463.1m and -461.3m ...... 92

54 Core photograph of saddle dolomite at -460.4m in well #3564 ...... 93

55 Photomicrograph of silicified fossil shell and dolomite at -460.4m in well #3564

(under plane polarized light) ...... 93 xiv

56 Photomicrograph of late stage chertification at -454.1m in well #3564

(under plane polarized light) ...... 93

57 Illustration of different stages of dolomitization in Trenton Limestone ...... 94

58 Fibrous silica (chalcedony, cross cutting dolomite fabric. Core no 3564, depth

-463.1m (under cross polarized light) ...... 95

59 Glauconite replacement of shell and later dolomitization, depth -455.5m

(under plane polarized light) ...... 95

60 Dedolomitization calcite replacement of dolomite by calcite at a depth of

-463.1m ...... 96

61 Dedolomitization calcite replacement of dolomite by calcite at a depth of

-463.1m ...... 96

62 Spectral profile of the cathodoluminescent planar-e dolomite. Sampled from Core no.

3564...... 100

63 Spectral profile of the cathodoluminescent calcite cement. Sampled from

Core no. 3564 ...... 101

64 Dull and bright cements in dolomite. The zoned cement is type 1 cement ...... 103

65 A diagram of the paragenetic sequence in the studied section of Trenton

Limestone ...... 107

66 SEM image of dolomite exhibiting microporosity in core no. 3564 at -463.1m ...... 108

67 Pyritized dolomite observed under SEM in core no. 3564 ...... 109

68 Microporosity in dolomite observed under SEM in Core no. 3564 at a depth

of -443m ...... 110

69 Schematic diagram of Upper Ordovician depositional environment in the study

area ...... 113 xv

LIST OF TABLES

Table Page

1 List of study methods for the studied cores ...... 33

2 Samples collected from core no. 3564 for petrographic study ...... 37

3 Thin section used for petrographic study and cathodoluminescence analysis ...... 40

4 Lithofacies table of Trenton Limestone ...... 61

5 Lithofacies Association of Trenton Limestone ...... 76

6 Microfacies of Trenton Limestone ...... 80

7 Types of cathodoluminescent cements ...... 102 1

INTRODUCTION

Carbonate Depositional Environments

Carbonates can originate in three major environmental settings: continental, transitional, and marine depositional environments (Flugel, 2004). In addition, marine carbonates can originate in both shallow and deep water. At present, about 90% of total carbonate production takes place in the deep-sea environments compared to only 10% in shallow or transitional marine environments. However, this is not the case prior to the Cenozoic. Accordingly, the majority of the research on modern carbonate rocks is carried out on shallow marine environments because of the economic importance of these environments in the past (Ali et al., 2010).

Shallow marine environments are generally classified on the basis of vertical and horizontal extent with respect to sea level and resultant water depth. One vertical classification scheme recognizes the subtidal, intertidal and supratidal zones. In contrast, one classification recognizes the neritic and pelagic zones.

As opposed to the clastic sedimentary rocks, carbonate sediments are said to be “born but not made” (James and Kendall, 1992). While clastic sediments are made from the weathered products of pre-existing rocks, carbonate sediments are biological or biologically mediated deposits. Hence, the basic difference between carbonate and terrigenous sediments is that the properties of the hydraulic regime are not always reflected in the texture of the deposited sediments. As the deposition of carbonate sediments take place only in certain environments, the major areas of carbonate sediment accumulation in platforms and ramp settings (Figure 1).

It is critical to properly assess the depositional geometry of carbonate deposits to properly appreciate the relation between depositional setting and facies. Ahr (1973) first introduced the 2

Figure 1: A schematic diagram showing the location of the shallow subtidal carbonate factory in platform, shelf and ramp settings (from James and Kendall, 1992).

difference of depositional geometry between rimmed shelf and ramp. Later Read (1985)

classified different types of carbonate platform facies models for better understanding of relation

between facies and depositional setting, which is discussed in detail in the next paragraph. An

illustration of the detailed subdivision of carbonate ramp environments is shown in figure 2 and

3. In figure 3 the distribution of different types of carbonate sediments in different sub- environments are shown.

According to Read (1985), the following types of facies occur in a carbonate depositional environment: (1) Tidal-flat complex. The tidal flat complex generally consists of meter-scale

shallowing upward from 1-10 meters in thickness. It consists of burrowed limestone and

cryptalgal laminites. (2) Lagoonal facies. The lagoonal facies is generally found behind barrier

complexes and consist of lime mudstone, pellet limestone or cherty, burrowed skeletal packstone to mudstone. Minor peritidal fenestral and cryptalgal carbonates can also occur which represents

shallowing from lagoonal to tide level. (3) Shoal water complex of banks, reefs, and shoals. This

type of facies assemblage is found in shallow ramp skeletal bank and shelf-edge skeletal reefs.

The deposits can be transported downslope and can be found in the deep ramp facies. (4) Deep

shelf and ramp facies. This facies assemblage consists of cherty, nodular bedded, skeletal packstone and wackestone. Abundant fossils and open marine biota are often observed. Storm

generated beds are common. The water depth range is from 10 to 40m and deposition takes place

below fair weather wave base. (5) Slope and basin facies. It is characterized by abundance of breccias and turbidites interbedded with terrigenous mud. Basinal deposits mainly consist of shale with an increase in carbonate upslope towards the platform.

Figure 2: Subdivisions of a carbonate ramp environment (modified from Burchette and Wright,

1992).

Figure 3: Carbonate ramp depositional model (modified from Tucker and Wright, 1990 and

Jones and Desrochers, 1992).

Carbonate Sediments

According to Tucker and Wright (1990), most of the carbonate sediments originate from biogenic activity. Hence the distribution of carbonate sediments depends on environmental parameters which are favorable for these biogenic activities (Moore and Wade, 2013). Schlager

(2005) described three types of mechanism of production of carbonate sediments. First, carbonate sediments can be produced from abiogenic processes such as direct precipitation from sea water, but this is relatively rare. Second, carbonate sediments can be precipitated from seawater by biologically induced processes due to bacteria or algae. The sediments produced in this way might resemble those produced by direct chemical precipitation. Third, the final type of carbonate sediment is completely influenced by organisms, such as production of shells. This type of carbonate rock is generally composed of the skeletal remains of the organisms.

Organisms that affect carbonate production can be divided into two major groups, autotrophs, and heterotrophs. The autotrophs depend directly on sunlight to produce carbonates while the heterotrophs produce the carbonate material by consuming food (Moore and Wade, 2013).

Examples of autotrophs are cyanobacteria, green algae, and red algae, while examples of heterotrophs are gastropods, brachiopod, bryozoans, and echinoderms. An illustration of carbonate grain types and their depositional environment are given in Figure 4.

Common Storm Deposits

Tempestites are storm deposits in the depositional record. Difference in the tempestite sequence signifies the location of the deposit in the depositional environment, for example, whether it is near shore, proximal, mid, or distal ramp. According to Aigner (1985), a basic difference between ramps and rimmed platforms is that carbonate ramps are more susceptible to 6

minor hydrodynamic changes such as storms. Storms develop as a result of special climatic and

geographic conditions, leading to storm deposits that can be characterized by their distinct

appearance. Seilacher & Aigner (1991) observed that tempestites form sheet-like sand beds which have considerable lateral extent. Owing to the environment of deposition there are a wide variety of tempestites observed. In carbonate sequences, tempestites can range from wackestones, packstones to grainstones or rudstones. Based on stratigraphy, the depositional zone in which the tempestite has been deposited can be identified. Einsele (1985) noted that in the proximal zone of carbonate deposition, tempestites are commonly amalgamated and cannibalized. The general difference between proximal and distal tempestite is the increasing amount of mud in the tempestite sequence. The trends of proximal and distal tempestites are shown in figure 4 from Einsele (1985).

Tucker and Wright (1990) noted that the base of tempestite beds are sharp and have varieties of sole marks such as gutter casts and tools marks. Internal structures may vary widely depending on the specification of the environment, but the most common features are graded bedding with concentration of bioclasts at the base. Figure 4 shows a typical carbonate tempestite sequence (Einsele, 1985).

Carbonate Minerals

The main components of carbonate rocks are carbonate minerals, which typically constitute more than 50% of the composition of carbonate rocks (Tucker and Wright, 1990). The main types of carbonate minerals in carbonate rocks are calcite (CaCO3), aragonite (CaCO3), and

dolomite (Ca.Mg(CO3)2). The crystal structure of these minerals is an important characteristic to

understand the growth, dissolution and replacement of these minerals. 7

Figure 4: A shallowing upward calcareous tempestite sequence (modified from Einsele, 1985).

The basic crystal structure of all carbonate group minerals consists of the CO3 (carbonate)

functional group. Though the chemical composition of calcite and aragonite are essentially

similar, they have different crystal structure due to the difference in arrangements of the anions

and cations. Calcite and dolomite crystals are rhombohedral while aragonite is orthorhombic.

Figure 5 shows the relation between different types of carbonate grains and their depositional

setting. As for example it relates the occurrence of algal stromatolites with supratidal environment of deposition, skeletal boundstone with reef, oolites with high-energy shoals. This

may act as a reference for attaining idea about the type of depositional environment from the

carbonate grain type.

Carbonate Cements

Carbonate cements of different morphology are characteristic of different diagenetic

environments. Figure 6 illustrates different types of diagenetic cements. These cements are

representative of different burial diagenetic conditions. In Figure 7, relation between the

morphology of some of these cements and various diagenetic environments are shown. It also

illustrates the mechanism of diagenetic processes and their environments.

Carbonate Diagenesis

Dissolution, cementation, compaction, and recrystallization are the common processes of

carbonate diagenesis. The rate and sequence of diagenetic change depends on the original

mineralogy, texture and, porosity. Each sequential diagenetic step produces new conditions and

products, resulting in a diagenetic pathway. Because all these changes are marked by physical

and chemical signatures it is possible to interpret the specifics of the diagenetic environment. For 9

example, the degree of change in the mineralogy can be calculated using the composition of

fossils compared to their known original chemical composition (Flugel, 2004).

The original carbonate materials consist of aragonite (CaCO3) and high-magnesium

calcite (>4 mol % MgCO3). The diagenetic products are low-magnesium calcite (<4 mol %

MgCO3) and dolomite. In addition, both also contain trace amount of iron, for example calcite

with >6 mol % FeCO3 is called ferroan calcite. There are other trace elements, such as Sr and

Mn, which also substitute in calcite and dolomite (Land, 1980).

Diagenetic Environments

Because aragonite is unstable on the Earth’s surface, carbonate sediments are more

reactive than their siliciclastic counterparts, undergo chemical alteration right after deposition.

The degree and nature of alteration depends on the depositional and burial processes. A number

of diagenetic pathways are possible in case of gradual burial of carbonate sediments (Carozzi,

1988), but in general it starts off with bioturbation by organisms, inversion, and early

cementation. The types of cements and their morphology are characteristic of certain diagenetic environments, the major types of environments being meteoric, marine, or subsurface (Moore and Wade, 2013).

Depending on the diagenetic pathways (Figure 8) the carbonate sediments undergo, it is possible for the previous diagenetic texture to be partly destroyed. Hence the relative influence of all the diagenetic processes is that the carbonate sediments undergo change at different stages which play a role in ultimate product formed by diagenesis. It is possible to establish a sequence of these alteration events by petrographic examination of the sediments and cements. This sequence of diagenetic events is called paragenesis. 10

Dolomitization

Dolomitization is an important diagenetic process where the original calcite or aragonite

undergoes reaction with a fluid containing magnesium to form the mineral dolomite. The basic

chemical reaction for dolomite formation can be summarized as:

2+ 2+ 2CaCO3(limestone)+Mg CaMg(CO3)2 (dolomite)+Ca

The exact mechanism of formation of dolomite is still a subject of debate, but according to

Boggs (2001), the widely accepted models are the hypersaline model, the mixing zone model,

and the seawater model. Among these models, the most widely accepted model is the mixing

zone model which is also known as the Dorag dolomitization model. According to Badiozamani

(1973) and Folk and Land (1975), dolomitization is initiated by the mixing of sea water with

fresh meteoric water at the transitional zones at the interface of continental and marine

environment. As the salinity is lowered by mixing, the magnesium/calcium ratio required for the forming of dolomite is achieved. It is presumed that dolomite formation is more likely to take place in mixed water conditions compared to sea water because of presence of other ions in high salinity brines which affects the kinematics of crystal growth.

The hypersaline dolomite model takes place in evaporitic environments. The hypersaline dolomite model takes place in takes place in evaporitic environment. Shin et al. (1965) described example of supratidal dolomites in Andros Island of Bahamas and later Shinn (1968) described similar supratidal dolomites in Sugarloaf Key of Florida. The saline water is introduced into the evaporitic environment by mechanisms of storms and high tides. Afterward the saline water gradually turns hypersaline because of continued evaporation. Another model of evaporation induced dolomite formation is called evaporative pumping (Hsu and Seigenthaler, 1969). In this model, hydrodynamic movement of saline pore water leads to transportation of magnesium rich 11

fluid which could lead to dolomite formation of the carbonate minerals under suitable conditions

the newly formed dolomite may become stable.

The concept of the sea water model of dolomitization is that in shallow subtidal

conditions, if sufficient amount of sea water is pumped through the pore spaces of carbonate

rocks in such a way that the saline water in pore space is constantly renewed with new sea water, dolomite formation can take place by enrichment of magnesium ions. Carballo et al. (1987) observed that relatively thin layered high permeability carbonates are more susceptible to dolomitization by tidal pumping. The proposed mechanism is that flow of new sea water supplies enough magnesium ions for replacing the calcium ion and at the same time the flow of pore water helps in removing the replaced calcium ion which may hinder the dolomitization process. Carballo et al. (1987) also discusses an example of the above mechanism which takes place in the Sugarloaf Key area in Florida.

Apart from the abovementioned mechanisms of dolomitization, evidence of bacterially mediated dolomitization and burial dolomitization have been observed. Baker and Kastner

(1981) observed that the presence of sulfate ion in water is not conducive to the formation of dolomite. Owing to this reason, dolomite deposits have been observed to have formed where bacterial biogenic activity creates a reducing condition for sulfate ions (Vasconcelos and

McKenzie, 1995). Burial dolomitization is another mechanism of dolomitization which takes place as a result of circulation of magnesium-rich water in the subsurface. This can happen long after the deposition of carbonate sediments in the subsurface. One example of burial dolomitization is in the Upper Devonian Miette buildup in Alberta, Canada (Mattes and

Mountjoy, 1979). 12

Figure 5: Carbonate grain types and their depositional setting (Harris et al., 1985).

According to Steinsund and Hald (1994), dissolution occurs in near surface regions where undersaturated fluid interacts with metastable minerals and dissolves the minerals. This saturated solution ultimately percolates through rocks and is precipitated in veins and pore spaces which lead to cementation.

Importance of Carbonate Diagenesis in Petroleum Exploration

The majority of the world’s oil reserves come from diagenetically altered limestone reservoirs

(Ali et al., 2010). Diagenesis in carbonate rocks is a complex process. The cycle of diagenesis depends on various environmental variables which are responsible for the end product. A schematic diagram of carbonate diagenesis is shown in figure 8. This illustrates chemical pathways of chemical conversion in a profile view of the surface and subsurface realm. Figure 9 shows a schematic diagram of the profile view of this realm. In spite of this complexity, the diagenesis of carbonate rocks is widely studied because of the potential of carbonate rocks to act as reservoir rocks. The high initial porosity of carbonates makes them susceptible to diagenesis by percolating fluids (Selley, 2000). Cementation reduces the primary porosity of carbonate rock to a significant extent, but secondary porosity is produced by dissolution and replacement reactions. Extensive exposure to diagenetic fluids can give rise to extensive vugs and fissures which may enhance the porosity of the formation. According to Csoma and Goldstein (2013), the nature of diagenetic alteration largely depends on the chemistry which is ultimately controlled by sea level and climatic fluctuations. The importance of the study of diagenesis in petroleum exploration lies in the fact that the diagenetic porosity helps to form reservoir rocks and the analogs of porosity creation by diagenesis helps greatly in the exploration of new carbonate reservoirs. 14

Figure 6: Types of diagenetic cements (From James and Choquette, 1983).

Figure 7: Carbonate cements in various diagenetic environments (modified by Harris, 1985 after

Folk, 1959).

Figure 8: Schematic diagram of carbonate diagenesis (modified from Selley, 2000).

Figure 9: Schematic diagram of major diagenetic environments (modified from Flugel, 2004).

Purpose

The primary purpose of the research is to correlate occurrence of porosity with

depositional facies and diagenetic alteration in Trenton Limestone in Northwestern Ohio.

Although there has been a number of previous studies carried on Trenton Limestone in

Northwestern Ohio, authors have noticed the depositional complexity (abrupt facies changes),

and complexity in diagenetic alteration at different locations, which leaves room for further study

to address these issues. Work carried out by Wickstrom et al. (1992) interpreted a subtidal origin

of Trenton Limestone in Northwestern Ohio, but more recent work by Patchen et al. (2006)

interpreted a peritidal carbonate ramp depositional environment. In terms of facies analysis, the

work done in the above publications are mostly large scale, carried out over a larger extent of

area. The earlier studies did not focus on detailed microfacies analysis at a smaller extent.

Moreover, newer research on carbonate sedimentology are based on interpretation of depositional and diagenetic features on sequence stratigraphic framework. Owing to these above factors, a detailed look at Trenton Limestone in Northwestern Ohio will help in enhancing present knowledge on lithofacies and petrography. Integrated knowledge of centimeter-scale depositional features combined with detailed paragenetic sequence and their geophysical log response will be valuable for future secondary recovery attempt, whether it be acid stimulation or artificial waterfront build up. To summarize, in terms of depositional environment, this research

will study the depositional features at a closer scale, and paragenesis study will help understand

the stages of porosity development. The results will be valuable for future exploration work.

GEOLOGIC BACKGROUND

Regional Geologic Setting

Precambrian Geology

The Precambrian geology of the Ohio is dominated by the effect of Grenville Orogeny.

The basement rock of Ohio consists of the Grenville Province and the Granite-Rhyolite Province

(Janssens, 1973). The Granite-Rhyolite Province in western Ohio represents a vast sheet of granite and rhyolite, about 11 km thick (Hansen, 1997), which formed by uprising of the earth’s mantle in what is also known as a superswell. The date of this event has been estimated to be around 1400 to 1500 Ma. The doming of the superswell resulting in the formation of a rift system characterized by normal faulting and subsidence of the thin magma which is called the

East Continent Rift (Drahovzal et al., 1992). The rift was later infilled by a sequence of sediment and basalt flows called the Middle Run Formation. Rift formation ceased at about 1000 Ma. The

Grenville orogeny was the major event that took place in this region as a result of the collision between North America and a continent on the east. As a result of compression, a zone of imbricated thrust sheets developed, known as the Grenville Front Tectonic Zone, which marks the western limit of the Grenville mountains. After the formation of Grenville Mountains, a 300- million years long erosion interval occurred, resulting in a low-relief paleotopography and exposing the metamorphic rocks of the Grenville orogen.

Cambrian Transgression

During the late Cambrian, there was a marine transgression across the eroded topographic surface, which later influenced the depositional history of the region. Paleozoic sediments covered most of eastern and midcontinent portion of North America, but the Precambrian is 19 gradually exposed toward the northeast, such as in Ontario or the Adirondack mountains of New

York. Precambrian rocks are also locally exposed in the Appalachian basin as a result of erosion.

A homoclinal ramp characterizes the Precambrian rock from Findlay Arch to the Allegheny structural front (Ryder et al., 2008).

Taconic Orogeny

Eastern Laurentia (which is the present north-central North America) underwent tectonic changes which transformed it into a foreland basin. The geographic location is shown in Figure

10. Collision with another continent (Avalonia) gave rise to the complex volcanic arc (The

Ammonoosuc Arc) which modified the paleogeography of the region (Hay and Cisne, 1988). At that time the majority portion of the present North America was under shallow sea water. Along the continental boundary covered by shallow sea water, carbonate deposition occurred. A cross section of Eastern North America showing the geological setting of this carbonate deposition is shown in Figure 11. Intermittent volcanic eruption from the volcanic arc gave rise to volcanic ash layers which got deposited between the layers of carbonate sediments. During Late

Ordovician, closure of the Taconic foreland basin resulted in the end of carbonate production. A

Late Ordovician paleogeographic map is given in Figure 14. According to Hansen (1997), the

Taconic Orogeny culminated during the Late Ordovician, and is recorded in the rocks from

Newfoundland to Alabama.

Regional Stratigraphy

According to Hay and Cisne (1989), the deposition pattern in the Taconic foreland basin changes from west to east. As a result of a deepening trend from west to east, the facies vary

Figure 10: Geographic position of the continents in Late Ordovician showing location of North

America and Ohio (from Coogan, 1996).

Figure 11: Ordovician Cross section of Eastern North America showing Trenton shelf and foredeep basin (modified from Shanmugam and Lash, 1982).

21 from shallow water carbonates to interbedded limestones and shales, to calcareous black shale, to gray silty shale. Titus (1989) attributed this facies change from west to east as a result of downwarping of the basin. The downwarped nature of Cambrian and Ordovician succession from west to east is illustrated in Figure 11. The consequences of this downwarping is evident in the cross section of Precambrian and overlying rocks in Figure 12. The east-west extent of the

Taconic foreland basin was about 120 km from the distal shelf to the basin floor. It has been established from the previous studies that there was connectivity between the Taconic Foreland basin and the ocean, but the degree of connectivity has not been ascertained (Thomas, 1985). The stratigraphic succession of the lower Paleozoic rocks in Ohio is given in figure 13.

Knox Group

The Knox Group is a mixed carbonate-siliciclastic sequence which was deposited in a tidal-flat to shallow marine environment along continental shelf (Riley et al., 2002; Ryder and Repetski, 1992). Read (1989) stated that deposition of Knox Group took place on a rimmed shelf environment. The period of deposition was from Late Cambrian to Early Ordovician. An unconformable contact with Wells Creek Formation marks the upper boundary of Knox

Dolomite (Hansen 1997). In central and eastern Ohio, the lithology consists of heterogeneous interbedded siliciclastic and carbonate intervals (Riley et al., 2002). The Knox Group can be subdivided into three units, namely Copper Ridge Dolomite, Rose Run Sandstone, and

Beekman- town Dolomite. The Copper Ridge Dolomite consists mainly of dolomite, but with 22

Figure 12: Geologic cross section of the Precambrian and overlying rocks in Ohio (from Hansen, 1997).

Figure 13: Stratigraphic Column of Lower Paleozoic rocks in Ohio (from Hansen, 1997)

Figure 14: Upper Ordovician paleogeographic map of study area (modified from Coogan, 1996).

some sandstone intervals. Original features that can be recognized include ooids, algal mounds,

mud rip-up clasts, and cryptalgal laminae (Riley, 1993).

The Rose Run Sandstone consists of sandstone, dolostone, and shale where the sandstone

consists of quartz arenite and feldspathic arenite (Enterline, 1991; Nwaodua, 2008; Shah, 2013).

The Beekmantown Dolomite consists of gray to brown, mottled crystalline dolomite (Riley,

1993). However, in northwestern Ohio, the Knox Group is undifferentiated (Janssens, 1973).

The lithology of Knox Group in northwestern Ohio consists of white to light gray dolomite and

sandstone (Wickstrom and Gray, 1989). Fossil fragments are not individually identifiable but

stromatolitic, and burrow-mottled structures are observed from well cuttings (Wickstrom and

Gray, 1989).

The top of the Knox Group is marked by an unconformity (“Knox unconformity”) below

the Wells Creek Formation. Janssens (1973) noted that localized karst topography might be

present along with localized drainage patterns due to subaerial exposure after deposition of Knox

Group.

The Knox Group in Ohio has served as a major reservoir for hydrocarbons. According to

Riley et al. (2002), 300 wells were drilled before 2002, and that an extent of 322 kilometers from

south-central to northwestern Ohio has a proven estimated ultimate recovery (EUR) of 62.9

MBBOE and 347 BCF gas. Hence it is a very important formation for oil and gas exploration.

But in Northwestern Ohio, no oil or gas discoveries have been made in the Knox Group.

Wells Creek Formation

The Knox Dolomite in Northwestern Ohio is unconformably overlain by the Wells Creek

Formation (Wickstrom and Gray, 1989). The thickness of the formation (0-18m) varies because 25 of the variable relief on the Knox unconformity (Hansen, 1997; Wickstrom and Gray, 1989). The lithology of the Wells Creek Formation consists of waxy, dolomitic, and pyritic, green, gray, and black shales, argillaceous limestone, and dolomite (Wickstrom and Gray, 1989). The Black River

Limestone sits on top of the Knox Group where Wells Creek Formation is absent. In terms of petroleum geology, the Wells Creek Formation acts as a seal rock for the hydrocarbon reservoirs of Knox Group (Wickstrom et al., 2005).

Black River Limestone

The Black River Limestone, which overlies the Wells Creek Formation and underlies the

Trenton Limestone, consists of fine-grained, tan-gray limestone which is about 91 meters thick in northwestern Ohio and about 152 meters thick in eastern Ohio (Hansen, 1997). Wickstrom et al. (1992) noted that the Black River Limestone consists of tan to light brown or gray micritic, fine-crystalline limestone. Minor amount of chert is also present in the Black River Formation in northwestern Ohio. Mottling and burrowing are common with fossiliferous zones which include brachiopod, gastropod, ostracod, mollusks, and trilobites (Wickstrom et al., 1992).

The depositional environment is interpreted to be a shallow subtidal to supratidal carbonate environment (Wickstrom et al. 1992) or a shallow subtidal to peritidal carbonate environment (Patchen et al., 2006). The Black River Limestone in the Appalachian basin was deposited in a broad and stable shallow water ramp at a time when the basin architecture was transitioning from a passive or extensional regime to a compressive regime (Patchen et al.,

2006). This shallow water carbonate ramp extended from the Michigan Basin through the central

Appalachians towards the Rome Trough.

Trenton Limestone

The Trenton Limestone consists of fossiliferous limestone within light gray matrix, with minor thin gray to black shale beds, and bentonite layers. Much of the unit has been dolomitized. The depositional strike of the Trenton Limestone is dominantly northwest to southeast in northwestern Ohio (Wickstrom et al., 1992). According to Cohee (1948), the limestone and dolomite in the Northwestern Ohio and adjacent region range from less than 61m to 274 m. The thickness of all these stratigraphic units gradually increases towards the

Appalachian basin, while these units thin towards the aforementioned arches. Oil and gas shows in the past have been attributed to the fracturing and dolomitization of the limestone.

Origin of Name. The nomenclature of Trenton Limestone comes from the literature by

Vanuxem (1838) who used this name to designate the rocks exposed at the Trenton Falls in

Oneida County, New York. He defined the rock unit to be a 30 m thick sequence of light gray

fossiliferous limestone. Newberry (1869) then the chief geologist of Ohio, first used the term

Trenton Limestone to describe a lower Silurian strata of limestone exposure in Ohio. The basic

idea was to correlate Ordovician limestone strata from Ohio to the Mohawk River Valley in New

York. To date, the name “Trenton Limestone” has been used in Ohio to refer to the Middle to

Upper Ordovician limestones, and in Indiana and Michigan for the Upper Ordovician carbonate

strata.

Depositional Environment. The properties of Trenton Limestone such as textures,

sedimentary structures and fossils have been used to divide it into three primary facies, the open

shelf facies, platform facies, and platform margin facies (Wickstrom et al., 1992). These facies have been interpreted to have resulted from a marine transgression from the southeast to 27 northwest. Due to the transgressive nature of deposition, lateral and vertical delineation of the abovementioned facies is difficult. Wickstrom et al. (1992) described the facies using the carbonate sedimentation models proposed by Wilson (1975).

Open-shelf Facies. It has been described to be a relatively thin basal facies which is found throughout the full extent of the Trenton Limestone interval. The open-shelf facies constitutes the full thickness of the Trenton Limestone in the southeastern part. It is an important marker for the reconstruction of the overall Trenton Limestone sedimentation framework. The rocks of this interval were deposited in shallower to restricted environment which is analogous to the underlying Black River Group (Wickstrom et al., 1992).

The depositional environment transitions upward from open-shelf to shallow platform environment. In the southeastern part of the study area, a transition is observed from the Black

River Group shelf environment to a shallow, subtidal, open-shelf environment in the Trenton

Limestone, and then to deeper water, basinal sediments of the Point Pleasant Formation. The open shelf facies is approximately 6-30 meters thick and is dominated by the presence of wispy to nodular bedded, gray to light-brown bioclastic limestone. Wackestone and packstone dominate the overall lithology with minor presence of carbonate mudstone and grainstone.

Extensive bioturbation is observed, which has homogenized the facies at many places. The fossils found in this facies are predominantly partially abraded brachiopods and bryozoans with lesser amounts of crinoids and trilobite fossils are also present in minor amount. The presence of abundant ostracodes is reported at the top of the formation to the southern and eastern extremities (Wickstrom et al., 1992). 28

Platform Facies. This facies overlies the open-shelf facies in the northwestern part of the

formation, and constitutes the majority (~30 to 68 meters) of the total thickness of Trenton

Limestone in the study area. The constituents are predominantly massive light to dark-brown

grainstones and packstones. There are also the minor presence of wavy bedded wackestone and

dark shales as partings and thin layers.

The dominant fossils are brachiopods and crinoids, with rare bryozoans and trilobites.

These are found in abraded condition and their sizes range from microscopic to 5 centimeters

across. Abundant brachiopod and crinoid debris form a thick sequence of brachiopod and crinoid

grainstone. The base of this facies grades to a zone of bryozoan mounds in the open shelf facies.

Wickstrom et al. (1992) interprets the platform facies to have been deposited in shallow,

open, normal marine conditions which are common on carbonate platforms. This is because the thick sequence of crinoid-brachiopod grainstone in this facies represent platform edge sand bars which shifted above the shallow sea floor.

Platform-margin Facies. This is the third primary facies of the Trenton Limestone which

occurs along a zone which thickens from Darke County to Ottawa County. This facies is highly

variable in both thickness and rock types. Rock types include grainstone and mudstone which

comprises of the similar fossil assemblages as the other two primary Trenton facies. This facies

is characterized by scour features, and lag concentrations of fossils, and lithoclasts.

Wickstrom et al. (1992) proposed that the platform of Trenton Limestone in northwestern

Ohio developed penecontemporaneously with the basinal deposits of the Point Pleasant

Formation to the south and east. This resulted to a very gentle slope on the platform margin. As

this environment was susceptible to sea floor disturbance by waves and storms, the subsequent 29

deposits are diverse in lithology and sedimentary features. The fossils found in this facies are

assumed to have been washed into this setting from adjacent facies.

Petroleum Geology. The Trenton Limestone in Northwestern Ohio acted as a major

reservoir for hydrocarbons (Wickstrom et al., 1992; Caprarotta et al., 1988). Traps include

anticlines, faulted anticlines, updip facies changes, drapes, and fractured reservoirs (Coogan and

Parker, 1989). Hydrocarbons in the Trenton Limestone are associated with the formation of white saddle dolomite (Prouty, 1989) and vuggy macrocrystalline dolomite (Caprarotta et al.

1988) which contains adequate porosity conducive to hydrocarbon accumulation.

The Lima-Indiana oil and gas trend was the second giant oilfield discovered in North

America and is of immense interest still today because of the secondary recovery potential.

Orton (1888) was the first to study the Trenton Limestone as a major reservoir for oil and gas.

Commercial quantities of oil were discovered in 1884 (Wickstrom et al., 1992) and subsequently, production rose to substitute the declining production from the Pennsylvania fields. In this way, the focus of the oil and gas industries shifted to northwestern Ohio. Production of oil and gas from this region rose to 20 million barrels of oil equivalent (MBBOE) in 1896 which was the peak of production and which fell abruptly by the middle 1930s. The decline of production in the

Lima Indiana trend followed by discoveries in Texas resulted in the shifting of focus of oil and gas industries to the south (Wickstrom et al., 1992).

Production was carried out in a very large scale and in an abrupt manner during this period, without any detailed study. Hence the use of the secondary recovery potential cannot be ruled out in spite of some unsuccessful attempts by water flooding (Wickstrom et al., 1992). 30

Though it is anticipated a large amount of original hydrocarbon is still in place, recovery is not

easy due to the heterogeneous nature of the reservoir. This problem is further enhanced by many

unaccounted wells which were not plugged after production discontinued. However, new

methods of secondary recovery are being discovered, such as the gravity drainage method. But it

is still not economically feasible because of the high investment cost of these operations

(Wickstrom et al. 1992; Wolfe, 1996).

In spite of the loss of interest due to inadequate production, 60,000 wells are still producing oil and gas (Wickstrom et al., 1992). Most of these wells are strippers wells which produce less than 10 barrels equivalent of oil equivalent a day (Wickstrom et al. 1992).

METHODS

This study involved stratigraphic analysis of three wells (with cores + geophysical logs), two wells with geophysical logs only, and two wells with thin sections only. The studied cores and wells were selected from three counties of northwest Ohio namely Wood, Hancock and,

Wyandot counties (Figure 15). Well preserved entire sections of two cores were available, the

third core had certain sections available. One of the cores was split transversely, which helped

for more detailed study. An evaluation of depositional environment and diagenesis was made

using thin sections (51 samples from 4 wells), SEM analysis (15 samples from one well), and cathodoluminescence analysis (15 samples from well no. 3564). The description of the source

wells and method of study is listed in Table 1.

Core Stratigraphy

Cores were examined at the H.R. Collins Core Laboratory in Delaware, Ohio. About 27

m of core sections were studied from Core no. 3564 (figureA3) and 30 m of core section was studied from Core 3374 (figure A2). Core stratigraphic analysis was carried out using visual identification of lithology, sedimentary structure, color, and texture. Graphical sections of the core logs were prepared using image editing in the Adobe Illustrator program.

Geophysical Logs

Geophysical logs (gamma-ray, density, and borehole compensated sonic logs) from five wells (3374, 3564, 2878, 2973, 20239) were evaluated. The geophysical logs were used to

Figure 15: Location map of the study area (modified from Ohio, DNR Oil and Gas GIS Maps-

2014) showing the studied wells. For the grey wells, only geophysical logs were available.

Table 1: List of study methods for the studied cores

Core Location Core Thin Cathodolu SEM Geophys Depth

No. (UTM) Secti Secti - mi - Study ical Log Interval

-on on nescence (Paper)

3564 4530025.5N √ √ √ √ √ 442-470m 312132.2E (Gamma (15 (15 (15 ray and Sonic) Sam- Samples) Samples) From ples) 20349

3374 45098713N √ None None None 391.6- 29706.5E (Gamma Ray and 408.4 Density) 2878 4432765.9N None 11 None None (Gamma 395-404m 289482.1E Ray and samples Density)

2971 4586844.5N None 21 None None None Unknown 269604.9E samples

2972 4591615N None 4 None None None Unknown 271634.9E samples

2973 4521842.7N None None None None 530.3- 315610.6E (Gamma 579.1m Ray and Density) 20239 4593485.8N None None None None (Gamma 350.5- 273565.5E Ray) 443.5 20347 4593493.3N None None None None (Gamma 357.23- 273570.1E Ray) 443.18

34 match porosity calculated from the geophysical logs either micro scale (thin sections), mesoscale

(hand samples) or macroscale (well cores) samples in well 3564. Different petrophysical properties of rocks can be identified using geophysical log records. Geophysical logs provide continuous records of lithology, porosity, fluid content and their type, presence of hydrocarbon.

Cross-plots of different parameters obtained from geophysical logs can be interpreted to ascertain subsurface geology (Poupon et al., 1971). For example, the combination of high gamma-ray values and low-density log values can denote shale. Or, in the case of carbonate rocks, use of geophysical logs may help identify diagenesis and distribution of porosity.

Diagenetically altered zones of porosity and cementation have distinct log response compared to other zones because of differences in density logs. A brief description of the types of logs to be used in this study are given below (Rider, 1986).

The gamma-ray log measures the natural radioactivity of a formation which is caused by the presence of 40K (potassium), 238U (uranium), and 232Th (thorium). Generally, shale has high gamma-ray due to the abundance of 40K in shale. Carbonate rocks and most sandstones typically show low gamma-ray response, which may help in identifying their presence in contrast with shale. The gamma-ray log is also used for lithocorrelation between spatially distributed wells

(Whittaker, 1998).

The neutron log is generated due to the interaction between fast neutrons emitted by radioactive elements in the log tool with the nuclei of hydrogen and other atoms in the rocks. The count of backscattered neutrons is converted to the hydrogen index or concentration of hydrogen atoms, which can ultimately be used to determine porosity in water-filled reservoir rocks (Rider,

1986). The neutron log values can be directly converted to neutron porosity units using after 35 necessary corrections using the porosity equation (Brown and Bowers, 1958). It is best to calibrate neutron porosity calculations by correlating the log data to core samples.

The density log is created by measuring the backscattering or attenuation of high energy bombarded gamma radiation between the tool source and the detectors. This attenuation, known as the Compton scattering, is a function of the number of electrons in the formation or the density of electrons (Rider, 1986). Compton scattering is measured with the help of density logging tool which contains a gamma ray emitter and two detectors placed at different distances

(two detectors are used to compensate for the borehole effects). The detectors are essentially scintillation counters that are placed at distance from the source. Electron density measured by this method is closely related to the formation density. Hence readings from the density log is presented as bulk density of the formation at different depths. If the matrix density and nature of fluid present in the formation is known, porosity can be calculated from the density (Rider,

1986).

Multiple types of geophysical log responses can be combined together to construct

‘electrofacies’ which is the combined response of multiple log responses. Electrofacies and core samples can be used in conjunction to interpret depositional environment.

Petrographic Study

The samples for thin section study were selected so as to cover all types of lithology observed while studying core sections. Petrographic study was performed by the analysis of 51 thin sections under polarized microscope. I prepared 15 thin sections from core 3564 (Table 2) and vacuum impregnated 2 of the thin sections with blue epoxy to distinguish porosity. In addition, thin sections from cores 2878, 2971, and 2972 were supplied from the H. R. Collins 36

Core Laboratory (Table 3). Thin sections were etched with 1% HCl and then stained with

Alizarin Red S solution and potassium ferricyanide solution which revealed the distinction

between calcite, ferroan calcite and dolomite. Alizarin red solution was prepared by using the

protocol formulated by Evamy (1969) where the solution contained 0.2% hydrochloric acid,

0.2% potassium ferricyanide and 0.2% alizarin red S.

For the purpose of this research, petrographic examination was performed along with

core lithologic descriptions in order to enhance the validity of interpretations made from

lithofacies analysis. With the help of petrographic analysis, the composition of grains, and

matrix, depositional fabric, and diagenetic features were studied. Detailed petrographic analysis

is a useful way to relate potential reservoir rock properties to their depositional fabric.

Petrographic study is also one of the most effective methods of studying the diagenetic history of sedimentary rocks. Diagenesis is mainly characterized by multiple stages of cementation,

dissolution, replacement, recrystallization, and/or neomorphism. Those processes cause secondary porosity to be developed, which may help the rock act as reservoir rock, subject to hydrocarbon emplacement.

The rocks of the Trenton Limestone were classified using system of Folk (1959) and

Dunham (1962). Terminology from both classification systems were used for the convenience of description. Dolomite texture was classified on the basis of Sibley and Gregg’s (1987) classification scheme (Figure 16).

Mineralogy and Constituents of Carbonate Rocks

A wide range of constituents are encountered while studying the petrography of carbonate rocks. The constituents can broadly be classified into skeletal and non-skeletal grains. 37

Table 2: Samples collected from core no. 3564 for petrographic study

Sl. No. Sample No Depth (ft) Depth(m) 1 W3564-7 1446.9 441.0 2 W3564-6 1451.5 442.4

3 W3564-8 1453.3 443.0 4 W3564-5 1460.5 445.2 5 W3564-9 1463.2 446.0 6 W3564-4 1466.8 447.1 7 W3564-10 1489.6 454.0 8 W3564-11 1494.5 455.5 9 W3564-12 1497.5 456.4 10 W3564-13 1501.5 457.7 11 W3564-3 1511.5 460.7

12 W3564-2 1519.5 463.1 13 W3564-14 1534.8 467.8 14 W3564-1 1541.8 469.9 15 W3564-15 1543.5 470.5

Non-skeletal grains are the type of carbonate grains which are not derived from skeletal remains of organisms. According to Folk (1959) four types of skeletal grains are recognized which are not derived from skeletal remains these are coated grains, peloids, aggregates and clasts

(reworked fragments). On the other hand, skeletal grains are the ones which are derived from skeletal remains of organisms. Many types of marine organisms are capable of producing skeletal material. Tucker and Wright (1990) stated that correct identification of skeletal grains is critical to the correct interpretation of depositional environment. Skeletal grains can be identified by their shape, mineralogy, and microstructure. The mineralogy of skeletal materials is a good indicator of the stages of diagenetic alteration because the original composition of the skeletal grains consists primarily of aragonite or high-Mg calcite. These minerals can be altered by the process of diagenesis which may include replacement, recrystallization, dissolution and dolomitization. Apart from non-skeletal and skeletal grains, carbonate rocks may contain matrix materials which are the fine grained component of a carbonate rock. According to Tucker and

Wright (1990) the materials which are less than 62 µm in diameter are considered as matrix.

Matrix materials which are composed of microcrystalline calcite are known as micrite. Calcite and dolomite are the two main constituents of carbonate rocks. As dolomite occurs in different forms and different classification methods can be adapted to describe these, the classification proposed by Sibley and Gregg (1987) will be used in describing dolomites.

Figure 16: Planar and non-planar dolomite (Sibley and Gregg, 1987).

Table 3: Thin section used for petrographic study and cathodoluminescence analysis

Core No. No. of Thin Petrographic Cathodoluminescence Depth Section Study Study Intervals

3564 15 √ √ 442-470m

2878 11 √ X 395-404m

2971 21 √ X Unknown

2972 4 √ X Unknown

Cathodoluminescence

The cathodoluminescence method uses the stimulation of a polished rock surface by electron bombardment which emits characteristic luminescence inherent to the property of the material. Cathodoluminescence helps in making fabrics visible that are not visible by standard petrographic and electron microscopy, such as recrystallization nuclei and fronts in skeletal carbonates. Cement stratigraphy is another application of cathodoluminescence imaging which is the correlating of cemented carbonate rocks by interpreting equally luminescing zones as diagenetically coeval, and by inferring cement sequences (Meyers, 1974). Origin of dark and bright cements in carbonates can be related to the presence of activator and quencher ions in the carbonate. In carbonate cements, Mn acts as an activator and Fe acts as quencher. The presence of higher proportions of Fe will make a carbonate cement dull, while the presence of higher amounts of Mn will make the cement bright. In figure 17 the effect of activators and quenchers in cathodoluminescence are shown.

The instrument used was a Technosyn cold cathode luminescence cathodoluminescence microscope fitted with a Nikon light microscope, Optronics camera, and computer for capturing images at the Geology Department of Bowling Green State University.

Cathodoluminescence was used on 15 samples from well 3564 to establish a cement stratigraphy.

Each sample was prepared by polishing a thin section and then observing it under cathodoluminescence. Cathodoluminescence helped to identify the different episodes of cementation and type of carbonate cements. The spectral profile helped in distinguishing between dolomite and calcite cements. Based on the relative brightness and zoning, dolomite

Figure 17: CL zones in calcite and dolomite, luminescent and non luminescent zones arise from dominant concentration of Mn and Fe respectively. Modified by Pagel et al. (2000) from Machel and Burton (1991).

43 cements were classified. Origin of dark and bright cements in carbonates can be related to the presence of activator and quencher ions in the carbonate.

Scanning Electron Microscopy/ Energy Dispersive X-Ray Analysis

Scanning electron microscopy (SEM/EDAX) was used as a tool to identify mineral composition, nature of pore space and elemental composition of minerals. The principle of analysis involves bombardment of free electrons at high voltage on the samples and catching the secondary and backscattered electrons with detectors. Two types of imaging are mostly used in

SEM study. Secondary electron imaging is used for observing the topography of the samples while backscattered electron images are used for compositional study (Reed, 2005).

This study used 15 samples (representative of all the lithologies observed) prepared for the SEM study by mounting the samples on aluminum stubs and coating them with Au-Pd (gold- palladium) with Hummer VI-A sputter coater. The samples were taken from the same billets which were used for thin section preparation (Table 4). Images were captured using Hitachi S-

2700 scanning electron microscope at 20 kV accelerating voltage and suitable working distance for viewing the appropriate features at the Center for Microscopy and Microanalysis at Bowling

Green State University.

Energy dispersive X-ray analysis (EDAX) was carried out on selected areas of the SEM samples for obtaining elemental composition, to confirm the presence of certain minerals. For example, dolomite can be identified from its crystal morphology, but for confirmation, its composition has to be known, as so, presence of magnesium in the EDAX scan confirmed visual observations. The presence of minerals such as Si was used to confirm the presence of chert, and 44

the presence of Fe and S was used to confirm the presence of pyrite. EDAX was carried out by using the EDAX microanalysis instrument installed with the Hitachi S-2700 microscope.

RESULTS

Lithology

The lithologic descriptions were prepared by hand samples, thin sections, SEM/EDS, and

Cathodoluminescence analysis. The primary types of lithology observed are limestone, dolostone

and shale. It was not always possible to identify dolomite in hand samples, hence the term

‘dolostone’ was only applied to pervasively dolomitized sections exhibiting abundance of saddle

dolomites and hydrothermal dolomites which could be easily ascertained in hand samples.

Some of the representative lithology of the studied cores are illustrated in figure 18.

Figure 18A shows skeletal packstone with nodular mud. Abundance of fossil fragments are

observed in this section. Grain size distribution is polymodal. Brachiopod and echinoderm

fragments are identifiable. Figure 18B shows wackestone on the bottom portion and mudstone

on the top. Figure18C shows alternating mudstone and wackestone. Figure 18D shows skeletal

packstone from core 3374 with prominent presence of brachiopod shells.

Different types of dolomite textures were noted in the studied thin section from cores

3564, 2878, 2971 and 2972. SEM images were also taken to observe the composition of cements.

Cathodoluminescence study revealed three types of cement in the studied thin sections of core

no. 3564. The results are given in Table 7 in the diagenesis section.

LimestonU e

Dunham (1962) and Folk (1959) classification systems were used to describe the petrography of Trenton Limestone. These classification schemes combined together helped in

properly describing the petrography of Trenton Limestone from hand specimen and thin sections. 46

The composition of the Trenton Limestone in the studied sections consisted principally of

calcite and dolomite. In well #3564 the composition was principally dolomite but in the studied

thins sections from well #2971 and well #2972 considerable proportion of calcite was present.

Petrographic study helped in describing the microfacies and in predicting the depositional conditions under which it was deposited when combined with information gained from lithological and sedimentary structure study. Some of the lithologies observed under polarized

microscope are described as follows. Figure 19 shows blocky calcite cement which may have

deposited under meteoric or burial conditions. The photomicrographs were taken under plane

polarized light and cross polarized light respectively. Figure 20 shows skeletal (crinoidal)

packstone where discoidal crinoid plates and circular cross sections are visible. The grainy

appearance is from the micritic composition of the crinoid fragments. In figure 21, a brachiopod

fragment and a crinoid fragment is observed in micrite matrix. Figure 22 shows bioclastic

packstone with seams of insoluble clay and other organic matter. The bioclasts are mostly

crinoid fragments. Some non-homogeneous micro-spar cements are observed. From the cross-

cutting relationship it can be inferred that the insoluble clay seams formed after deposition of the

packstone. The clay seams resemble pressure solution phenomena where organic matter is also

present in association with the insoluble seams. The crinoid fragments show dusty appearance

because of the inherent porous nature of their surface. Figure 23 shows bioclastic wackestone

with moderately sorted, mud to micro-spar matrix around shell fragments. The color is medium

to light gray. The fossil fragments range from 50 to 300µm in size which include bivalve shells.

Figure 24 shows the picture of a bioclastic packstone with crinoid and brachiopod fragments

ranging from 50µm to 2.5mm. The matrix material is composed of micrite, calcite cement is also

observed besides micrite matrix. In Figure 25, peloidal wackestone is observed which is mostly 47

Figure 18 : (A) Skeletal packstone with nodular mud in well #3564 (B) Alternating mudstone and wackestone in well #3564 (C) Alternating mudstone and wackestone (varved) in well #3564 and (D) Skeletal packstone in well # 3374 on the top separated by abrupt contact in the middle. 48

U composed of micrite and microsparite. The size of the peloids range from 100 to 300 µm in

diameter.

Dolostone

The term ‘dolostone’ was applied to the studied hand samples and thin sections where

more than 50% of the constituents was dolomite. The occurrence of dolostone gradually

increases from bottom towards top of the studied wells but is not restricted to the top portion.

Dolostone in the studied wells are mostly associated with vuggy and altered porous regions. In

general, the grain size corresponds to the limestones as well as fossil content. Iron and

manganese content are relatively common, as observed from SEM/EDS.

A wide range of dolomite crystal morphology is observed in the studied sections. For

describing dolomite microfacies, the classification proposed by Sibley and Gregg (1987) was

used as mentioned earlier. On the basis of geometry, planar and non-planar dolomite was observed. The size of the dolomite generally ranges from 0.05mm to 0.7 mm, with occasional occurrence of larger crystals. In some of the thin sections multiple stages of dolomitization is

observed, where neomorphic dolomite replaces earlier dolomite. Distribution of dolomite types

is wide ranging, in general nonplanar dolomite is dominant, followed by planar-s and planar-e varieties. In figure 26 planar-e and planar-s dolomites are observed which have replaced the calcite in brachiopod shells. In figure 27 subhedral (planar-s) dolomites are observed along with a silicified crinoid shell. Figure 28 shows planar-s dolomite with relict peloidal texture and vuggy and moldic porosity. Dolomite fault breccia is observed in figure 29 where the original composition cannot be perceived due to extensive fracturing. Porosity is observed in between the brecciated fragments. Saddle dolomite with wavy extinction ranging from 0.3mm to 1mm in size is observed in figure 30. This type of saddle dolomite has occasional patchy occurrence in 49

between the commonly occurring planar and non-planar dolomites in the studied section. In

figure 31, a dolomitized brachiopod shell is observed under plane and cross polarized light

respectively. Some dissolution porosity is observed in the surrounding area of the brachiopod

shell with adjacent organic matter. Several phases of earlier cementation are observed, including

two phases of isopachous fibrous cement on the outer periphery of the shell. Blocky dolomite

cement is observed on the interior portion of the shell. Recrystallization porosity with

pyrobitumen stains is observed in figure 32. In figure 33 planar-e dolomite is observed with

some recrystallization porosity and argillaceous material. The distribution of the types of

dolomites in the studied section is wide ranging among which the planar-e dolomite exhibits

more intercrystalline porosity, which is virtually absent in the nonplanar dolomite. However, the

nonplanar dolomite occasionally exhibits vuggy dissolution porosity.

ChertU

Chert is observed in the studied sections in the studied wells. Abundant chert nodules

were observed in core no. 3374. Two types of chert were observed in the hand samples, the first

one being light colored chert replacing fossils, the second type being darker and more indurated with spherulitic texture. Under plane polarized light the chert associated with fossils appear very fine grained having spherulitic texture and the chert that are not associated with fossils tend to be coarse grained with the grain size between 0.25 to 0.35mm. Examples of chert replacement can be seen in figure 27. Most of the silicification observed occur after early calcite but some also appears to have formed after late stage dolomite replacements which signifies many episodes of silicification.

Shale

The shale observed in the studied sections are dark colored thinly laminated and are

calcareous in nature. The thickness of the beds ranges from a few millimeters to few centimeters.

Shales are mostly associated with dolomitic grainstone and packstone. Thin section study of shale revealed microcrystalline dolomitic composition within the shale laminae.

Bentonite

Prominent bentonite beds of thickness ranging from 0.5 centimeter to 3 centimeters were observed in core no. 3374. The bentonite beds are very conspicuous in appearance, homogenous,

and lacks any laminations. In geophysical logs they have high gamma ray response which is

known as bentonite kick. Due to the scale of the geophysical log from well 3564, the bentonite

kicks could not be individually identified because of the low resolution of the geophysical log.

However, from the appearance of the geophysical logs the presence of bentonite could be

implicitly ascertained. An example of bentonite can be seen in figure 34.

Lithofacies and Microfacies

Depositional features were not always clearly observable on hand samples as a result of

diagenetic overprint. Hence thin section study helped in identification of microfacies and

diagenetic events. Since lithofacies were described from hand samples and core photographs, for

microfacies interpretation only thin sections were used. Both depositional and diagenetic

microfacies were taken into consideration. The lithofacies identified were described in table 4

and microfacies types in table 5. Microfacies types were identified and interpreted according to

Wilson (1975) facies description scheme.

The following lithofacies were identified and described (Table 4). 51

Figure 19: Photomicrograph (under plane polarized light and cross polarized light respectively) of blocky calcite cement (C) and replacing dolomite (D) Depth -395.3m (Core 2878). 52

Figure 20: Bioclastic packstone with micrite matrix microfacies. Crinoid and mollusk fossils observed under plane polarized light. Depth -398.4m (Core 2878).

Figure 21: Photomicrograph of crinoid (C) and brachiopod (B) shells in micrite matrix, bioclastic packstone microfacies observed under plane polarized light. Depth -396.8m (Core 2878). 53

Figure 22: Bioclastic packstone microfacies with crinoid (C) and other matrix material observed under plane polarized light Depth -397.5m (Core 2878).

Figure 23: Bioclastic wackestone with mollusk and other shell fragments observed under plane polarized light. Depth -396.6m (Core 2878). 54

Figure 24: Bioclastic packstone microfacies showing brachiopod (Br) and other fragments observed under plane polarized light. Depth -403.3m (Core 2878).

Figure 25: Mudstone/wackestone microfacies showing peloidal structure (P) observed under plane polarized light. Depth -402.4m (Core 2878). 55

Figure 26: Skeletal packstone with brachiopod shells (Br) being replaced by dolomite. Stylolitic seam containing pyrobitumen observed under plane polarized light. Depth -431m (Core no.

2971).

Figure 27: Chert (Ch) replacement of micritized crinoid shell observed under plane polarized

light. Depth -450.6m (Core no. 2972). 56

Figure 28: Planar-s dolomite with vuggy porosity with relict peloidal texture observed under plane polarized light. Depth -460.8m (Core no. 3564).

Figure 29: Dolomite fault breccia observed under plane polarized light. Depth -460.8m (Core no.

3564). 57

Figure 30: Saddle Dolomite with wavy extinction observed under cross polarized light. Depth

-445.2m (Core no. 3564).

Figure 31: Pseudo-punctate brachiopod shell with antitaxial and blocky dolomite cement on the inside an isopachous fibrous cement on outside observed under plane polarized light (left) and cross-polarized light (right). Depth -454m (Core no. 3564).

Pyrobitumen

Figure 32: Planar-s dolomite with recrystallization pores hosting pyrobitumen observed under plane polarized light. Depth -463.1m (Core no. 3564).

Figure 33: Planar-e dolomite with surrounding organic matter and argillaceous substances observed under plane polarized light. Depth -442.4m (Core no. 3564). 59

Figure 34: Bentonite bed in between packstone in core no 3374 at a depth of -406.5m.

Carbonate Packstone (Cpm)

This lithofacies consists of gray to light gray, parallel to wavy bedded skeletal packstone

with associated mudstone. The mud content is significantly lower than that of Cps lithofacies.

The skeletal fragment consists of brachiopods and crinoids. Scour features are often observed between the individual beds. Thickness of individual beds range from a few millimeters to few centimeters. An example of this lithofacies is given in figure 35.

Highly Bioturbated Wackestone Lithofacies (Cws)

This lithofacies consists of light gray to tan wackestone with uniform grain size. It is very common throughout the lower section of the studied core in well #3564. Intense bioturbation is observed in some portions of the studied section. The thickness ranges from a few centimeters to several meters. Generally, it is found in association with skeletal packstone (lithofacies Cps).

Some relict parallel to wavy laminations are observed. Bioturbation may resemble stromatactis structure at times. A figure of the lithofacies is given (Figure 35).

This lithofacies was possibly deposited in back ramp environment above the fair weather wave base or in a shoal environment (Burchette and Wright, 1992).

Massive Carbonate Grainstone Lithofacies (Cgm)

This lithofacies consists of massive medium-to coarse-grained carbonates (grain supported) interbedded with parallel to wavy laminated grainstone, and occasionally grades to packstone. The lithofacies consists of bioclasts and lithoclasts with minor bioturbation. This lithofacies is very common throughout the entire section of the studied cores. The thickness ranges from a few centimeters to a few meters throughout the entire section. The bioclasts are dominated by brachiopods and crinoids. An example of this lithofacies can be seen in figure 36. 61

Table 4: Lithofacies table of Trenton Limestone

Sedimentary Lithofacies Lithology Interpretation structures

Carbonate Cml Laminated Tidalites. mudstone. Massive with shrinkage, Subaerial exposure in subtidal to Cmm Gray mudstone desiccation, intertidal energy conditions. fenestrae. Carbonate mudstone to Shallow subtidal to intertidal Cms Stromatactis wackestone with conditions. stromatactis Grainstone Massive or Shallow subtidal conditions, Cgm (with interlaminated with below FWWB. intraclasts) mudstone Wavy bedded Shallow subtidal to intertidal, Skeletal Cps accompanied by intermediate energy condtions, Packstone shell hash. below fair weather wave base. Shallow, subtidal to intertidal Dmm Dolomudstone Massive conditions. Shallow subtidal condtions. Dgm Dolograinstone Massive Shallow ramp, below FWWB Alternation of planar laminated Planar laminated Cmw carbonate Tidalites. with bioturbation mudstone and wackestone Alternation of skeletal Cpm packstone and Wavy bedded Tempestites.- skeletal mud/wackestone Cmf Alternation of Flaser bedded Tidal rhythmites with ripple packstone and heterolithic ripples marks. mudstone. and mud drapes Cws Carbonate Bioturbation and Tidal rhythmites. wackestone lamination

Massive Dolo-grainstone Lithofacies (Dgm)

This is one of the most abundant lithofacies found in the studied section of Trenton

Limestone. This lithofacies consists of massive medium-to coarse-grained dolograinstone interbedded with parallel to wavy laminated grainstone, and occasionally grades to packstone.

The lithofacies consists of bioclasts and lithoclasts with occasional bioturbation. This lithofacies is common throughout the entire section of the studied cores. The thickness ranges from a few centimeters to a few meters throughout the entire section. The bioclasts consist of brachiopods and crinoids where the shells are occasionally micritized or replaced by dolomite.

Prior to dolomitization, these grainstones can be interpreted as shell hash that formed above the storm wave base in a region characterized by constant wave action and winnowing

Burchette and Wright’s (1992). Though depositional features are overprinted by pervasive dolomitization, there is still evidence of fine lamination. A figure of the lithofacies is given

(Figure 37).

Gray Carbonate Mudstone Lithofacies (Cmm)

This lithofacies consists of massive, fine-grained, light gray, mudstone. Fossils are absent. The thickness of this lithofacies ranges from a few centimeters to a few meters. This lithofacies is generally overlain and underlain by skeletal packstones interpreted as storm beds

(lithofacies Cpm)

According to Burchette and Wright (1992) this type of deposits is found in outer ramp environment below the storm wave base where sediments fall from suspension in a calm depositional setting. A figure of the lithofacies is given (Figure 37 and Figure 38).

Alternation of Calcareous Shale and Carbonate Mudstone Lithofacies (Cml)

This lithofacies unit consists of alternation of carbonate mudstone and shale which varies in color from light brown to dark gray. Shrinkage or desiccation features are observed in some of the sections. The thickness ranges from a few centimeters to about a meter and it amalgamates with mudstone (lithofaies Cmm) and wackestone (lithofacies Cmw). A figure of the lithofacies is given (Figure 39).

This lithofacies could have been deposited in a tidal environment of deposition where each of the thin laminations are event beds for high and low tides. This lithofacies may have deposited in the back ramp peritidal inner ramp environment according to Burchette and Wright

(1992).

Skeletal Packstone (Cps)

This lithofacies consists of gray to dark gray packstone with mud matrix. It contains interbedded lenses of brachiopods, crinoids, and bryozoan fragments. The thickness of the lenses ranges from a few millimeters to a centimeter. The composition of the lenses is relatively mud rich and may transition to lithofacies Cmm. A figure of the lithofacies is given (Figure 38).

According to Brookfield and Brett (1988) this lithofacies may have been deposited in a shelf or lagoonal environment which represents agitated conditions and subsequent non-deposition. In the carbonate ramp classification system of Burchette and Wright (1992) this lithofacies possibly belongs to the inner ramp region.

Carbonate Mudstone to Wackestone with Stromatactis (Cms)

This lithofacies is characterized by the presence of stromatactis structure. The

composition ranges from well cemented and indurated mudstone to wackestone which ranges

from light yellow to gray in color. An example of this lithofacies is given in figure 40.

Dolomudstone Lithofacies (Dmm)

This lithofacies consists of fine-grained massive, dolo-mudstone. The color ranges from

light gray to dark gray (Figure 41). Nodules of chert are observed at different intervals. Due to

dolomitization this lithofacies often appear to be homogenous but ghosts of burrows are often

observed. This lithofacies is dolomitized version of lithofacies Cmm. It is interpreted to have

been deposited in mid to outer ramp facies in subtidal environments below the fair weather wave

base.

This lithofacies might correlate with the “basal chert-bearing lithofacies” observed by

Budai and Wilson’s (1991) description of upper Black River Formation in the Michigan basin.

Alternation of Planar Laminated Carbonate Mudstone and Wackestone Lithofacies (Cmw)

This lithofacies consists of alternating dolomitized mudstone and wackestone arranged in parallel layers of about one millimeter to several millimeters thickness. It is characterized by vertical burrows, and absence of fossil remains, and crude laminations that is light gray to tan in color.

This lithofacies was possibly deposited in outer ramp calm water conditions below the

SWWB because of the presence of burrows. Burrows represent calm conditions under which the

Cpm

Figure 35: Lithofacies Cpm and Cws at -447.4m and 465.6m in core no. 3564.

Figure 36: Core photograph of lithofacies Cgm at -450m in core no. 3564.

Figure 37: Lithofacies Dgm at -404.62m in core no. 3374 and Cmm at -453.4m in core no. 3564.

Figure 38: Lithofacies Cps and Cmm at -454.8m in core no. 3564.

Figure 39: Core photograph of lithofacies Cml at -447.8m in core no. 3564. 68

Figure 40: Lithofacies Cms and Cmm at -464.8m in core no. 3564.

Figure 41: Lithofacies Dmm at -470.74m in core no 3564.

Figure 42: Lithofacies Cmw at -464. 4m in core no. 3564.

Figure 43: Lithofacies Cmf (Heterolithoc ripples and mud drapes) at -454.1m in core no. 3564

organism escaped and subsequent infilling by sediments. A figure of the lithofacies is given

(Figure 42).

Alternation of Packstone and Mudstone (Cmf)

This lithofacies consists of mudstone to wackestone exhibiting heterolithic ripples and mud drapes sedimentary structures. The color ranges from tan to light gray. The thickness of individual laminae ranges from 1-3 millimeters. A figure of the lithofacies is seen in figure 43.

This lithofacies is considered to be ‘tidalite’ deposit which have been deposited under the

influence of tidal current as characterized by its sedimentary structure (Lasemi et al., 2012).

Microfacies

The petrographic observation was matched with common microfacies from Wilson

(1975) and were listed in Table 6. Petrographic observation was used as a source of mircrofacies

classification. While the carbonate microfacies were matched with the Wilson (1975)

microfacies classification, the diagenetic facies were listed in terms of their abundance in the

studied sections.

Lithofacies Associations

Back Ramp (lagoonal) Lithofacies Association

This lithofacies association consists of the lithofacies Cmm (desiccation features) and

lithofacies Cms. The lithofacies Cmm represents alternation of planar laminated carbonate

mudstone and wackestone. Associated Cms lithofacies appears to be composed of the same

composition but with intense bioturbation which creates stromatactis (subtidal/intertidal)

structure. 72

The supratidal/back ramp region is the zone above the sea level which is intermittently flooded by high tides. Calm energy condition prevails in this environment within the shallow water. The depositional environment consists of lagoon and tidal flats. Rhythmitic deposits are deposited during changing tide in lagoons. Similar lithofacies has been described to be present in the

Trenton Limestone in New York by Brett and Caudill (2004). Figure 18C is an example lithofacies association.

In the lower portion of the studied section the lithofacies Cmm and Cms association is very abundant. Bioturbation indicates that depth of deposition was shallow and hence livable by burrowing organisms.

Tempestite Facies Association

This lithofacies association consists of the skeletal packstone lithofacies (Cps), grainstone lithofacies (Cgm), and heterolithic ripples and mud drapes lithofacies (Cmf). Skeletal packstones and grainstones are deposited in intertidal depositional environment. The heterolithic ripples and mud drapes are interpreted as tidal rhythmite deposits.

In figure 44, a typical proximal tempestite association is observed. This association is abundant in the middle portion of core no. 3564. The scour base on the grainstone is succeeded by skeletal lag deposits followed by ripples and repetition of scour base and skeletal lag. Similar successions are abundant in the middle part of the studied core section. In the tempestite sequence in figure 44, scour base is succeeded by skeletal packstone facies (Cps) followed by grainstone (Cgm) and herterolithic ripples and mud drapes. In figure 45, a similar sucesssion is observed where skeletal packstone (Cps) is succeeded by mudstone/wackestone followed by 73

skeletal packstone, heterolithic ripples and mud drapes. These associations are interpreted to be proximal ramp tempestite deposits.

Amalgamated Tempestite Association

This association is seen in the middle and upper portion of the studied well #3564. The

constituent lithofacies are skeletal packstone (Cps) and alternation of carbonate mudstone and

wackestone (Cml). The appearance of this lithofacies is rather jumbled because of the reworking of the previously existing layers.

The general meaning of amalgamation is the reworking or erosion of the upper part of a preexisting event layer (Einsele, 1998) prior to deposition of another layer. Amalgamated tempestites form when one storm event takes place shortly after another, or when one tempestite sequence is stacked over another. This type of type of tempestite sequence is observed in the

middle and upper portion of the studied well core no. 3564. According to Dattilo et al. (2008),

storm events can rework the previously deposited storm beds to form amalgamated beds which

become gradually cleaner towards distal ramp as a result of attenuation of energy required for

reworking. This implies that the distally deposited amalgamated tempestite will retain some of

the original depositional characteristics due to lack of reworking. This idea is further supported

by Brandt and Elias (1989) who stated that the proximal tempestitites are more likely to be

amalgamated and will show complex internal laminations. The amalgamated tempestite found in

the studied well core no. 3564 shows exactly the same characteristics with complex internal

characteristics. In figure 46 an amalgamated tempestite association is observed. This type of

sequence is common throughout the lower top part and upper middle part of studied well #3564.

Proximal Deep Ramp Lithofacies Associations

According to Tucker and Wright (1990), the deep ramp ranges from the fairweather wave base to the storm weather wave base. The proximal portion of the deep ramp depositional environment is a combination of lower energy calm depositional conditions with intermittent transition to higher energy conditions. This type of depositional environment has also been described by Brett and Caudill (2004) when describing lithofacies association of Trenton

Limestone in New York. As the location is well within the photic zone different types of fossil assemblages are observed. The deposits of this region include storm deposits, muds and interlaminated shale. Hence interlaminated shale, mudstone and skeletal packstone constitute this lithofacies association. This lithofacies association is abundant in the upper portion of core no.

3564.

Depositional Sequence

From studying the lithofacies association it has been observed the lower portion of the studied well core #3564 consists mainly tidally influenced lagoonal deposits. Tidal influence can be inferred from the rhythmic appearance of laminated strata. Intense bioturation is often observed in the fine-grained mudstone of this portion signifying suitable biogenic conditions.

Cmm and Cms are the dominant lithofacies which exhibit evidences of subaerial exposure. Some of the evidences of subaerial exposure are relicts of mudcracks, collapse breccia etc. The fine grain dominated lower portion slowly graduates to relatively coarser grained middle portion which is characterized by abundance of tempestites which are interpreted as storm deposits. This portion contains frequent scour marks, skeletal lags, wavy laminations and beds. This section is represented by the tempestites and amalgamated tempestites lithofacies associations. The dominant lithofacies constituting the tempestites and amalgamated tempestites are Cps, Cgm and 75

Cml. The upper portion is dominated by proximal deep ramp association which consists of alternation of skeletal packstone (Cps) and interlaminated mudstone and shale (Cml). This section is abundant in thinly laminated wavy shale and mudstone which are interbedded with packstone. From the succession of lithofacies from bottom to top, a general deepening upward trend is observed in the studied well core #3564.

Stratigraphy

Top and bottom contacts of the Trenton Limestone were demarcated by identifying changes in lithology and/ or geophysical log signatures. The contact between Trenton Limestone and overlying Utica Shale was easy to identify on the gamma ray log which exhibits an abrupt decrease in magnitude (Figure 47). The contact between underlying Black River Limestone and

Trenton Limestone was gradational as corroborated by previous work (Wickstrom et al., 1992 and Patchen et al., 2006) with alternating lithology characteristic of each units (Figure 48).

Gamma-ray log when cross-matched with borehole compensated sonic log provided a more reliable demarcation. Well card information available at the Ohio Department of Natural

Resources lab aided in determining the transition point. Later the demarcation was delineated on the geophysical logs roughly at the transition zone.

The stratigraphic trend of Trenton Limestone as inferred from the studied well sections shows variable thickness over the study area. At the studied well #3564 the thickness of Trenton

Limestone is 27.43 m. Figure 49 shows the stratigraphic correlation between the studied wells.

The correlation was done by peak matching technique of gamma ray logs.

Table 5: Lithofacies Association of Trenton Limestone

Association Facies Interpretation

Tempestite Association Cps, Cgm, Cmf Proximal tempestite above

FWWB

Amalgamated Tempestite Cps, Cml Proximal tempestite above

Associations FWWB

Back Ramp Association Cms,Cmm, Cmf Lagoonal/tidal facies

Proximal Deep Ramp Cml, Cps Deep ramp facies

Association

Tempestite

Figure 44: Proximal tempestite sequence at -454.1m (top) in core no. 3564. 78

Tempestite

Figure 45: Proximal tempestite sequence at approximately -452 m (top) in core no. 3564. Scour base not present. 79

Reworked skeletal packstone and laminated mudstone

Scour Base

Preserved internal laminations

Figure 46: Amalgamated tempestite sequence approximately at -451.7m.

Table 6: Microfacies of Trenton Limestone

Microfacies Type Wilson (1975) Interpretation

microfacies Code

Bioclastic packstone with SMF 12 Intermediate energy micrite matrix conditions

Bioclastic wackestone SMF 9 Low energy conditions

Peloidal mudstone + SMF 19 Restricted low energy wackestone conditions

Dolomitized mudstone - Diagenetic facies

Recrystallized dolomite - Diagenetic facies

Monomict dolomitic breccia - Diagenetic facies

The thickness of Trenton Limestone is highest in the northwestern section of the study

area (Wood County) which is about 76.2 m in well #20239 which thins towards south (Hancock

county) to about 8.2 m in well #2878 and thickens again in well #3564 and 2973 in Wyandot county. Top of Trenton Limestone in gamma ray logs were picked by abrupt decrease gamma

ray value. But the bottom contact between Trenton Limestone and Black River Group was

identified from the well cores and well cutting description as it was not very conspicuous.

However, when gamma-ray logs are cross matched with other geophysical logs such as the borehole compensated sonic log the bottom demarcation with Black River Group can be done with more confidence (figure 50). Gamma-ray response of Black River group is characterized by gamma-ray spikes at regular intervals. This character was used to delineate the boundary between Black River Group and Trenton Limestone.

Graphic logs for well no. 3564 were prepared and were compared with the gamma-ray log and borehole-compensated sonic log (figure 50). The low-density zones picked up by the borehole compensated sonic log corresponded with higher gamma ray responses in some of the intervals which were matched with red lines with the graphic log. Similar procedure was followed for well no. 3374 which is seen in figure 51. It was observed that, apart from the transition between overlying Utica Shale and Trenton Limestone, the gamma-ray response was

not always consistent with the lithofacies in well no 3564. But in well no. 3374, the abrupt

increase in gamma ray signals could be attributed to the sharply transitioning and unaltered

bentonite beds. But due to the low resolution of available gamma-ray log in well no.3374,

Figure 47: Contact between Utica Formation and Trenton Limestone in well #3564 at -442.5m

depth (Geophysical log scale does not correspond to hand sample scale).

Figure 48: Contact between Black River (bottom) and Trenton Limestone in well #3564 at

-469.95 m depth (Geophysical log scale does not correspond to hand sample scale). 83

Well no.3374 Well no. Well no.20239 Well no.2878 3564(20347)

Well no. 2973

Figure 49: Gamma ray log correlation of the studied wells of Trenton Limestone.

Figure 50: Correlation between geophysical logs and graphic log in the top portion of well

#3564. Higher interval transit time are correlated with porous zones.

Figure 51: Correlation between geophysical logs and graphic log in the top portion of well

#3374.

precise correlation with graphic log was not possible. For well no. 3564, the gamma-ray response within the section was not very consistent, possibly because of diagenetic alteration of clay material, where the radioactive minerals in clay were dissolved and mobilized by the percolating hydrothermal fluid through the fractures. But borehole compensated sonic log offered a good representation of the density. For example, the sonic log had more interval transit time in the

porous zone, while densely cemented and chertified zones showed less interval transit time.

Hence it can be inferred that the borehole-compensated sonic log can be used as a tool to identify

porous zones.

Diagenesis

The Trenton Limestone in Northwestern Ohio has been subjected to various stages of

diagenetic changes. Due to the diagenetic alteration, depositional features are often obscured. It

has also been observed in this research that most of the early diagenetic features have been

overprinted by late diagenetic features. Hand-sample analysis, polarized light microscopy,

cathodoluminescence microscopy, and SEM/EDS were employed to study diagenesis. Cement

stratigraphy was studied with the help of cathodoluminescene microscope. From the studied

evidence of diagenesis, the relative timing and nature of diagenetic features were recognized.

The diagenetic processes which acted on the Trenton Limestone are discussed below.

BurialU and Moderate Compaction

This is the mode of diagenesis that the carbonate rocks of Trenton Limestone initially

underwent. The initial burial took place in shallow subsurface region due to continued

sedimentation.

U 87

Calcite Cementation

Calcite cementation is the next step of diagenesis after burial cementation. The type of

cement occurs in both marine and meteoric environment. The type of calcite cements observed in

the Trenton Limestone are syntaxial, blocky (meteoric phreatic) or drusy calcite cement, fibrous

calcite cement, micro spar and blocky calcite. Calcite cementation after dolomitization which

replaces the dolomite (dedolomitization at much la ter stage) is also observed.

MicritizationU

U Fairly considerable amount of micritization is observed in the studied thin sections of

Trenton Limestone. Micrite rims form around fossil shells or many of the times the complete

fossil shell is micritized. Micritization is a bacterially mediated process taking place in the

shallow marine environment (Alexandersson, 1972). In figure 27 micritization of a shell is

observed.

Marine Hard Grounds

Marine hard grounds are lithified seafloor (Palmer, 1982) which are formed as a result

of combination of various physical and chemical processes in the seafloor. Several hardgrounds

were identified on the basis of their cemented appearance. The general characteristics contrasting

colored cements and some glauconite peloids (1-4mm), layers of compacted glauconite peloids underlain and overlain by different lithologies (Figure 52). The characteristics of hardgrounds match the ones observed by Fara and Keith (1988) in Trenton Limestone in Northern Indiana.

Pyritization

The process of pyritization involves the replacement of original calcium carbonate material by pyrite, which may include filling of vugs, void lines or original shell material (Wolf and Chilingar, 1994). In the studied well #3564, several pyrite crystals (authigenic) were observed in vugs and SEM (energy dispersive x-ray) analysis was carried out on a sample from at -452.5 meter (Figure 67). Given the nature of occurrence of pyrites in the studied core, it is

interpreted be a post dolomite reservoir mineralization product similar to the ones found in the study of Budai and Wilson (1991).

SilicificationU

Silicification in the form of chert nodules and silica replacement of vugs and fossils

was observed at different intervals of the studied section of Trenton Limestone. The silicification

in the entire section of the studied core occurs as late diagenetic process. Silicification process is also very fabric selective where the fossil fragments are preferentially silicified over the other cement and matrix. In figure 27 silicification of a crinoid shell is observed. Other examples can be seen in figures 55, 56, and 58. The chert found in the studied cores are mostly in nodular form, white colored and moderately porous. Several chertified intervals show that the void space in skeletal packstone were replaced by chert (figure 56).

Dolomitization

Dolomitization is the most dominant diagenetic feature in the studied well sections of

Trenton Limestone. Several stages of dolomitization are seen in Figure 57. Different modes of dolomitization have acted on the Trenton Limestone after deposition. These include, cap dolomitization, facies dolomitization, fracture dolomitization. Facies dolomitization as described 89 by the previous authors is the dolomitization that occurred as a result of lateral migration of magnesium-rich fluid from the Sebree Trough (Patchen et al., 2006). Cap dolomitization has been described to have occurred due to migration of magnesium-rich fluid from the overlying the

Utica Shale and the Point Pleasant formation. This type of dolomite form by mechanism described in McHargue (1982). Fracture dolomitization has occurred due fractures created by fault movement in the Bowling Green fault zone. The Bowling Green fault zone experienced recurrent tectonic movement throughout the Paleozoic (Onasch and Kahle, 1991). In this study, different types of dolomites were observed which denoted different episodes and timing of dolomitization. Almost the entire section studied was dolomitized in well #3564. In contrary, thin sections from well #2971 and 2972 showed almost equal distribution of dolomite and calcite. Three different modes of dolomitization were identified in this research, the first and most dominant type is the extensive type of dolomitization which is regarded as the facies dolomite in the previous literatures (Patchen et al., 2006; Wickstrom et al., 1992). In figure 54 saddle dolomite is observed. In figure 57 several episodes of dolomitization is observed as a result of fracturing and fluid migration through the fracture.

Evidence of collapse breccia was observed at different intervals in core no. 3564 is observed along with evidence of evaporite. Collapse breccia may have formed in response to fluid moving through fracture conduits. It may also have been formed as a result of solution weathering during eogenetic diagenesis. Presence of evaporite signifies subaerial exposure

(Figure 53). Similar presence of Karst collapse breccia in Michigan Basin has been documented by Ekberg (2008).

Figure 52: Marine hard ground with characteristic cemented appearance at depth of -464.4m (A) and -453.4m (B) respectively in core no. 3564.

Dedolomitization

Dedolomitization or replacement of dolomite by calcite was observed in the upper part of Trenton Limestone in well #3564. The dedolomites were formed at later stage probably resulting from late Paleozoic uplift and subsequent change in the chemistry of formation fluid.

The criteria for understanding dedolomitization was the evidence of corroded boundaries in dolomite where calcite crystals have replaced the pre-existing dolomite. In figure 60 and 61 the examples of dedolomitization are seen.

Cathodoluminescence

Cathodoluminescence study was performed to identify and classify types of cements.

Compared to luminescent dolomite cements, the very minor amount of luminescent calcite cements found in the samples from well #3564 were mostly associated with dedolomites. The dedolomites had significantly different appearance (color) compared to the luminescent dolomites. The modal peak used was to identify the difference between bright calcite and dolomite cements. The color of the cements were distinguished on the basis of spectral histogram where modal peak in case of dolomite was ~91 (figure 62) and for calcite it was ~46 (figure 63).

Three distinct types of luminescent cements were identified based on their appearance and occurence from cathodoluminescence images which are listed in Table 7 and shown in figure 64.

As mentioned earlier, the cathodoluminescent minerals observed are predominantly dolomite which are of bright and dull variety. In the upper portion of the studied section in well #3364 dull cements are observed which may have sourced from the overlying iron rich Utica Formation.

The planar-e luminescent dolomite showed 4-6 stages of zonation and zoned calcite showed up to 10 stages of zonation (Figure 63). The dolomite crystals exhibiting zoned 92

Figure 53: Karst collapse breccia at with angular chert fragments in well no 3564 at depths of

-463.1m and -461.3m.

Figure 54: Core photograph of saddle dolomite at -460.4m in well #3564.

Figure 55: Photomicrograph of silicified fossil shell and dolomite at -460.4m in well #3564.

(under plane polarized light).

Figure 56: Photomicrograph of late stage chertification at -454.1m in well #3564 (under plane polarized light).

Figure 57: Illustration of different stages of dolomitization in Trenton Limestone.

Figure 58: Fibrous silica (chalcedony, cross cutting dolomite fabric. Core no 3564, depth -

463.1m (under cross polarized light).

Figure 59: Glauconite replacement of shell and later dolomitization, depth

-455.5m (under plane polarized light). 96

Figure 60: Dedolomitization calcite replacement of dolomite by calcite at a depth of -463.1m.

Dedolomitization porosity is observed which is marked with arrow. The calcite is stained pink. The void space is filled with blue epoxy (under plane polarized light).

Figure 61: Dedolomitization calcite replacement of dolomite by calcite at a depth of -463.1m.

Dedolomitization porosity is observed which is marked with arrow. Image was taken under plane polarized light. Void space appears colorless as epoxy impregnation was not performed on this thin section. 97 cathodoluminescence are generally emplaced within non-luminescent or slightly luminescent groundmass matrix. The numerous banding pattern possibly happened because of frequent changes in chemistry of precipitating fluid which can be possibly established as individual cement stratigraphic unit with further study. The size of the cathodoluminescent dolomite crystals range from 50 to 300 µm and the size of cathodoluminescent calcite range from 300 to

500 µm. Apart from the luminescent crystalline grains there are luminescent fracture filling dolomite cements which have filled the fractures at later stage possibly due to diagenetic remobilization of manganese rich cement. Contrary to the fracture filling dolomites, the zoned dolomites have pronounced crystal faces which is reflected in the zoning.

Scanning Electron Microscopy/Energy Dispersive X-Ray Analysis Scanning electron microscopy aided in studying the nature of pores and their geometry along with mineral and chemical composition. Higher resolution combined with ability to infer mineral type from crystal morphology and elemental analysis resulted in more reliable interpretation when results were compared with petrographic microscopy results. Micropores were also identified from SEM images which were beyond the resolution capacity of light microscope. The major mineral composition in order of their abundance are dolomite and calcite, clay minerals, chert and, pyrites. Organic matters were aggregated with clay minerals. Dolomite and calcite crystals ranging from 0.002 mm to 0.5 mm were observed under SEM which appeared to be lighter on the edges and darker in the center. Smaller dolomite crystals formed around boundaries of larger calcite crystals wherever there was presence of calcite. As for dolomite crystal morphology, incipiently formed crystals with poorly defined crystal faces to well-formed planar-e types dolomites were observed. Calcite cements and crystals were observed which was lesser in abundance than dolomite. Pyritization observed under SEM images (figure 98

67) was confirmed by their elemental EDAX chart whitish appearance because of higher atomic

number. When EDAX was used to analyze elemental composition in dolomites a higher

proportion of iron and magnesium were evident. In terms of dolomite composition, scanning

electron microscope images revealed marine, ferroan origin with high magnesium contend as

seen from the EDAX graphs. In terms of porosity, SEM images revealed several types of microporosity, e.g., porosity in the form of vugs which occurred due to dissolution and weathering, intercrystalline pores which are angular in appearance. In figure 66 and 68 that the dolomite crystals showing intercrystalline pore space. The smallest micropores and fractures in

terms of scale were observed in clay rich samples when size of the pores of fracture measures

less than 1µm. The smallest micropores associated with clay did not appear to be interconnected.

Mesopores and macropores observed under secondary electron images which were mostly

associated with crystalline dolomite and calcite were mostly interconnected.

Paragenesis

The relative timing of the occurrence of the diagenetic events can be used to create the

paragenetic sequence. The paragenetic sequence was created based on the timing of diagenetic

events.

The paragenetic sequence can be listed as follows.

(1) bioturbation (2) authigenesis (glauconization and pyritization) (3) burial and

moderate compaction (4) calcite cementation (5) micritization (6) silicification (7) dolomitization

(8) late pyritization and, (9) dedolomitization

The sequence was developed on the basis of hand sample and petrographic observation

and following the law of cross cutting relationship. As the relative timing of each of the 99 depositional and diagenetic events are not mutually exclusive in the time domain, a schematic diagram (figure 65) was produced to show the observed temporal relationship of the diagenetic events. Description of the diagenetic events in the paragenetic sequence and their relationship is given below.

Bioturbation

The primary structures like bedding and lamination were frequently disturbed subjected to bioturbation. It can be inferred that the bioturbation took place immediately after deposition from activities of organisms near the surface. An example can be seen in vertical escape burrows in figure 18C.

Authigenesis (Glauconization and Pyritization)

U The glauconization and pyritization follows as prolonged period of sub-aqueous exposure. Authegenesis is evidenced by the presence of marine hard ground in the hand samples

(Figure 52) where glauconite minerals occur with characteristic green color. Example of glauconization under thin section can be observed in figure 59. Glauconite was also observed in thin section cross-cut by later formed calcite and dolomite cements. U

Burial and Moderate Compaction

The carbonate rocks of Trenton Limestone represent deposition in a peritidal environment which is characterized by its depositional texture. In Trenton Limestone, bedding which includes primary depositional structures such as bedding, cross-bedding, and, lamination, were retained in majority of the studied section. Initial compaction followed deposition resulting in minor decrease of initial depositional porosity. 100

100 µm

Figure 62: Spectral profile of the cathodoluminescent planar-e dolomite. Sampled from Core no. 3564. 101

Figure 63: Spectral profile of the cathodoluminescent calcite cement. Sampled from Core no. 3564. 102

Table 7: Types of cathodoluminescent cements

Cement Type Characteristics Interpretation

Type 1 Zoned Cement dark and bright rims Zones showing alternating dark and

bright cements denoting multiple

cement phases.

Type 2 Bright cement Mn rich cement

Type 3 Dull cement Mn poor/Fe Rich cement 103

Depth -442.5m

Dull ferroan cement in dolomite.

Depth -454m

Type 2 bright Type 3 dull

cement

Figure 64: Dull and bright cements in dolomite. The zoned cement is type 1 cement.

104

Early Cements (Calcite Cementation)

Cementation occurred as a result of precipitation of calcium rich pore fluids. This happened before other types of calcite cementation took place. Isopachous cements were not directly observed. In contrary, late cementation occurred as a result of burial diagenetic environment, this type of blocky calcite cementation is prevalent throughout the entire studied interval.

SilicificationU

Silica cementation and replacement took place after calcite cementation which is evidenced by replacement of calcite cements by silica. The reasoning behind placing silica cementation and replacement after calcite cementation was the observed cross-cutting relationship between the calcite and silica cements. The origin of silica has been anticipated to be from sponge spicules and volcanic glass (Fara and Keith, 1988). A corroborating photomicrograph is seen in figure 27.

Dolomite Replacement and Dolomite Neomorphism

This is the most important diagenetic step which took place after calcite cementation. The temporal distribution of dolomite replacement and neomorphism is widespread as different factors over time were responsible for different stages of dolomitization. Without the exception of late stage dedolomitization in the diagenetic history of Trenton Limestone, dolomitization postdates calcite cementation. The pervasive cross-cutting relationship between calcite cementation where calcite is replaced by dolomite is observed in the studied section.

Neomorphism of dolomite crystals is observed which took place as a result of recrystallization of dolomite under favorable conditions. All three modes of dolomitization observed in this study 105

namely, cap dolomitization, facies dolomitization and, hydrothermal dolomitization took place

after the calcite cementation.

DedolomitizationU U

Dedolomitization took place after dolomitization where dolomite was replaced by calcite

which is evidenced by the cross-cutting relation between calcite and dolomite.

While studying the sequence of diagenesis it was observed that some of the diagenetic

features like pyritization and pressure solution were not single episode events but happened

throughout the geological history of Trenton Limestone.

Porosity

Primary porosity in the Trenton Limestone is virtually non-existent in the studied section due to the multiple stages of cementation and dolomitization. Any depositional porosity (1o

porosity) was filled out by early diagenetic cementation. These cements were the in the form of

isopachous blocky cement, fibrous cement, and micrite rims. Subsequent porosity was created by

replacement reactions such a micritization. This porosity was eventually lost by 2Po P calcite

cementation (mosaic cements). This was followed by subsequent porosity gain due to dissolution

and fracturing, and porosity loss by dolomitization. Intercrystalline porosity was observed in

dolomite with organic matter between the intercrystalline spaces (Figure 32 and Figure 33).

Microposrosity was observed in the form of intercrystalline porosity in dolomite under

petrographic microscope and SEM (figure 66 and 68). The significance of microporosity was

that bitumen stains was evident in the dolomite intercrystalline microporosity which signifies oil

migration or in-situ production in the microporous zones. The size of the micropores ranged

from 3-8µm. The occurrence of microporosity is ubiquitous in mudstone and wackestone 106 consisting intervals of the studied sections. Vuggy or moldic porosity was observed in many intervals both in hand samples and thin section. Examples of macro-vuggy porosity is seen in figure 54 and another example of vuggy meso-porosity can be seen in figure 28.

Dedolomitization porosity (figure 60 and 61) was observed in thin section study but was not as abundant as the other types of porosity observed. The timing of porosity development with respect to petroleum migration is important in order to relate the porosity to petroleum prospect.

107

Bioturbation

Authigenesis (glauconization and pyritization)

Burial and moderate compaction

Calcite cementation

Micritization

Silica cementation and replacement

Dolomite replacement and dolomite neomorphism

(planar-s and planar-e)

Late Pyritization in vugs

Pressure solution (late burial) (late solution Pressure

Dedolomitization

Figure 65: A diagram of the paragenetic sequence in the studied section of Trenton Limestone. 108

Intensity(counts)

Figure 66: SEM image of dolomite exhibiting microporosity in core no 3564 at -463.1m.

109

Figure 67: Pyritized dolomite observed under SEM in core 3564.

110

Figure 68: Microporosity in dolomite observed under SEM in Core no. 3564 at a depth of -443m.

111

DISCUSSION

Depositional Environment

The Trenton Limestone was deposited in a peritidal carbonate ramp environment, which could be inferred from the stratigraphy and depositional texture. Evidence of back ramp depositional environment in the studied core no. 3564 includes bioturbated fine grained limestone, shell hash in wackestone and packstone, and dolomite associated with evaporites. In contrast evidence of rimmed shelf was not observed. This finding is in accord with previous literature which does not state the presence of the rimmed platform. Several transgressive phases in the studied section were observed which were evidenced by gradual shifting from fine-grained burrowed limestone to coarse-grained storm deposits. The presence of indurated glauconite is interpreted as condensed sections deposited during transgressive phases. Both top and bottom of the Trenton is marked by erosion and abrupt topography in Northwestern Ohio.

Stratigraphic Trends

The basal part of Trenton Limestone in the studied section consists of chert-bearing carbonate wackestone, finely laminated mudstone to wackestone, evaporitic dolomites which is interpreted to have been deposited in a tidal flat environment. The succession of tidal rhythmites with bioturbation are interpreted to have been deposited in a lagoonal environment. Abundance of chert and evaporitic solution collapse breccia chert indicated prolonged subaerial exposure in a supratidal environment. Hence it can be concluded that in the basal part most of the deposition took place in shallow water to subaerial environment.

The middle part of the studied core no. 3564 consists of several tempestite sequences and the tempestites are interpreted to have been deposited in a proximal ramp environment (Reed,

1985) because of the presence of interbedded skeletal packstones and wackestones with 112

prominent scour bases. The proximal ramp environment was being more susceptible to wave

action and hence deposits of this type have generally been related with moderate energy environment associated with inner ramp environment (Aigner, 1985).

The upper part of the Trenton Limestone in the studied section is characterized by interbedded grainstone/packstone with laminated shale which is indicative of shallow subtidal environment in back ramp area. Generally, this region is influenced by occasional tidal energy

which gives rise to such deposits. A schematic diagram of the depositional environment of

Upper Ordovician Trenton Limestone deposition in the studied area is given in figure 69. The

diagram was developed on the basis observed stratigraphy and interpreted depositional history.

Depositional Architecture

It is important to know the depositional architecture of a potential reservoir rock. The

depositional architecture of Trenton Limestone is in accordance with the western distal shelf

deposition in the Upper Ordovician Taconic Foreland basin. The Trenton Limestone in

Northwestern Ohio underlain by Black River Group on the bottom and the Utica and the Point

Pleasant Formation above. Unconformities at top and bottom of Trenton Limestone denotes

periods of erosion before and after the deposition of Trenton Limestone. The thickness of

Trenton limestone varies greatly possibly because of erosion before and after the deposition of

Trenton Limestone. Depositional thickness variation has been attributed to the Findlay Arch,

which is known to have formed after the deposition of Trenton Limestone due to differential

subsidence and probably did not have any influence on the depositional architecture. The decrease in thickness from northwest to southeast in the studied area can be attributed to the

113

Figure 69: Schematic diagram of Upper Ordovician depositional environment in the study area.

114

regional depositional environment which was gently sloping carbonate ramp environment.

Paragenesis

The paragenetic sequence was inferred from detailed observation of hand samples thin

sections and, cathodoluminescence study. Early diagenesis included bioturbation, isopachous

blocky calcite cementation (meteoric phreatic zones), isopachous fibrous and micrite rims

(marine phreatic zones). Evidence of stratiform karst or evaporite dissolution is confirmed by

presence of chert collapse breccia. Micritization of fossil shell is also observed which denotes

early replacement. In the middle stage of diagenesis evidence of burial environment is observed

which is evidenced by burial calcite cements. Silicification is observed in different types.

Dominant types of chert include fragmented chert and spherulitic chert. Chert fragments are

sometimes surrounded or cross-cut by dolomites. The occurrence of inversion can be inferred from the early diagenetic signatures are almost non -existent in core no. 3564 while it is somewhat clearly present in core 2878 and 2971 and 2972. Burial diagenesis involved compaction of the deposited limestone which reduced the porosity to almost zero at almost all locations. Frequent stylolitization is observed which is an indicator of deep burial.

Dolomitization may have occurred by interaction of marine and meteoric water during times of exposure but burial dolomitization through fault induces fractures is evident throughout the entire section. Saddle (vuggy) dolomites were deposited along the fractured zones which created porosity.

Porosity

Various types of porosity play role in helping the Trenton Limestone act as a reservoir for petroleum. Micro-, meso-, and macro-pores were observed in this study. While some of the 115

micro porosity are fabric selective, meso- and macro-pores can be attributed to dissolution or

fracturing, which is not related to fabric. Structural control along with the observed presence of

collapse breccia (stratiform evaporites) which gave rise to macroporosity may have also played role in compartmentalization of reservoir characteristics in the affected areas. Another aspect of porosity creation is the dolomitization porosity, which is still under frequent debate because the mass balance equation from calcite to dolomite seldom plays out in the real world scenario

(Lucia, 2004; Machel 2005). These authors have suggested the theoretical decrease in mass of rocks and subsequent porosity creation due to dolomitization is more of a theoretical assumption and cannot be backed by real world experiments. Ehrenberg et al. (2012) has debated the genesis of burial porosity by the generally agreed upon mechanism of mesogenetic dissolution. Hence, we conclude that the fluid responsible for fracturing and dolomitization had a more abundant source of Mg-rich connate water than formation pore fluid. This leads to the inference that fracture and dissolution porosity far outweighs the role of dolomitization matrix porosity.

Previous studies conducted by Keith (1986), Caprarotta et al. (1988) and, Prouty (1984) also corroborated the higher significance of fracture related porosity compared to mesogenetic dissolution and dolomitization. While we do not totally discount the role of mesogenetic dissolution in reservoir porosity development, we think it is not responsible for the bulk of the observed porosity.

Timing of Petroleum Migration

Petroleum migration apparently occurred in the final stage. Presence of pyrobitumen and saddle dolomite may be interrelated (both may have formed due to elevated temperature). This observation is bolstered by the fact that numerous oil inclusions are observed in the saddle dolomite. Migration may have occurred in both vertical and horizontal direction. Several oil 116 stains were observed in the vuggy and porous hand samples which can be inferred as an evidence of oil migration. Petroleum accumulation generally occur along fractured and vuggy dolomitized zones which was confirmed from oil stain in this type of porous zones. The stages of diagenesis were listed in the previous sections which were inferred from petrographic study.

117

SUMMARY AND CONCLUSIONS

The depositional environment and diagenetic sequence study of Trenton Limestone in the studied well core #3564 was undertaken in this study. This helped us understand the depositional history, diagenesis and petroleum migration. The lithofacies and lithofacies associated in well core #3564 indicated that the deposition took place in a peritidal portion of a carbonate ramp environment. Platform or platform margin facies were not observed in the well core #3564.

However, thin section analysis from wells in Wood County and Hancock County showed abundant presence of bryozoans which may be indicative of proximal ramp organic buildup. In well core #3564 deposition storm deposits were observed from which it can be inferred that the deposition was influenced by storm waves. The frequent storm layers are found in the middle and upper part which means that deposition took place under frequent storm influence above the storm weather wave base. The shift from shallow back ramp lagoonal environment to storm dominated proximal ramp environment suggest rise in relative sea level, which was reflected in the depositional conditions. Many workers have described the depositional and diagenetic characteristic of Trenton Limestone. Previous workers agree that the unit represents carbonate ramp depositional environment. The presence of a carbonate ramp depositional environment because gradual change in facies without the evidence of reef or rimmed shelf /shelf break deposits or organic build up facies (Fara and Keith, 1988).

The bottom portion of the core is characterized by back ramp facies which includes tidally influenced lagoonal deposits. This part of the Trenton Limestone is sparse in skeletal material, but the tidally influenced lagoonal deposits show evidence of intense bioturbation, which gave rise to stromatactis structure. Some evidence of subaerial exposure in the bottom portion was observed which includes stratiform solution collapse breccia and angular chert 118

fragments. This portion of Trenton Limestone has been through extensive cementation so the

portion has resemblance to lithographic limestone.

The middle portion of the studied well core #3564 is dominated by carbonate storm

deposits. These can be recognized by scour marks, skeletal lags, and wavy laminated heterolithic

lamination and beds. In this portion, and in the lower portion of the upper section, these tempestite sequence and amalgamated tempestite sequence are very common. These were deposited above storm weather wave base, and in the proximal portion frequent amalgamation was observed because of the reworking of previously deposited storm deposits.

The upper portion of the core is characterized by the presence of thinly laminated wavy

shale and mudstone interbedded with skeletal packstone. This lithofacies is the result of

alternating calm and agitated energy conditions. The mudstone layers were deposited during calm periods and the skeletal packstone was deposited during higher energy periods. This lithofacies association was interpreted to be proximal deep ramp lithofacies below the fair weather wave base which is characterized by alternation of calm and agitated energy conditions.

It has been observed from this study that the petroleum prospect of the studied section of

Trenton Limestone does not depend on the depositional facies but it depends on the diagenetic alteration which creates secondary porosity. Secondary dolomitization and dissolution are responsible for creation of porosity in Trenton Limestone which allowed for the migration of petroleum into the pores. It has been observed that the vuggy dolomitization and dissolution porosity relates to the abundant fractures present in the well core section #3564. The fractures are interpreted to be solution and fault related because core #3374 did not have any vuggy dolomitized porosity as it was not affected by the fractures. The Bowling Green fault is probably 119

one of the major catalysts for fracture generation besides solution fracturing in core no# 3564

which has been subjected to recurrent tectonic movement. The timing of fracture generation is

interpreted to be after burial and compaction because the fractures cut through the compacted

fabric of Trenton Limestone.

The source of diagenetic fluid is interpreted to be hydrothermal because of the presence

of abundant saddle dolomites along the fractures. Saddle dolomite forms at high temperature,

hence the hydrothermal origin of saddle dolomite can asserted. Moreover, a fluid inclusion study of saddle dolomites by Patchen et al. (2006) in the region suggested high temperature origin of

the saddle dolomite.

In terms of dolomite cement history, three types of dolomite cements were observed. The darker concentric cement is interpreted to have originated from the iron-rich dolomitizing fluid originating from the overlying Utica Formation. The other types of cement were concentric bright cement and fracture related bright cement. The fracture related to bright dolomite cement is interpreted to have formed by dissolution and subsequent re-precipitation of dolomite by hydrothermal fluid movement. This is interpreted to be the latest episode of dolomite cementation.

Evidence of dedolomites and porosity arising from dedolomitization was observed where dolomites were replaced by late calcite, but the volume percent of this type of porosity is almost negligible. This may have occurred either after most of the diagenetic events or at a shallow depth from the same fault related fluid migration but at lower temperature compared to their deep-seated counterpart. Same goes for pores formed by mesogenetic dissolution along the 120

stylolites. Hence it can be concluded that late-stage hydrothermal dolomitization is the main

source of porosity creation in the Trenton Limestone of Northwestern Ohio.

The Trenton Limestone was deposited in a shallow subtidal gently sloping carbonate ramp in the Upper Ordovician. It acts as a reservoir rock for petroleum owing to the secondary porosity that was generated from faulting, dissolution and meteoric diagenesis. This study found that the scope of connected porosity creation was not all pervasive in the studied area but was strictly restricted to the zones which had undergone dissolution and fracture. Besides conventional porosity, microporosity was observed under SEM and thin section study which may

have played a role in Trenton Limestone’s property as a reservoir rock. Relics of primary

porosity was observed but secondary porosity was the only porosity present. Determination of

the actual cause of porosity development is still speculative to certain degree. For reservoir

identification and delineation purpose it is not always possible to isolate a single variable and

correlate with porous and permeable intervals, hence an integrated approach is the best way to

solve this problem. 121

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Institut Bulletin, v. 7, p. 201-236.

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APPENDIX A: LITHOLOGS

Limestone

Faint parallel bedding

Parallel lamination

Wavy Lamination

Fracture fill

Fossils Chert

Grain Size Stylolite

M W P G B

M=mudstone, W= wackestone, P=packstone, G=Grainstone, B=Boundstone

Figure A1: Legends. 134

1283

Utica Shale

Trenton Lst.

1295

1286

1298

1289

1301

1292

M W P G B M W P G B Figure A2: Core log of well no 3374 (Con’t.). 135

1304

1316

1307

1319

1310

1322

1313

Bentonite

M W P G B 1325 Figure A2: Core log of well no 3374 (Con’t.). M W P G B 136

1325

Bentonite bed

1328

1331

1337 Trenton Lst.

Black River Group

1334 1340

M W P G B M W P G B Figure A2: Core log of well no 3374 . 137

1455 1458

Fossiliferous packstone with fracture lls of dolomite veins

1456 Dolomite fracture ll

Fossiliferous grainstone Fossiliferous wavy laminations dolomite fracture lls with vuggy Packstone with intraclasts porosity

1457

Fossiliferous grainstone Fossiliferous packstone with fracture lls dolomite fracture lls with vuggy porosity

1458 1461 M W P G B M W P G B

Figure A3: Core log of well 3564 (Con’t.). 138

1461

Fossiliferous packstone

1465

1462

Interlaminated fossiliferous mudstone and wackestone

Fracture lls 1466

1463

Fossiliferous packstone

Lamianted wackestone

1467

1464

Chert Nodules

M W P G B M W P G B

Figure A3: Core log of well 3564 (Con’t.). 139

1471

1468

Hard Ground 1472

1469

1473 Stylolite

1470

1474

Cherti ed

1471 M W P G B M W P G B

Figure A3: Core log of well 3564 (Con’t.). 140

1478

Chert nodule Scour Surface 1475 Shell hash layer with fenestral pores

1479

Chert fragment with fossil fragments

1476

1480

1477

Stylolite

1481 M W P G B M W P G B Figure A3: Core log of well 3564 (Con’t.). 141

1481

Chert nodule

1485

Chert nodule 1484

1486

1485

Cherti ed shell hash

1487

1486

Marine hard ground

M W P G B M W P G B

Figure A3: Core log of well 3564 (Con’t.). 142

1491 Massive mudstone

1488

1492

1489

Chert nodules

1493

1490

Brecciated packstone

1494

Chert fragments 1491 M W P G B M W P G B

Figure A3: Core log of well 3564 (Con’t.). 143

Chert fragments 1498

1495

Hardground

1499

1496

Marine hard ground

1500

1497

1501 M W P G B M W P G B Figure A3: Core log of well 3564 (Con’t.). 144

1501

Scour surface

1505

1502

1506

1503

Brecciated grainstone with vuggy dolomite

1507

1504

M W P G B Figure A3: Core log of well 3564 (Con’t.). M W P G B 145

1511

1508 Fossiliferous packstone with dolomitized vuggy porosity

Bioturbated grainstone

1512

1509

Lithographic packstone with occasional laminations and microstylolites

Chert nodules 1513 Shell Lag Collapse breccia with brecciated chert 1510 Glauconite

1514

1511

M W P G B M W P G B Figure A3: Core log of well 3564 (Con’t.). 146

1518

1515 Chert nodules

Bioturbated packstone

1519

1516

Chert nodules

Sporadic chert nodules 1520

1517

1521

Bioturbation

1518 M W P G B M W P G B Figure A3: Core log of well 3564 (Con’t.). 147

Bioturbation 1525

1522 Alternation of bioturbated mudstone and wackestone

1526

1523

Dissolution posrsity Bioturbated wackestone (Stromatactis)

Marine hard ground 1527

Chert nodules

1524

1528 M W P G B M W P G B Figure A3: Core log of well 3564 (Con’t.). 148

1528

Bioturbated mudstone with ne lamination 1532

1529

1533

1530 Bioturbation

1534

1531

M W P G B M W P G B Figure A3: Core log of well 3564 (Con’t.). 149

1538

1535

1539

1536

1540 Shell hash

1537

Bioturbation

1541

1538 M W P G B 1542 M W P G B M W P G B Figure A3: Core log of well 3564 . 150

APPENDIX B: CATHODOLUMINESCENCE PHOTOMICROGRAPHS

Figure B1: Cathodoluminescent zoned planar-e dolomite at -467 m depth in core no. 3564.

Figure B2: Cathodoluminescent zoned calcite at -463.2 m depth in core no. 3564.

Figure B3: Cathodoluminescent fracture filling dolomite cement at -460.7 m depth in core no. 3564. 151

Figure B4: Cathodoluminescent planar-s dolomite at depth -467 m depth in core no. 3564.

Figure B5: Cathodoluminescent planar-e dolomite at -445.16 m depth in core no. 3564.

Figure B6: Cathodoluminescent planar-e dolomite at -442.5 m depth in core no. 3564. 152

Figure B7: Cathodoluminescent planar-e dolomite at -441.04 m depth in core no. 3564.

Figure B8: Cathodoluminescent planar-e dolomite -443 m depth in core no. 3564. 153

Figure B9: Cathodoluminescent planar-e dolomite at -454m depth in core no. 3564.

Figure B 10: Cathodoluminescent planar-s dolomite at -460.7 m depth in core no. 3564.

Figure B11: Cathodoluminescent dolomite -469.9 m depth in core no. 3564. 154

Figure B12: Cathodoluminescent planar-s dolomite with incipient crystal faces at -470.5 m depth in core no. 3564. 155

APPENDIX C: SEM PHOTOMICROGRAPHS

Figure C1: SEM photomicrograph of dolomite with associated clay minerals at depth of -441m in core no. 3564.

Figure C2: SEM photomicrograph of close –up view of associated clay minerals at depth of -441m in core no. 3564. 156

Figure C3: SEM photomicrograph of weathered dolomite with associated clay minerals at depth of

-442.4 m in core no. 3564.

Figure C4: SEM photomicrograph of fractured and weathered calcite at depth of -442m in core no. 3564. 157

Figure C5: SEM photomicrograph close-up view of the surface of fractured and weathered calcite at depth of -442m in core no. 3564.

Figure C6: SEM photomicrograph of calcite crystals at depth -445.2m in core no. 3564. 158

Figure C7: SEM photomicrograph of weathered calcite crystal surface at depth -445.2m in core no. 3564. 159

Figure C8: SEM photomicrograph of dolomitic chert at depth -446m in core no. 3564.

Figure C9: EDAX spectroscopic chart of dolomitic chert at depth -446m in core no. 3564. 160

Figure C10: SEM photomicrograph of weathered calcite crystals at depth of -446m in core no. 3564.

Figure C11: SEM photomicrograph of fractured dolomite crystals at depth of -447.1m in core no. 3564. 161

Figure C12: SEM photomicrograph of partially developed dolomite crystals with associated chert as quartz crystals at depth of -454m in core no. 3564.

Figure C13: SEM photomicrograph of rhombohedral dolomite crystals at depth of -454m in core no.

3564. 162

Figure C 14: SEM photomicrograph of quartz crystals from chert at depth of -454m in core no. 3564.

Figure C 15: EDAX spectroscopic chart of chert (square area in SEM image) at depth -454m in core no.

3564. 163

Figure C 16: SEM photomicrograph of weathered calcite (calcite recognized by remnant traces of rhombohedral cleavage) at depth of -455.5m in core no. 3564.

Figure C 17: EDAX spectroscopic chart of weathered calcite (square area in SEM image) at depth of

-455.5m in core no. 3564. 164

Figure C 18: SEM photomicrograph of weathered calcite (calcite recognized by remnant traces of rhombohedral cleavage) at depth of -456.4m in core no. 3564.

Figure C 19: SEM photomicrograph of weathered calcite (this micrograph was taken at a higher magnification in the same general area as the photomicrograph above) at depth of -456.4m in core no.

3564. 165

Figure C 20: SEM Photomicrograph of dolomite crystals with minor silica at depth of -456.4m in core no.

3564.

Figure C 21: EDAX spectroscopic chart of dolomite crystals with minor silica at depth of -456.4m in core no. 3564. 166

Figure C 22: SEM Photomicrograph of calcite at depth of -457.7m in core no. 3564.

Figure C 23: EDAX spectroscopic chart of calcite with minor silica at depth of -457.7m in core no. 3564. 167

Figure C 24: SEM Photomicrograph of weathered calcite/dolomite at depth of -457.7m in core no. 3564.

Figure C 25: SEM Photomicrograph of weathered calcite/dolomite at depth of -457.7m in core no. 3564. 168

Figure C 26: SEM photomicrograph of weathered calcite at depth of -460.7m in core no. 3564.

Figure C 27: SEM photomicrograph of crystalline dolomite at depth of -463.1m in core no. 3564. 169

Figure C 28: SEM photomicrograph of weathered calcite at depth of -467.8m in core no. 3564.

Figure C 29: SEM photomicrograph of weathered calcite exhibiting distinct cleavage pattern at depth of

-467.8m in core no. 3564. 170

Figure C 30: SEM photomicrograph of incipient dolomite crystals at depth of -467.8m in core no. 3564.

Figure C 31 EDAX spectroscopic chart of crystalline dolomite at depth of -467.8m in core no. 3564. 171

Figure C 32: SEM photomicrograph of weathered and fractured dolomite at depth of -496.9m in core no.

3564.

Figure C 33: SEM photomicrograph of weathered dolomite at depth of -469.9m in core no. 3564. 172

Figure C 34: SEM photomicrograph of weathered calcite and dolomite at depth of -470.5m in core no.

3564.

Figure C 35: SEM photomicrograph of close up view of surface of dolomite crystal and at depth of

-470.5m in core no. 3564. 173

Figure C 36: SEM photomicrograph of close up view of dolomite and associated clay minerals at depth of

-470.5m in core no. 3564.