Diagenesis and Reservoir Characterization of The

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Theses and Dissertations

12-2012 Diagenesis and Reservoir Characterization of the Pennsylvanian Middle Atoka Formation, Sebastian and Logan Counties, West-Central Arkansas Elvis Chekwube Bello University of Arkansas, Fayetteville

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DIAGENESIS AND RESERVOIR CHARACTERIZATION OF THE PENNSYLVANIAN MIDDLE ATOKA FORMATION, SEBASTIAN AND LOGAN COUNTIES, WEST-CENTRAL ARKANSAS

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology

Elvis Chekwube Bello Delta State University Bachelor of Science in Geology, 2001

December 2012 University of Arkansas

ABSTRACT

The Middle Atoka Formation evolved from a stable passive margin during the early

Pennsylvanian time to a rapidly subsiding basin, with the sedimentary fill thickening greatly southward during the middle Pennsylvanian time. The basin dips in north-south direction. An east-west anticlines and synclines were observed.

The purpose of this study is to establish the stratigraphic units, reservoir geometry and distribution, and infer the depositional environments and reservoir quality. Well correlation and petrographic studies are used to achieve the goals.

Sandstone Point Count method was used. The Point Counts was divided into five categories. They include the framework grains (quartz, feldspar, and lithic fragments), accessory minerals (muscovite and biotite), cementing materials (quartz overgrowths, feldspar overgrowths, dolomite, and calcite cements), pore-spaces (primary and secondary), and the

“Other” (minerals that cannot be identified under the microscope, and matrix and pyrite).

Vertical sequences of sand bodies that are closely spaced and separated by thick marine shale intervals, and sandstones that blocky signatures and abrupt bases and tops, were correlated as genetically related sand bodies. Four stratigraphic units; Borum, Turner, Nichol, and Basham

Sandstone Units were identified.

The sandstones of the Middle Atoka Formation are composed of very fine silt to coarse quartzarenites, subarkoses, sublitharenites, and litharenites. The reservoirs are heterogeneous and were divided into two: the amalgamated reservoirs (proximal and medial submarine fans), and the overbank reservoirs (levee-overbank deposits, crevasse splays, and distal lobes). The geometries of the reservoirs are elongate and radial depending on the stratigraphic units. Quartz overgrowths and clay cements are intense and advanced and variable within the two reservoirs.

Porosity loss was significantly caused by compaction and quartz cementation. Secondary porosity, where it occurs, resulted from the dissolution of the labile grains, probably from the interaction with migrating organic acids or as a result of the increasing geothermal gradient.

Diagenetic processes either enhance the porosity by dissolving mineral grains, or reducing the porosity by stimulating growth of clay and quartz minerals.

Amalgamated reservoirs contain higher amount of dissolution, clay cements, and lower quartz overgrowths. Dissolutions are filled with clay cements. Dissolution when present in overbank reservoirs are better preserved than is in amalgamated portions. Dissolutions and clay cements are also higher in the south than is in the northern part. Higher clay content in the south ensures that the dissolution is almost effectively occluded by clay cements. Consequently there seem to be no net-gain in porosity despite in the area.

This thesis is approved for recommendation to the

Graduate Council:

Thesis Director:

Dr. Xiangyang Xie

Thesis committee:

Dr. Doy L. Zachry

Dr. Walter Manger

THESIS DUPLICATION RELEASE

I hereby authorize the University of Arkansas Libraries to duplicate this thesis when needed for research and/ or scholarship.

Agreed

Elvis Chekwube Bello

Refused

Elvis Chekwube Bello

ACKNOWLEDGEMENTS

I would like to give special thanks to Dr. Xiangyang Xie, my thesis advisor, who suggested the study area, procured the data used for the study from Stephens Production

Company, and arranged the meeting between me and the company personnel. Furthermore, his ideas and thorough timely review of my work several times have immensely improved the quality of this thesis.

I also appreciate Dr. Doy L. Zachry for helping me develop my knowledge of diagenesis, and his invaluable guidance and support. He had been tremendous, explanatory, helping me whenever and wherever he could. I would also like to thank Dr. Walter Manger for reviewing and providing technical suggestions that added color to this thesis.

I would like to thank Stephens Production Company, Fort Smith, for the opportunity and resources provided for this project. I wish to express special thanks to two people at Stephens

Production Company, Robert Liner and Luke Martin, who traveled between Fort Smith and

Fayetteville in order to train me on the use of the Petra-IHS WorkStation. The training was critical to the development of this thesis and I enjoyed and valued what I learned so much.

Thanks to all my friends in the Geology Department who made my stay at the University of Arkansas an enjoyable one.

Finally, I extend appreciation to my family who believed, encouraged and supported me in my efforts to complete this project. Special thanks to my mother Mrs. Comfort John Bello and

Dr. Benjamin John for their consistent support of my graduate studies.

Thank you all.

TABLE OF CONTENTS

ABSTRACT

ACKNOWLEDGEMENTS

LIST OF FIGURES

LIST OF PLATES

LIST OF TABLES

CHAPTER 1. INTRODUCTION

1.0 Study Area 1

1.1 Purpose of Study 1

1.2 Previous studies 1

1.3 Methodology 4

1.3.1 Well log correlation 5

1.3.2 Petrographic microscopic study 6

1.3.3 Diagenesis 7

1.3.4 Rock texture 10

CHAPTER 2. The ATOKA FORMATION 12

2.1 Geological setting 12

2. 2 Regional stratigraphy of the Atoka Formation 20

2.2.1 Lower Atoka Formation 20

2.2.2 Middle Atoka Formation 21

2.2.3 Upper Atoka Formation 22

CHAPTER 3. WELL LOG CORRELATION 23

3.1 Well log correlation 23

3.2 The depositional model 25

3.3 The depositional environments 27

3.3.1 Amalgamated deposits 28

3.3.1.1 Proximal deposits 28

3.3.1.2 Medial deposits 29

3.3.2 Overbank deposits 29

3.3.2.1 Levee-overbank deposits 29

3.3.2.2 Crevasse splays 30

3.3.2.3 Distal lobes 30

3.4 The Stratigraphic Units 31

3.4.1 Borum Sandstone Unit 35

3.4.2 Turner Sandstone Unit 37

3.4.3 Nichol Sandstone Unit 38

3.4.4 Basham Sandstone Unit 39

3.5 Evaluation of the sandstone reservoir units 40

3.5.1 Borum sandstone reservoir 40

3.5.2 Turner sandstone reservoir 41

3.5.3 Nichol sandstone reservoir 42

3.5.4 Basham sandstone reservoir 43

CHAPTER 4. PETROGRAPHIC STUDY 45

4.1 Composition 45

4.2 Frame work grains 46

4.2.1 Quartz 46

4.2.2 Feldspar 49

4.2.3 Lithic Fragments 49

4.3 Accessory minerals 50

4.4 Cementing materials 51

4.4.1 Quartz and feldspar overgrowths 51

4.4.2 Clay minerals 52

4.5 Grain sizes 53

4.6 Texture 54

4.7 Diagenesis 55

4.8 Tectonic provenance 57

4.9 The reservoir quality 60

CHAPTER 5. CONCLUSIONS 65

REFERENCES 67

APPENDICES 78

LIST OF FIGURES

Figures Page

1. Regional geological setting of Arkoma Basin 3

2. Study area map showing townships and ranges 4

3. Type of well logs used for the study 9

4. Top of the Middle Atoka marker 10

5. Diagrammatic cross sections showing the tectonic development 13

6. Foreland structural features and tectonic elements showing Mississippi Valley 15

7. Major foreland structural features and tectonic elements 16

8. Fan models proposed by Shanmugam and Moiola (1988) 19

9. Types of well log motifs 24

10. Conceptual submarine fan depositional model 26

11. Simplified depiction of a slope-basin environment 27

12. Well log signatures depicting the reservoir types 32

13. Showing clean sands for the four stratigraphic units 33

14. Contour map of the study area 34

15. North-South cross section A-A’ 34

16. East-West cross-section A-A’ 35

17. Index map showing the two cross sections 35

18. Interval isopach map of the Borum Sandstone Unit 36

19. Isopach map of Turner Sandstone Unit 37

20. Interval isopach map of Nichol Sandstone Unit 38

21. Interval thickness of Basham Sandstone Unit 39

22. The isopach map of Borum Gross Sands 40

23. The isopach map of Turner Gross Sands 41

24. The isopach map of Nichol Gross Sands 42

25. The isopach map of Basham Gross Sands 43

26. The isopach map showing sediments dispersal patterns 44

27. Folk’s sandstone classification (Folk, 1980) 45

28. QFL diagram of the Middle Atoka Formation 46

29. Paragenetic Sequence 55

30. Clay minerals distribution in the reservoir rock 56

31. Triangular QmFLt plot showing mean framework grains 58

32. QFL distribution of sedimentary rocks in various tectonic regimes 58

33. Triangular QFL plot of the middle Atoka Formation 59

34. Triangular QmFLt plot middle Atoka Formation 60

35. The percentage abundance of the primary and secondary porosities 78

36. The composition of the framework grains 78

37 Percentage abundance of the component of cementing materials 80

38 showing the distribution of the accessory minerals 80

39. Sanderson 3-9H well modal thin section composition 81

40. State 2-22 well modal thin section composition 82

41. Carter 2-26 well modal thin section composition 83

42. Hale 5-19 well modal thin section composition 84

43. Holcomb 1-36 well modal thin section composition 85

44. Schultz-Teeter 3-18H well modal thin section composition 86

LIST OF PLATES

Plate Page

1. Photomicrographs of Quartz and Others 47

2. Photomicrographs of Textures and Others 48

3. Photomicrographs of Overgrowths, Sorting and Others 50

4. Photomicrograph of Authigenic Minerals and Others 51

5. Photomicrographs of Plant Fragments, Muscovite and Others 52

6. Photomicrograph of Deformation and Others 54

LIST OF TABLES

Tables Page

1. Stratigraphic column of the Atoka Formation 6

2. Informal sandstone units of middle Atoka Formation 6

3. Petrographic Data of Samples of Schultz-Teeters 3-18H 87

4. Petrographic Analysis of Thin-Sections obtained from Holcomb 1-39 well 88

5. Petrographic Analysis of Thin-Sections obtained from Hale 5-17H Well 89

6. Petrographic Analysis of Thin-sections obtained from Hale 5-17H well cont. 90

7. Petrographic Analysis of Thin-Sections obtained from Carter 2-26 well 91

8. Petrographic Analysis of Thin-Sections obtained from Carter 2-26 well cont. 92

9. Petrographic Data of Samples of State 2-22H Well 93

10. Petrographic Data of Samples of Sanderson 3-9H Well 94

11. Well Names and Gross Sands of the four sandstone units 95

12. Percentages of the framework grain components for all the depths 96

CHAPTER 1

INTRODUCTION

1.0 Study area

The study area is located on the southern part of the Arkoma Basin (Figure 1). It is situated north of the Ouachita Mountains and south of the Backbone and Washburn anticlines.

Geographically, it is limited to the west-central Arkansas, encompassing eight townships area. It covers a total area of 288 square-miles, encompasses Townships 7N-5N and Ranges 32W-27W in Sebastian and Logan Counties (Figure 2).

1.1 Purpose of Study

The purpose of this study is to establish the stratigraphic units, determine the reservoir distribution and geometry, depositional systems, and document the reservoir quality of the

Middle Atoka Formation.

1.2 Previous studies

As one of the main gas producing units in the Arkoma Basin, the Atoka Formation has been a major research topic and exploration target since 1930s. Croneis (1930) defined the surface extent of the Atoka Formation within Arkansas, including the distribution, thickness, lithology, fossils, and the petrologic characters. Caplan (1957) determined the thickness of the

Atoka Formation within northwestern Arkansas using subsurface data. Scull (1961) using well log correlation, provided three-fold division of the Atoka Formation in a series of cross sections for the Arkoma basin. Buchanan and Johnson (1968) produced the structural cross section of the

Bonanza Field with lettered correlation markers A-C (descending order) to describe the tectonic history of the Arkoma basin. They designated marker B and C as the middle and lower Atokan boundaries respectively.

Merewether (1961) discovered the southward thickening trend of the Atoka Formation.

However, Buchanan and Johnson (1968) attributed the thickening to the growth faulting in the

Middle Atoka Formation. Thomas (1983) while working on the southern Arkoma basin in western Arkansas was able to determine the southward thickening trends of the three members of the Atoka Formation. Gilbreath and Haley (1972) mapped the thickness of the Cecil series, an informal Lower Atoka member of the Atoka formation in Arkansas. Merewether (1961) and

Merewether and Haley (1972) were able to identify and describe the formation contacts, folds and structures of the Atoka Formation during the mapping of the Delaware and Knoxville quadrangles.

The investigation of the Bloyd-Atoka boundary by Zachry and Haley (1975) showed that

Henbest (1953) and other previous workers had misidentified the Middle Bloyd Sandstone as the basal sandstone of the Atoka Formation. In 1975, Zachry and Haley established the proper position of the Bloyd-Atoka boundary and published detailed reports on the position of the

Bloyd-Atoka boundary. Other works in the area include studies that established the sedimentation and plate tectonics of the Ouachita Mountains and Arkoma Basin; the distribution and the evidence of normal faulting in the East Central portion of the Arkoma Basin; and the sedimentation transition from the shelf to the basin in the Oklahoma – Arkansas area (Shields,

1961; Briggs and Roeder, 1975; Sutherland and Manger, 1979).

Ozark Plateau

Arkoma Basin

Ouachita Mountains

West Gulf Coastal Plain

Figure 1. Regional maps showing geological setting of Arkoma Basin (modified from

Encyclopedia of Arkansas).

•West central Arkansas •Sebastian & Logan Crawford Counties Franklin Johnson •Townships 5 N – 6 N Pope Sebastian •Ranges 28 W – 31 W Logan •288 square miles Study area Yell Perry Scott

Garland Polk Montgomery

T6N R31W T6N R30W T6N R29W T6N R28W

T5N R31W T5N R30W T5N R29W T5N R28W

Figure 2: Study area map showing the townships and ranges.

Additionally, there are many unpublished Master theses at the Department of

Geosciences, University of Arkansas on the middle Atoka Formation. They include Williams,

(1983), Fritsche, (1981), Normand, (1986), Harris, (1983), Mollison, (1983), and Gatson, (1985).

1.3 Methodology

Gamma Ray, Resistivity Logs, and Conductivity Logs are used for the correlation of wells in the area (Figure 3). The stratigraphic units, distribution, geometry, and the depositional environments of the sandstone reservoirs are made through the correlation of fifty wells.

Petrographic studies of fifty thin-sections obtained from six-wells are combined with the geophysical studies to determine the reservoir quality.

1.3.1 Well log correlation

All the well logs used for the study are converted to raster logs (Figure 3). The determination of the lithologies, well correlations, and the construction of the cross sections are accomplished by the use of IHS-Petra-Workstation. The top of the middle Atoka Formation is marked by low resistivity shale. It is recognized on well log by the blocky-type signature of the

Conductivity Log (Figure 4). This blocky type signature is recognized and correlatable across the study area. The vertical variation of the log signatures was used to infer changing depositional environments. Wells that have full/partial penetration of the middle Atoka Formation were selected for the correlation. In total, ninety-eight wells are selected. Fifty well logs are finally used for the correlation of wells and the construction of isopach and contour maps. A series of cross sections are made across the area to ascertain the dips, structures, and the stratigraphic units of the middle Atoka Formation. Gross isopach maps are used to determine the stratigraphic thickness, geometry, and the distribution of the sandstone reservoirs in the area.

In this study, four sandstone units of the middle Atoka Formation are identified. They are the Borum, Turner, Nichol, and the Basham Sandstone Units. These units are the parallel equivalence of the Morris, Areci, Bynum, Freiburg and Casey Sandstone Units used in other previous studies. In addition, the Basham Sandstone Unit is composed of both Morris and Areci

Sandstone Units of the previous studies.

System Series Formation Sandstone Unit Desmoinessian Hartshorne Carpenter A Upper Alma Carpenter B Morris Areci Middle Bynum Freiburg Atokan Atoka Pennsylvanian Casey Sells Jenkins Dunn B Lower Dunn C Paul Barton Cecil Spiro Bloyd Morrowan Hale

Table 1. Stratigraphic column of the Atoka Formation, Arkoma Basin, Arkansas (after

Zachry 1983).

Sandstone unit System Formation Member Sandstone unit (This study) Upper Atoka Formation Morris Areci Basham Middle Atoka Bynum Nichol Pennsylvanian Freiburg Turner Casey Borum Atoka Lower Atoka Formation Morrowan Bloyd

Table 2. Informal sandstone unit of middle Atoka Formation used by petroleum companies and this study.

1.3.2 Petrographic microscopic study

Fifty thin-sections were made from the well cuttings obtained from four vertical and two horizontal wells. The thin-sections were impregnated with blue epoxy in order to reveal the porosity. Glycerin liquids were applied on the surfaces of the thin sections and covered with the 6

Glass-Microscopic-Cover-Slip. A hand operated mechanical counting stage was attached on the stage of the Nikon-Transmitted-Light Microscope. This was used to construct the counting grid.

Irregular grids of 91-300 were made on each thin sections based on their quality. It was necessary to repeat grid lines in some cases. Magnifications of 10X and 20X were used for point counts, 20X and 40X for the photography, while 40X was used for the grain size measurements.

Depending on the quality thin sections each contained 250-300 point counts. Therefore, some of the data may not reflect the true composition characters.

The point-counts were divided into five categories. They include the framework grains, accessory minerals, cementing materials, pore-spaces, and “others”. The framework grains include quartz (monocrystalline quartz and polycrystalline quartz), feldspar (orthoclase, plagioclase, and microcline), and the lithic fragments (cherty fragments, metamorphic and the sedimentary fragments), cementing materials (clay minerals, quartz overgrowth, feldspar overgrowth, dolomite, and calcite cements), “others” (pyrite, matrix, and other minerals that cannot be identified under the microscope), accessory mineral (muscovite and biotite), and the porosity.

The percentages of each category was calculated and tabulated in Tables 4 to 10. The occurrences of some particular constituents did not coincide with the cross-polar point during counting and are therefore recorded as zero percent. This is not an indication of the absence of such constituent minerals, but should rather be considered as occurring in trace amounts.

1.3.3 Diagenesis

Diagenesis is defined as modifications made to rocks prior to burial, as well as their complete destruction during continued weathering (Burley et al, 1984). The modifications include changes occurring immediately after deposition, through compaction, lithification,

7 alteration, and total transformation of the mineral grains. Diagenesis persists toward the onset of metamorphism. The diagenetic processes have negative impact on the reservoir quality by altering the porosity and permeability.

Holcomb 1-36

Conductivity Log

Resistivity Log

Gamma-Ray

Figure 3. Type log used for the study

Gregory

Top of middle Atoka marker:

Top of low blocky conductivity motif toward the track

Figure 4 Top of the Middle Atoka marker that is capped by low resistivity shale.

1.3.4 Rock texture

Grains were randomly selected on thin-sections for grain size measurement. The long axes of such minerals were measured to determine the grain size and distributions. Pore throats

10 were also measured to obtain the pore throat diameter. All the measurements were then converted to millimeter using Nikon-Microscopic-Calibrated-Values of the Microscope

Objective and the Ocular Mike Michrometer. The degree of sorting, roundness and sphericity were visually estimated. Ternary plots were made for quartz, feldspar and lithic fragments to determine the composition of the sandstones (Folk, 1980) and the tectonic provenance

(Dickinson, 1985).

CHAPTER 2

ATOKA FORMATION

The Atoka Formation is lower Pennsylvanian in age. It is composed of sequences of alternating sandstone and shale units. It is overlain by the Hartshorne Sandstone of Desmoinesian age, and unconformably lies on the top of the Morrowan Series. It is divided informally into lower, middle, and upper Atoka Formation. The Atoka Formation is one of the main gas producing units in the Arkoma Basin.

2.1 Geological Setting

The Arkoma Basin is 250 miles long and 20 to 50 miles wide (Branan, 1968). It lies immediately north of the Ouachita Mountains to which it is closely related. Previous studies suggest that the development of the Arkoma Basin and the Ouachita Orogenic Belt involves the collision of continental plates (Viele, 1979; Houseknecht and Kacena, 1983; Lillie et al 1983).

Houseknecht (1986) proposed five different tectonic-stratigraphic stages of the development of the basin (Figure 5) with the middle Atoka Formation forming between stages three and four; from early Mississippian/earliest Atokan to the middle Atokan times.

North Arkoma Basin Ouachita Mountains South

Sabine ? uplift

Figure 5: Diagrammatic cross sections showing the tectonic development of the southern margin of North America and the Ouachita orogenic belt much shortened from north to south

(from Houseknecht, 1986). (A) late Precambrian- earliest Paleozoic records the onset of extension. (B) During the late Cambrian-earliest Mississippian, the Ouachita Ocean is at its widest and deepest. (C) In early Mississippian-earliest Atokan times, the Ouachita Basin is segmented by the development of an accretionary wedge, into a forearc basin to the south and a trench basin to the north. (D) During early to middle Atokan time, thrusting of the accretionary wedge onto the southern margin of North America occurred. (E) In the late Atokan-

Desmoinesian, thrusting of the Ouachita orogeny onto North America continued.

The first stage records the onset of the basin extension and the development of a Proto-

Atlantic Oceanic Basin on the southern margin of the North America Continent during late

Precambrian and Cambrian time. The sediments associated with this rifting include carbonates, shale and sandstone. These sediments were deposited on a stable shelf environment. Walper

(1977), and Houseknecht and Weaverling (1983) maintained that the onset of the Anadarko and the Reelfoot Rift Basin development also began at this time. The Anadarko Basin evolved into an aulacogen, because of the intrusive and extrusive igneous activity. The Reelfoot Basin was inactive before the igneous activity (Walper, 1977).

The southern margin of North American Continent preserves a belt of deformed

Paleozoic strata extending from the eastern margin of the Mississippi Embayment in west-central

Alabama to the Marathon and Solitario Uplifts of west Texas, and from there into the adjacent northern Mexico. This folded and faulted belt is the Ouachita Orogen and only about 20 percent of its nearly 2500 kilometers is actually exposed. The bulk of the Ouachita Orogen is in the subsurface beneath the Mesozoic and Cenozoic successions of the Gulf Coastal Plain and

Mississippi Embayment. Consequently, the only significant outcrops of the deformed strata comprising the orogen are the Ouachita Mountains of southwestern Arkansas and southeastern

Oklahoma, and the Marathon Uplift, which is actually a fenster through the Cretaceous of the

Edwards Plateau in west Texas. (A fenster is formed when an underlying block of thrust-faulted strata is exposed in a small area by erosive action - http://www.answers.com/topic/thrust-fault).

The Ouachita Orogen resulted from the collision of northern South America (as part of

Gondwana) with the southern midcontinent of North America (as part of Laurussia). In addition to the Ouachita-Marathon Fold-Belt Mountains, the collision also formed a series of foreland basins between the deformed orogenic belt and the stable cratonic region.

Most of these basins produce petroleum, particularly the shale resource plays of the

Arkoma Basin, and the Fort Worth Basins.

Ozark Plateau

Ouachita Mountains ?

Ouachita Salient

Figure 6: Major foreland structural features and tectonic elements adjacent to the

Ouachita Fold-Belt as interpreted by Denison, 1989. Note that the Arkoma Basin is not shown to connect with the Black Warrior Basin across the Mississippi Embayment (modified after Miall,

2008).

Ozark Plateau

Arkoma Basin

Ouachita Mountains A Frontal belt

Metamorphic belt

Figure 7: Major foreland structural features and tectonic elements adjacent to the

Ouachita Fold-Belt as interpreted by Viele and Thomas (1989). Note that those authors indicate that the Arkoma Basin is continuous with the Black Warrior Basin across the Mississippi

Embayment (modified after Miall, 2008).

Nevertheless, it has been a common interpretation in many articles to extend the eastern

Arkoma Basin of Arkansas across the Mississippi Embayment and connect it to the Black

Warrior Basin in western Mississippi (Figure 3). While there is no question that the Ouachita fold-belt extends through this area as proved by drilling and geophysics (Ginzburg, et al, 1983;

Mickus and Keller, 1992), the Mississippi Embayment is part of an aulacogen that includes the

Anadarko Basin. That aulacogen represents failed Precambrian to Cambrian rifting of the southern North American craton, when it was part of the Precambrian supercontinent Rodinia, and later Pannotia. A remnant structure from that rifting is the Mississippi Valley Graben, also

16 known as the Reelfoot Graben, which remains tectonically active to this day. The Mississippi

Valley Graben disrupts any structural continuity between the Arkoma and Black Warrior Basins, which are not connected across the Mississippi Embayment.

The second stage of the basin development occurs from late Cambrian to earliest Atokan age. This stage is marked by the onset of the ocean basin closure, accommodated by southward subduction beneath Llanoria (Houseknecht and Kacena 1983). The occurrence of the detrital sediment in the basin is an indication of orogenic provenance (Morris 1974). The local abundance of volcanic fragments in the Stanley Formation of the Ouachita Mountain

(Houseknecht and Kacena 1983) is evidence that the subduction was underway in Mississippian time, although the exact time is difficult to determine. Houseknecht and Kacena (1983) and

Nicholas and Waddell (1982) documented Carboniferous volcanic fragments in the subsurface, south of Ouachita Mountains. This represents traces of the magmatic arc that developed along the northern margin of Llanoria, which marks the onset of the accretionary wedges that are associated with subduction zones. Carbonates, shale and sandstones are deposited on the continental passive margin. The sediment thickness of the Arkoma Basin increases gradually southward towards the Ouachita Mountains.

The third stage of the development of Arkoma Basin is composed of rocks of the Lower and the Middle Atoka Formation. The thickness increases from north (1000/1300 feet) to south

(over 15,000 feet) perpendicular to the axis of the basin (Zachry and Pierre-Orly, 2008). The passive margin shelf remains the site of deposition at this stage, which culminated with the deposition of the basal Spiro Sandstones. Houseknecht and Kacena (1983) and Dickson (1974) suggested that the more than 5 km thick, thin-bedded alternation of sandstones and shales were deposited in the deep remnant ocean basin in a relatively short time. Houseknecht and Kacena

(1983) and Thomas (1985) propose that some of the sediments deposited into the basin were derived from the east, transported westward in a longitudinal fashion, and ultimately deposited on the submarine fans settings and the surrounding abyssal plains (Figure 8).

The fourth stage occurred during early to middle Atokan time. At this stage, the closure of the remnant ocean basin by subduction was complete, and the northward advancing subduction complex was being thrusted over the rifted continental margin of the North America

(Houseknecht and Kacena 1983). The bending of the southern margin of the North American continental crust was caused by the weakening subduction of the continental crust and the thrusting of the northward advancing subduction complex (Houseknecht and Kacena 1983, and

Dickinson, 1974, 1976). This led to the widespread development of normal faults that strike parallel to the Ouachita Mountains. Houseknecht (1983) maintain that these normal faults broke the undeformed continental crust and the shelf-slope rise geometry that was prevalent along the passive continental margin since Precambrian/Cambrian time, and that this resulted in an abrupt increase in both subsidence and sedimentation.

This down-thrown normal faulting is syndepositional, and contributed to the abrupt thickenings of the sediments toward the south. The abrupt increase of the strata southward are syndepositional with the subsidence as well as the normal faults. The thickening represents the transition from fluvial, shallow marine facies to deep marine sedimentation from north to south.

It also marks the beginning of the development of the foreland basin.

Distal fan Medial fan Proximal fan

Several kilometers

Arkansas Oklahoma

Mississippi

Texas Louisiana

Figure 8: Fan models proposed by Shanmugam and Moiola (1988) showing (A) middle fan setting (channel dominated) for the DeGray section and outer fan setting (lobe dominated),

(B) longitudinal Fan system in a regional tectonic framework (Shanmugam, 2006)

The fifth stage is composed of rocks of the Upper Atoka Formation and Desmoinesian

Strata. It is composed of sandstones, shales and coals of shallow marine origin. In the late

Atokan-Desmoinesian, thrusting of the Ouachita orogeny onto North America continued and predominated as the subduction pushed northward against the strata deposited during Devonian to early Atoka time (Houseknecht and Kacena, 1983). The peripheral foreland basin was formed

19 at this stage due to the Ouachita thrust that culminated in the deposition of shallow marine, deltaic, and fluvial deposits (Dickenson, 1974, and Houseknecht and Kacena, 1983).

During late Atokan time, the closure of the Arkoma Basin was complete at the southern side of the basin (Viele, 1973). The deposits at this stage were mostly deltaic coals, which prograded southward across the muddy open shelf (Zachry and Sutherland, 1984). Deposition at the western end of the basin was composed of alternating shale and sandstone layers in a transgressive open shelf (Zachry and Sutherland, 1984).

2. 2 Regional Stratigraphy of the Atoka Formation

The Atoka Formation is informally divided into lower, middle, and upper Atoka

Formation (Tables 1 & 2). The division is based on the differences in sedimentary responses to tectonic processes that prevailed during the formation of the basin (Buchanan and Johnson, 1968;

Haley and Hendricks, 1972; Fritsche, 1981; Thomas, 1983; Zachry, 1983; and others).

2. 2. 1 The Lower Atoka Formation

The Lower Atoka Formation lies unconformably over the Morrowan Series (Buchanan and Johnson, 1968). It is composed of eight sandstone units (Tables 1 & 2) separated by thin- layers of shale. The Spiro Sandstone is the basal sandstone unit of Lower Atoka. The Spiro unit is widespread within the Arkoma Basin (Berry and Trumbly, 1968). This sandstone unit is fine- grain, calcitic, submature, and fossilized. The rocks of the Lower Atoka Formation are depositionally related to those of the Morrowan Series. They are believed to be deposited by southward progradation of high destructive deltaic systems and the related shallow marine processes (Scull, 1961; Berry and Trumbly, 1968; Uszynski; 1982; and Stephens; 1985). The thickness of the lower Atoka Formation is uniform and consistent across the basin (Zachry,

1983).

2. 2. 2 The Middle Atoka Formation

Four to five sandstone units (Table 1) separated by shale intervals characterize the middle

Atoka Formation (Zachry, 1983). It is overlain by the Carpenter “B” Unit of the upper Atoka

Formation and underlain by the Sells Sandstone Unit, which is the top of lower Atoka

Formation. The unit thickens from about 1500 feet southward around the Mulberry fault system to over 4000 feet in the central part of the basin. The outcrops of the lower sandstone units of the middle Atoka Formation are found north of Cass fault and are buried north of the Mulberry fault systems (Zachry, 1983).

The deposition of the middle Atoka Formation in the southern area, south of the growth faults, is created through submarine fan systems in a deep-marine setting. The sediments are transferred and transported in a north-south direction, and longitudinally in an east-west, and deposited as turbidities (Zachry, 1983; Stephens, 1985; Williams, 1983; and Shanmugam, 2006).

The deposits at the northern and central parts of the basin result from the processes of deltaic systems that prograded slowly on a subsiding shallow marine shelf (Stephens, 1985; and

Williams, 1983).

The comparison of the middle and lower Atoka Formation indicates that the Middle

Member displays higher variability in thickness, lithology, and also contain higher amount of shale (Zachry, 1983). Zachry (1983) and Fritsche (1981) described lobate and elongate geometries of the Middle Atoka Formation. The abrupt southward increase in the thickness of the unit suggests that the rate of subsidence and sedimentation increased in the middle Atokan time (Zachry, 1983). The nomenclatures used for the sandstone units of the middle Atoka

Formation are the same as that of the Petroleum Operators in the study area (Tables 1 & 2).

In this study, the sandstones of the middle Atoka Formation are divided into four sandstone units separated by shaly units. The four units are the Borum, Turner, Nichol, and the

Basham sandstone units. Most of the wells used for this study did not have complete penetration of the Borum sandstone unit. The Borum, Turner, Nichol, and the Basham sandstones units are the parallel equivalence of the Morris, Areci, Bynum, Freiburg and Casey sandstone units used in previous studies (Table 2). Basham sandstone unit is parallel equivalent of Morris and Areci sandstone units used in previous studies (Table 2).

2. 2. 3 The Upper Atoka Formation

This unit is composed of three Sandstone Units that thicken southward (Buchanan and

Johnson, 1968; Zachry, 1983). Toward the south, deposits of the Upper Atoka Formation are deposited in a prograding high destructive deltaic system (Zachry, 1983; Stephens, 1985). In the west, they are deposited in a shallow marine, inner and middle shelf of the deep-marine environments (Scull, 1961). There is gradual replacement of the deep-water deposits of the

Upper Atoka Formation by the deltaic system of the overlying Hartshorne sandstones unit. The gradual replacements of the upper Atoka sandstones by the deltaic deposits of the Hartshorne

Sandstone create localized erosional surfaces (Thomas, 1983).

Coal bed deposits cap the alternating deposits of shales and sandstones within the upper

Atoka Formation (Zachry, 1983; Gatson, 1985; and Scull, 1961). These coals are presence within shale beds and within few feet of the sandstone units (Haley, 1961). Six coal layers were identified in the Knoxville Quadrangle, and a very extensive one near Centerville in the Yell

County that is east of this study area (Merewether, 1967).

CHAPTER

Well Log Correlation

Forty eight wells with well logs were used for the lithology correlation. Three types of well logs were used. They include the Gamma Ray, Resistivity and the Conductivity Logs

(Figure 3).

3.1 The Correlation of Sandstone Units

Well logs in the area have a variety of log motifs, ranging from blocky, thin and symmetrical, thin-bedded to crescent-shaped signatures. Others include successions of fining upward and coarsening upward sequences. Most of the blocky signatures in the northern part of the area are serrated compared to the massive type-signatures at the southern portion (Figure 9).

The blocky log signatures usually have an abrupt top and bottom. Blocky signatures with gradational tops are common at the northern portion of the study area.

Top of the middle Atoka Formation is marked by low resistivity shale. It is identified on the well log by the blocky signature of the Conductivity Log, with an excursion toward the track on the log (Figure 4). It can be recognized and correlated laterally across the study area. The depth covered in this study is between 2500 ft. to 8200 ft.

Based on the log motifs, the reservoirs were differentiated into two broad groups. They include the amalgamated reservoirs and the overbank reservoirs (Figure 12). Blocky gamma signatures (serrated and non-serrated) are used to identify amalgamated reservoirs. Apart from the blocky signatures, other log types are the identification features of the overbank reservoirs, and they are dominated with thin-bedded signatures.

PROMINENT LOG SIGNATURES Holcomb 1-36 Carter 2

Blocky & serrated Thin & symmetrical

Tracks

Crescent shape

Blocky & massive

Fining upward Thin beds successions

Figure 9 Types of well log motifs depicting depositional environments in the area.

Amalgamated reservoirs are channelized deposits for the most part. They include the proximal and medial submarine fans. Overbank deposits, on the other hand, include levee-overbank deposits, crevasse splays, and the distal lobes. The two terminologies are for explanatory purposes in this study. Detailed description of the two reservoir units is included later in this chapter.

Individual bed thicknesses range from a few centimeters to about 150 feet in the area.

The western part of the study area is predominantly composed of crescent-shaped, thin and

24 symmetrical signatures. Other signatures include the blocky signatures (serrated) and the successions of coarsening upward signatures. These signature-types grade into blocky (massive) signatures, thin-bedded signatures, and the successions of fining upward sequences toward the central portion of the area. Toward the northeastern portion, the log type changes again to serrated blocky types, and to thin and symmetrical motifs. All the type of log signatures in the area is shown in Figure 9.

Individual beds are amalgamated. They have a great degree of lateral continuity in the north and at the central portion of the area. Gross sands in the northern portion are high, but lateral connectivity of the individual beds is poor compared to the central portion. There is a general thinning of individual beds away from the central portion of the area as seem on the well logs. The gross sand bodies change between the four different stratigraphic sand units and between the north and south of the area. There is an inconsistent and isolated thickening and thinning of beds throughout the basin. This makes the correlation between wells difficult.

3.2 The Depositional Model of the Middle Atoka Formation

The deposits of the middle Atoka Formation are described as turbidites, deposited by the submarine fan systems in a slope-basin deep-water environment (Figure 11). This is based on the type of the well log signatures. According to Normark (1978), Walker, (1979), and Shanmugam,

(2006), turbidite systems are described in terms of the submarine fan model.

The sediments deposited into the basin are derived from the north, east, and the south of the study area. This is based on the series of isopach maps, and the tectonic provenance studies.

Sediment failures at the continental shelf-margin gave rise to the gravity-driven processes, which transported the sediments away from the shelf regions, into the deep-water environments, where

25 they were deposited as turbidites. Gravity-transport processes include mass-transport and turbidity currents. Further description of these terms is not made in this study.

The failure of the shelf-margin sediments could be triggered by any of the following events according to Shanmugam (2006): 1) Eustatic changes in sea level, 2) submarine volcanic activity, 3) earthquakes, such as the 1929 Grand Banks tremor off the U.S. east coast, 4) over- steepening of the submarine slope, such as near the mouth of the Magdalena River, Colombia, 5) high sedimentation, such as in the Mississippi delta-front setting, Gulf of Mexico, 6) tsunamis, 7) storm waves, and 8) biologic erosion of submarine canyon walls.

Levee-overbank

Feeder

Channel 100’s of km of 100’s Crevasse splay

Channelized (Proximal)

Non-channelized (Medial)

Distal lobe

Figure 10 showing the conceptual submarine fan depositional model of the study area

(Kim et al, 2012).

The submarine fan systems are formed when changes of the slope gradient occur near the basin floor. When this happens, the transport energy of the medium drops, channels splits and then deposit their loads as submarine fans (Figure 11). Several episodes of this nature produce a lobe-shifting system of submarine fans (Figure 10). The proximal and medial ends of the fans are the sandier part of the submarine fans. On both sides of the channel, fine-grained materials in suspension are deposited as mud-dominated levee-overbank and crevasse splay systems (Figure

10). Beyond the proximal and medial depositional lobes are the distal fans. The distal fans are dominated by finer-grained sediments (Figure 10).

Shelfal margin Continental slope Deep-water

Several kilometers

Figure 11. Simplified depiction of a slope-basin environment

3.3 The Depositional Environments

The sandstone reservoirs in the study area are mud-dominated sandstones. As said earlier, they are grouped into amalgamated deposits and overbank deposits. This division is based on the well log signatures. Amalgamated deposits are identified on the well logs by blocky gamma-ray signatures that have abrupt bases. These blocky gamma signatures are both massive and serrated, and channelized and non-channelized deposits. The overbank deposits, on the other hand, are

27 recognized by every other signature-type except the blocky gamma-ray motifs. The overbank deposits are mostly non-channelized.

3.3.1 Amalgamated Deposits

The amalgamated deposits have very good vertical and lateral beds continuity. They are better developed in Nichol and Basham sandstone units. Within the Nichol sand unit, they occur as blocky and massive sand bodies, characterized by abrupt tops and bottoms as suggested by the well logs. The amalgamated deposits with the abrupt tops and bottoms are very prominent at the central portion of the study area and occur as isolated sand bodies with gradational upper tops and abrupt bottoms in the northern portion.

The abrupt contacts of the amalgamated deposits are attributed to the erosive activities of the deep-marine currents and/or sudden and rapid deposition from the sand influxes resulting from flood events. The amalgamated deposits are characterized by very poor to moderate sorting.

They are clean sands and usually overlain and underlain by the shaly overbank facies. The ratio of sand/mud is high as suggested from the well logs. The presence of interbedded shales is more noticeable at the north than in the south. This interbedded shale may serves as flow barriers, and at the same time as a sealing layer. They have the potential to occlude vertical and lateral flow continuity. On the whole, amalgamated deposits have great potential for significant hydrocarbon production in the study area.

3.3.1.1 Amalgamated Proximal Deposits

The amalgamated proximal submarine fans are recognized on the well log by blocky gamma-ray signatures that have abrupt tops and bases. The amalgamated proximal submarine fans are elongate, and are distributed within the central portion of the study area as shown on the gross sands isopach of the Nichol sandstone unit (Figure 24). They are best developed in the

Nichol sandstone unit, have relative poor development in Basham unit, and are absent in the rest of the other two units (Borum and Turner sandstone units). They have a very wide lateral continuity and good vertical stacking of sand bodies. They are underlain and overlain by shales which may represent seal rocks at both ends. The proximal sand bodies may still be sealed internally by the interstitial shale layers. Other forms of stratigraphic traps that are not seen on the well logs may result from the switching of the submarine fan lobes as shown in Figure 10.

3.3.1.2 Amalgamated Medial Deposits

These groups are recognized on the well logs by blocky gamma-ray signatures that have abrupt bases and gradational tops. The amalgamated medial fans are relatively thick, isolated, and composed of intermittent alternation of sandstones and shales as supported by the well logs.

Individual bed amalgamation is poor vertically and horizontally compare to proximal sand bodies. Medial sand bodies are more common at the northern portion of the area. The Basham sandstone unit is typically made up of the medial submarine fan type for the most part.

3.3.2 Overbank Deposits

The overbank deposits form moderate to well-sorted reservoirs. The ratio of sand/mud is low to moderate depending on the sandstone units. Thin-bedded deposits overlap all the reservoir types in the area.

3.3.2.1 Levee-Overbank Deposits

Levee-overbank deposits form the sandier type of the overbank deposits. They are recognized on the well logs by their crescent-shaped signatures. They form laterally to the channel (Figure 10). The levee-overbank reservoirs are sparse and uneven. They are difficult to correlate between wells, and formed compartmentalized reservoirs as suggested by their lateral discontinuities on the well logs. Slatt et al, (2004) suggested that horizontal wells offer the best

29 potential to achieve maximum production from the levee-overbank reservoirs because of the high degree of compartments, and the lack of good lateral bed connectivity.

3.3.2.1 Crevasse Splays

Crevasse splays, on the other hand, are recognized on the well logs as thinning and fining upward sequences. They are composed of mixed silt-shale sediments as suggested by the well logs. According to Walker (1979) thinning and fining upward sequences of sands formed as a result of the infillings of the distributary channels that are incised into the levee-overbank deposits. Accordingly, there is a gradual abandonment of the channels and gradual filling by thinner and finer siltstones and shales fractions. They are rare in the area and when present, are usually discontinuous laterally and vertically. Individual beds are thin, from a few centimeters to two feet.

3.3.2.3 Distal Lobes

Beyond the proximal and medial submarine fans are the distal fans. They are also rare and not well-developed. They are distinguished on the well logs by the vertical stacking sequences according to Slatt el al, (2004). They are thin individual amalgamated sand bodies.

They are both channelized and non-channelized fan complexes in the study area. When they are channelized, they formed lobes that are characterized by the successions of fining upward patterns. This type is only noticeable in the southeastern portion of the study area. When they formed non-channelized deposits, they form successions of coarsening upward sequences on the well logs. This type too is present in isolation at the northwestern side of the area. According to

Slatt et al, (2004), distal lobes form a stratigraphic trap as a result of the down-dip termination of the proximal and medial sands caused by the lateral continuous shaly layers of the distal lobes that were not eroded before subsequent deposition of another layers. Laterally continuous layers

30 of shales of the distal lobes may partly be responsible for the lack of productivity of the Holcomb well. This is because deposits that have the successions of fining upward signatures are present on the log obtained from the Holcomb well within the interval of the Turner sandstone unit.

3.4 Stratigraphic Units

Vertical sequences of sand bodies that are closely spaced and separated by thick marine shale intervals were correlated as genetically related sandstone deposits that resulted from one depositional cycle according to Allen (1975). In addition, blocky gamma-ray signatures that have abrupt bases are also mapped as genetically related sand bodies. On this basis, four informal stratigraphic units were identified and mapped. They include the Borum, Turner, Nichol and

Basham sandstone units in ascending order (Table 2). The division of the reservoirs into four different stratigraphic units is based mostly on the changes in the characters of the gamma ray, and then the resistivity and conductivity logs.

The characters and the thickness of these sand bodies vary vertically and laterally between wells across the study area. These variations make the stratigraphic correlation very difficult even over a short distance. The abrupt regional north-south thickenings of the sedimentary fills of the middle Atoka Formation were not observed at the study area. The basin dips in a north-south direction, and series of east-west anticlines and synclines can be observed along the east-west direction within the central portion (Figure 14).

Two type of sandstone reservoirs

Holcomb Hales

Amalgamated deposits

Overbank deposits

Amalgamated deposits

Overbank deposits

Figure 12. Well log signatures depicting the depositional environments (after Miall,

2008).

Hattabaugh Holcomb

TMA

Basham sands

Nichol sands

Turner sands

Borum sands

Figure 13: Showing clean sands (API 75) for the four stratigraphic units

Subsea (SS)

Schultz-Teeter - H Carter

Sanderson - H

Hale

State Holcomb

Figure 14: Contour map showing the location of the six wells used for study thin sections.

The top of the contour is the subsea, which is total depth minus the elevation (ground surface).

Figure 15: North-South cross section A-A’

Figure 16: East-West cross-section of selected wells A-A’

Figure 17: Index map showing the two cross sections

3.4.1 Borum Sandstone Unit

The Borum sandstone unit is overlain and underlain by the Turner sandstone unit and the lower Atoka Formation, respectively. Laterally persistent, thick shale units separated it from the overlying sandstone unit. Seventeen wells have full penetration through the Borum sandstone

35 unit. There is a bit of an uncertainty about the distribution and the geometry of the Borum sandstone unit. The most common well log signatures in this unit are the thin, symmetrical and crescent log motifs. Other type of log signatures includes a few blocky types, and the successions of fining upward sequences at the southeastern side of the area. The upward fining successions are rare, isolated and unevenly distributed. The lithology is mainly interbedded sandstone, siltstone and shale. Lateral bed connectivity is generally poor across the area.

Figure 18 Interval isopach map of Borum sandstone unit

The interval depth of the unit ranges from 6192 ft. to about 8200 ft. at townships T6N

R29W and T5N R29W, respectively. The thickness of this unit is up to 1750 ft. Individual bed thickness ranges from a centimeter or less to over 10 ft. The interval isopach map of this unit shows that the sediment dispersal is in the north-south trend, specifically at the township boundary between T6N R231W and T6N R30W, and at township T5N R28W.

3.4.2 Turner Sandstone Unit

Stratigraphically, the Turner sandstone unit is above the Borum sandstone interval. It is overlain by the Nichol sandstone unit. Most wells penetrated the entire Turner Sandstone interval. The thickest part of this interval is located at the northern portion. It is up to about 2000 ft. as shown in Figure 19. There is no observed general thickening trend. The depth at which the unit is encountered is between 5263 ft. to 8181 ft.

Figure 19. Isopach map of Turner sandstone unit

The most common log signatures of the Turner sandstone unit include the thin, crescent, and blocky log motifs. Blocky signatures are generally serrated within this interval. In addition, the crescent-log motifs are poorly developed, usually thin and serrated. Individual bed thickness ranges from a centimeter to about twenty feet. Blocky and interbedded units of sandstones and shales are prominent in the Tyler and Hattabaugh well logs situated in townships T6N-R31W &

T5N R29W. Successions of fining upward patterns are observed on some of the well logs at township T5N R29W. Combinations of the amalgamated reservoirs and overbank reservoirs are suggested by the log signature types. Distal lobes are suggested by the succession of fining

37 upward log patterns. The interval isopach map shows that the sediment dispersal within this interval is haphazard, but generally in north-south direction across the area (Figure 18).

3.4.3 Nichol Sand Unit

Figure 20. Interval isopach map of Nichol sandstone unit

The Nichol sandstone unit stratigraphically is above the Turner sandstone unit and lies below the Basham sandstone unit. The interval thickness is up to 1790 ft. The amalgamated deposits are well-developed and have good lateral connectivity. They are more prominent and laterally correlatable within the central portion of the townships T6N R29W and T5N R29W.

The unit is separated from the Turner sandstone unit by thin shale beds, and separated by a very thick shaly unit from the overlying Basham sandstone unit. The bases and tops are usually abrupt.

The most common log type is blocky and usually massive as suggested by the well logs.

The amalgamated deposit is the dominant reservoir in this unit, especially at the south-eastern portion of the study area. The log type changes from the blocky motifs at the eastern area to thin, symmetrical, and crescent-shaped signatures toward the western area. The changes in the log

38 signatures suggest changes in the depositional environments; from the amalgamated systems of deposition in the east to overbank sedimentation in the west. The interval isopach map of this

Unit shows haphazard sediment dispersal trends (Figure 20). The isolated thickenings of the interval suggest multiple accommodation spaces.

3.4.4 Basham Sandstone Unit

Figure 21. Interval thickness of Basham sandstone unit

This sandstone unit is above and below the Nichol sandstone and the Carpenter “B” units of the middle and upper Atoka Formation, respectively. It is separated vertically from the overlying and underlying sandstone units by thick shaly units that are laterally continuous across the study area. Lithologically, it is much more similar to the Borum and Turner sandstone units.

All well log types (blocky, crescent-shaped, thin and symmetrical signatures) are present in the

Basham sandstone unit. At the northern portion, serrated and blocky signatures, and the upward fining signatures are dominant. Thin and symmetrical signatures are common at the southern portion of the area. Crescent-log signatures are common and evenly distributed within the unit.

Successions of coarsening upward signatures are sparse and occur in isolation. Varieties of well

39 log signatures are suggest diverse depositional environments and systems. The thickness of

Basham sandstone unit is up to 1800 ft. as shown in Figure 21. The trend of the sediments dispersal is haphazard as reflected on the interval isopach map (Figure 21).

3.5 Evaluation of the Sandstone Reservoir Units

The individual clean sand bodies of each of the stratigraphic units were sum up to obtained the respectively gross sands. The gross sands were then used to generate gross thickness maps to determine the geometry and the distribution of reservoir of the four stratigraphic units.

The thicker portions on the isopach maps of the gross sandstones are indicative of amalgamated deposits. Conversely, as the gross sands become thinner, the overbank deposition becomes dominant and the ratio of sand/shale decreases

3.5.1 Borum Sandstone Reservoir

Figure 22: The isopach map of Borum gross sands

As stated earlier, seventeen wells penetrated this unit. The thickness of the gross sand is up to 300 ft. They are well developed at the eastern part of the area, at townships T6N-R29W and T5N-R29W. The thickenings extended across the eastern boundary into the western portion of the area (Figure 22). The Borum sandstone unit is sand-dominated reservoir. The ratio of 40 sand/shale sediment is low at the extreme southeast and southwest. It is higher toward the north and at the central portion. Both amalgamated and overbank reservoirs are identified in this unit.

Amalgamated reservoirs are common at the central portion of the area. Away from the central portion of the study area, the overbank sedimentation increases at both directions.

3.5.2 Turner Sandstone Reservoir

Figure 23: The isopach map of Turner gross sands

The reservoir of the Turner sandstone unit is isolated and unevenly distributed across the area (Figure 23). The thickness of the gross sand is up to 256 feet with elongate and radial geometry observed. Overbank reservoir is dominant in this interval. It is shaly, and poor to well- sorted reservoir. The ratio of sand/shale is low. Individual beds are thin commonly a few centimeters to two feet. The thickness of the individual bed varies laterally. Individual beds are laterally discontinuous and pinch out within short distances. The upper contact of the Turner sandstone unit is sharp to gradational. Their bases are gradational, usually made up of succession of fining upward sequences and coarsening upward sequences in most cases.

3.5.3 Nichol Sandstone Reservoir

Figure 24: The isopach map of Nichol gross sands

The thickness of the Nichol’s gross sand is up to 300 feet. This unit has the best developed amalgamated deposit. They are connected vertically and horizontally especially at the central portion, and at the extreme eastern part of the study area (Figure 24). The reservoir is sand-dominated and composed mostly of the amalgamated proximal submarine fans. The ratio of sand/shale sediment is high. Individual beds are in the range of a few centimeters to as high as

150 ft. Blocky well logs with abrupt bases and tops are the dominant type in this unit. The unit is poorly sorted and possesses more dissolution cavities. The geometry of the reservoir is elongate and is distributed within the central portion as shown in Figure 24.

3.5.4 Basham Sandstone Reservoir

Figure 25: The isopach map of Basham Gross Sands

The Basham sandstone unit is composed of an amalgamated reservoir and is usually interbedded layers of sandstones and shales. The upper contacts are mostly gradational contacts.

The degree of beds amalgamation is poor to moderate. Lateral bed connectivity is poor when compared to the Nichol sandstone unit, but much better than the other two units. The thickness of the gross sand is up to 280 feet. The Basham sandstone unit is sand-dominated reservoirs. The ratio of sand-shale fractions is high. The amalgamated medial deposits are dominant in this unit.

Individual beds are thin, ranging from a centimeter to about 8 feet. The geometry of the reservoir is elongate to radial, isolated and unevenly distributed across the area (Figure 25).

The elongate portions (Figures 22, 23, 24, and 25) of the four stratigraphic units were extracted from each gross isopach maps using a cutoff thickness of 150 ft.

Figure 26: The isopach map showing sediments dispersal patterns.

Then, the extracted elongate portions were superimposed on each other using the principle of superposition, in order to determine the sweet spots, the direction of the sediments dispersal, and the depositional systems in the study area (Figure 26). The sweet spots are located at the eastern side and within the central portion of the area (Figure 26). The sediments dispersals into the basin were from the northeastern side and the southern portion of the study area. Submarine fan systems were interpreted as a result of the fan shifting pattern of depositions (Figure 26).

CHAPTER 4

PETROGRAPHY STUDY

Fifty thin-sections made from well cuttings of four vertical and two horizontal wells were analyzed for the petrographic study. The sandstone point-counting method was utilized to document the composition and the tectonic provenance of the middle Atoka Formation.

4.1 Composition

Point-counting of the framework grains was in accordance with Folk’s method (Folk,

1968). The components of the point counts were divided into framework grains, cementing materials, accessory minerals, porosity, and the “others”. When the percentages of the components of framework grains were plotted on the QFL diagram (Figure 26), the compositions of the middle Atoka sandstone is quartzarenites (41%), sublitharenites (39%), litharenites (3%), and subarkoses (17%) in Figure 27.

Quartzarenite 10 Subarkose Sublitharenite

Feldspathic Arkose Lithicarkose litharenite Litharenite F L

Figure 27: Folk’s sandstone classification (Folk, 1980). Q-pole includes all types of detrital quartz grains (mono and poly crystalline) except cherty fragments. F-pole includes all

45 types of detrital feldspar grains (mono and poly crystalline rock fragments). L-pole includes all other rock fragments, including chert. Granite and gneiss fragments are not included.

Quartz Arenites Q Q 0 100

10 90

20 80

30 70

40 60

50 50

60 40

70 30

80 20

90 10

100 F 0 L FeldsparF 0 10 20 30 40 50 60 70 80 90 100 Lithic Fragements

Schult-teeter Holcomb Sanderson State Carter Hale

Figure 28: QFL Diagram of the Middle Atoka Formation

4.2 Framework Grains

These include quartz, feldspar and lithic fragments. Framework grains average 69% of the modal composition of sandstones as shown in table 5.

4.2.1 Quartz

Quartz is the major framework grain in the section. It averages 87% of the framework grains. Under the microscope, low birefringence, lack of cleavage, and extinction patterns were used to identify it. Extinction pattern was undulose to straight under cross polarized light. Two types of quartz, monocrystalline and polycrystalline quartz were recorded. Monocrystalline quartz is the most common type of quartz. It has an average of 92%. The numbers of quartz with

46 straight extinctions are more common. The percentages of undulose and straight extinction were not determined.

Polycrystalline quartz is less common. It has an average of 8%. Polycrystalline quartz displayed a varying degree of high birefringence under cross polarized light by the individual grains. Plates 1, 2 and 3 shows monocrystalline and polycrystalline quartz grains under the microscope.

Grain dissolution A B

Polycrystalline Quartz

Chert Fragments

Advance quartz overgrowth Undulose Extinction

Undulose extinction Grain edge dissolution C D

Straight Extinction

Grain Fractures

Plate 1: Photomicrographs of (A) Partial to complete grain dissolution, and advance quartz overgrowths. (B) Quartz grains with straight extinction, cherty fragment, and polycrystalline quartz grains. (C) Grain edge dissolution, and fractured grains. (D) Quartz grain exhibiting undulose extinction and straight extinction.

Most of the quartz grains exhibit straight extinction (Plate 1). They are free of inclusions and lack vacuoles. Some are fractured and cloudy in appearance (Plate 2). Grain-edge dissolution of quartz overgrowths is rare to common (Plate 3). Pressure-induced dissolution due to compaction has resulted in some suturing between the grains. Polycrystalline quartz ranges from grains with straight jagged borders to grains with elongate minerals.

Quartz cementation, well-compacted Grain replacement, alteration, & dissolution A B

C D

Grains alteration to clay

Polycrystalline, Lithic fragments, matrix

Plate 2: Photomicrographs of: (A) Well-compacted grains cemented by quartz cement.

Grain boundaries are from straight, angular, rounded to well-rounded grains (B) Grain replacement, alteration, and dissolution (C) Matrix, polycrystalline quartz grains, and lithic fragments. (D) Clay-filled pores.

4.2.2 Feldspar

Orthoclase, microcline and plagioclase feldspar are grouped together as feldspar. They average 4% of the framework grains. Orthoclase is by far the most common type. There are traces of feldspar recognized as partially replaced feldspar grains (Plate 2). Some feldspar grains exhibit straight and untwined extinction pattern. Orthoclase exhibits a blocky outline with undulose extinction. Some of the feldspar grains have undergone partial to complete dissolution, and while others have been completely altered to clay minerals (Plate 2, 3 and 4). Selective replacement of feldspar is common (Plate 2).

4.2.3 Lithic Fragments

Lithic fragments average 9% of the framework grains. They include all rock fragments and cherty fragments. No further differentiation of the lithic fragments is made in this study.

Most of the rock fragments are sedimentary materials and a few isolated cherty fragments. They are sub-rounded to rounded grains.

A B Undulose & straight extinction Mudstone Quartz edge dissolution

Mono-quartz Matrix

Muscovite

Moderately sorted Fractured grain Advance quartz overgrowth

D C

Fracture healed by quartz cementation Clay coatings before quartz overgrowth

Plate 3: Photomicrograph of: (A) Very fine, well to poorly sorted grains, composed of monocrystalline quartz grains with straight extinction, mudstone, matrix, fractured grains, and muscovite. (B) Advance quartz overgrowths, with quartz grains exhibiting undulatory and non- undulatory extinction. (C) Healed fractures by secondary quartz cementation. (D) Coating of grains before quartz cementation.

4.3 Accessory Minerals

Accessory minerals include biotite and muscovite. They average 3% of the modal components of the sandstones. Muscovite is by far the most abundant accessory mineral. It makes up 99% of the accessory grains. It is present in virtually all of the samples. Others that are present in traces include pyrite and biotite.

Clay mineral occluded porosity Partial Dissolution A B

Effective grain coatings

Clay-cemented grains, no quartz overgrowth Clay-filled voids

D C Isolated dissolution structures

Leached remnant

Poorly interconnected pores Grain to grain contacts

Plate 4: Photomicrograph of: (A) Clay linings prevented quartz cementation, but also occluded pore throats. (B) Partial dissolution of grains, and clay-filled voids. (C) Grain to grain contact, isolated dissolution voids, and leached grain. (D) Poorly interconnected pores.

4.4 Cementing Materials

Cementing minerals resulted from the diagenetic alteration of detrital material including cements that were carried in suspension. They occur as clay cements and quartz overgrowths on the most part. Others that occur in traces include feldspar overgrowths, calcite cements, and dolomite. They constitute an average of 17% of the sandstone modal composition.

4.4.1 Quartz and Feldspar Overgrowths

Quartz and feldspar overgrowths average 39% and 0.4% respectively of the cementing materials. Quartz overgrowths occur mostly as cementing materials on grains boundaries (Plates

1, and 3). Sometimes the grains and overgrowth are separated by clay rims (Plate 4). Secondary quartz overgrowths were also observed as fracture fillings in detrital quartz grains (Plate 3).

These fractures were probably deformed by compaction and were later healed by secondary quartz cementation. Sometimes overgrowths showed signs of partial edge dissolution (Plate 3).

Poorly sorted Poor plate A B

Plant fragments Deformed muscovite

D Angular edge C

Mudstone Broken muscovite

Plate 5: 10 X Photomicrographs of: (A) Poorly-sorted grains and a deformed muscovite fragment. (B) Plant fragments. (C) Mudstone with intercalated chips of shale. (D) Broken muscovite with sharp and angular edges.

4.4.2 Clay Minerals

Clay mineral has an average of 59% of the cementing materials. Clay mineralization in the reservoir occurred in three ways: laminar clays (disperse between interbedded units of sandstones and shales), dispersed clays (pore-lining and pore-filling clays between grains), and

52 structural clays (selectively replaced grains). Laminar and dispersed clays are both pre- and syndepositional. Structural clays, on the other hand, are post-depositional.

Pore-lining clays were identified as alteration products (rims) around detrital grains (Plate

4). In some instances, they also appeared as cement infilling the intergranular pore throats (Plate

4-A).

The composition of the sandstones that are classified as “others” makes up 5% of the composition. The constituent minerals in this category include matrix, pyrite, and those minerals that cannot be identified under the microscope.

4.5 Grain Sizes

Grain size is determined by measuring randomly selected grains along their long axes.

Quartz grains were used for the measurement as a result of their stability and abundance. The measured size of the grains ranges from 0.008 mm to 0.82 mm (Tables 3 to 9). The measured grains fall within silt and sand fractions. They range from very fine silt to coarse sand size fractions according to Wentworth (1922). The average of the total grains is 0.2 mm.

Polycrystalline grains B Well-sorted, well rounded & A compacted grains

D C Grain edge dissolution

Poorly sorted, angular to rounded grains Poorly sorted

Plate 6: 20 X photomicrograph of: (A) Sharp edges of polycrystalline quartz grains. (B)

Well-sorted, rounded to well-rounded grains. (C) Poorly sorted grains. (D) Poorly-sorted grains with quartz overgrowths, grains sutures, and dissolution cavity.

4.6 Texture

Sorting is poor, moderate and well-sorted. Roundness and sphericity were made by visual estimation (Plate 4). Grain contacts are sutured and broken at times (Plate 4 and 6). Muscovite is broken, bent, and angular (Plate 5). Quartz and feldspar grains are angular to rounded grains. The edge of the detrital grains may be sutured or overgrown by quartz. Some thin sections have some plant fragments (Plate 5).

4.7 Diagenesis

Diagenesis Early Late

Compaction

Pore-filling and pore-lining clays

Dissolution

Quartz overgrowths

Secondary cementation

(secondary clay and quartz cements)

Figure 29: Paragenetic Sequence

The paragenetic sequence was established from the petrographic studies. Some of the significant diagenetic processes that have impacted the reservoir tremendously include compaction, shale diagenesis, quartz overgrowths, clay cements, and dissolution.

The reservoir quality is the product of depositional and diagenetic processes. The source, transport systems, and the depositional environments controlled the initial sediment texture, composition, porosity, and permeability. These depositional characteristics together with early sediment compactions, shale diagenesis, and the acid nature of the pore waters, resulted in the dissolution of some grains, and the precipitation of pore-filling and pore-lining clay. Pore-filling and pore-lining clays were identified as alteration products, such as rims around detrital grains under the microscope (Plate 4). In some instances, they occur as the cement infilling the intergranular pore spaces (Plate 4-A). These early diagenetic processes impacted negatively on the reservoir quality by reducing the intergranular porosity and permeability of the reservoir.

As diagenesis continues with further burial and increased temperature gradient, intense compaction and dissolution of grains occurred as a result the increased temperature gradients.

Further addition of pore waters that drives the diagenetic events occurs through dehydration; that is, the pore waters derived from the destruction of organic matters, grain dissolutions, and the destruction and alteration of earlier formed clay minerals. Grain rotation, slippage, reorganization of grain fabrics, and plastic and brittle deformation of grains are some of the effects of this stage. Plastic and brittle grain deformations were recognized by grain-edge sutures, fractures, and distortion of grains under the microscope.

These diagenetic processes have further destroyed the reservoir quality, completely occluding the porosity within the overbank reservoirs. Other diagenetic effects include grain replacement, recrystallization, dissolution, intense and advanced quartz overgrowths and clay cementation. This stage will persist toward an onset of metamorphism (Burley, 1987).

Laminar clays Structural clays

Dispersed Clays as pore fillings Dispersed clays as pore coatings

Figure 30: Clay minerals distribution in the reservoir rock (modified after Boyd et al,

1995).

4.8 Tectonic Provenance

Dickinson et al, (1983) demonstrated that standard QFL and QmFLt diagrams that are used for plotting the framework modes of sandstones can also be used to determine the tectonic provenances of sandstones. On the QFL and QmFLt ternary diagrams shown in Figures 30 and

31, Q includes both the monocrystalline quartz (Qm), and polycrystalline quartz (Qp). Feldspar includes the totality of feldspar grains, which include the plagioclase, orthoclase, and microcline.

Lithic fragments are made of sedimentary, igneous, and metamorphic rock fragments, and include the cherty fragments. Volcanic fragments are absent in all the samples. Cherty rock fragments are scarce to absent. Lt is the totality of polycrystalline quartz fractions and the lithic fragments (Dickinson, 1985).

Three main classes of tectonic provenances are defined by Dickinson ternary diagram

(1983 and 1985) as shown in Figures 30 and 31. They include: the continental sources (craton interior and uplifted basement); the magmatic arc provenances (dissected and undissected arcs); and the recycled orogeny (subduction complex, collision and the foreland uplift).

Craton Interior Quartzose recycled Transitional continental Mixed

Dissected Transitional arc recycled

Basement a uplift Lithic recycled

F Lt Transitional Undissected arc

Figure 31. Triangular QmFLt plot showing mean framework grains derived from different provenances: Qm is monocrystalline quartz grains; F is total feldspar grains; Lt is total polycrystalline quartz, including lithic fragments (after Dickinson, 1985).

Craton interior

Transitional continental Recycled orogeny

Dissected arc Basement uplift

Transitional arc Undissected arc L F

Figure 32 showing QFL distribution of sedimentary rocks in various tectonic regimes

(modified from http://csmres.jmu.edu/geollab/fichter/SedRx/Sedtect.html); Qt = Qm + Qp; Qm =

58 monocrystalline quartz; Qp = Polycrystalline quartz; F = feldspars; L = Lithics (after Dickinson,

1985).

When the sandstones in the area were plotted on the QFL and QmFLt diagrams (Figures

32 and 33), they plotted within the fields of craton interiors, recycled orogeny, and quartzose recycled (Figures 30 and 31). According to Dickinson (1985), points that plotted within the field of craton interior are derived from either interior basins, platforms, oceanic basins or a combination. The areas that fit these descriptions on the geologic map (Figures 6 and 7) are the continental north and the eastern part of the Arkoma Basin.

Quartz Arenites Q Q 0 100

10 90

20 80

30 70

40 60

50 50

60 40

70 30

80 20

90 10

100 F 0 L FeldsparF 0 10 20 30 40 50 60 70 80 90 100 Lithic Fragements

Schult-teeter Holcomb Sanderson State Carter Hale

Figure 33. Triangular QFL plot of the middle Atoka Formation showing means of framework grains derived from different types of provenances. The sandstone falls in the field of continental block and recycled orogeny provenances.

Q MonocrystallineQmQm Quartz

0 100

10 90

20 80

30 70

40 60

50 50

60 40

70 30

80 20

90 10

100 F 0 Lt Feldspar 0 10 20 30 40 50 60 70 80 90 100 Lithic Fragments

Sanderson State Carter Hales Holcomb Schult-teeter

Figure 34. Triangular QmFLt plot middle Atoka Formation showing means of framework grains derived from different types of provenances. The sandstone falls in the field of craton interior and quartzose recycled.

In addition, the points that plotted within the fields of the recycled orogeny and quartzose recycled were derived from uplifted terranes, fold belts or both. Uplifted terranes and fold belts around the area are located south of the Arkoma Basin (Figures 6 and 7).

By integrating the results obtained from the tectonic provenances and the isopach map that depicted the direction of the sediments dispersal (Figure 26), I concluded that the sandstones of the middle Atoka Formation were derived from the northeastern and southern portions of the

Arkoma Basin. Because further studies were not made on lithic fragments, feldspar, and quartz fractions, the certainty of the exact sources was not made.

4.9 Reservoir Quality

Point-counted porosity averaged 8% of the sandstone modal composition. Porosity distribution is highly variable among the samples examined. The reservoir quality is also chaotic and heterogeneous as determined from the textural studies and the statistical analysis. Scatter

60 plots of the framework grains and cementing materials against total porosity respectively, were made to see if the trend of the reservoir quality can be determined or be predicted, and to assess where major differences in pore volumes are occurring. All regression coefficients fell below

0.36, which suggest very weak correlations, and therefore, fall within the error bars.

Porosity is depositional related and is dependent on sediments sorting and grain size distributions. The reservoir qualities were mainly destroyed by sediments compaction, clay and quartz cementations. Secondary porosity, where it occurs, resulted from the dissolution of the labile grains, probably from the interaction with migrating organic acid waters or as a result of the increasing geothermal gradient. These diagenetic processes effects are both negative and positive, either enhancing the porosity by dissolving mineral grains, or reducing the porosity by stimulating the precipitation and growth of clay and quartz minerals. Quartz overgrowths and clay cements are intense and advanced and variable within the two reservoirs.

Amalgamated reservoirs contain a higher amount of dissolution, clay cements, and lower content of quartz overgrowths. Most of the dissolution cavities within these reservoir types are filled with clay cements. Dissolution, when it is present in overbank reservoirs, is better preserved than in amalgamated portions as result of the lower clay cements. In addition, the degree of dissolution and clay cement is higher in the south than in the northern part of the study area. Higher clay content in the south ensures that the dissolution is almost effectively occluded by clay cements. This phenomenon leads to almost no net-gain in porosity in the south.

As said earlier, grain dissolution is significantly higher within the amalgamated portions of the reservoirs, and dissolution of grains generates silica cements. These silica cements are exported away from their sources (amalgamated reservoirs) into the overbank portions of the reservoir, where they precipitate as quartz overgrowths. It seems like the amalgamated reservoirs

61 tend to act as the silica cement generators and the exporters, while overbank reservoirs act as the recipients of the silica cements. These suggestions are based on the observations that higher concentrations of dissolution and quartz overgrowths are found in the amalgamated portions and the overbank reservoirs respectively.

One unique distinction between the amalgamated and overbank reservoirs is in the degree of heterogeneity in each of the reservoirs. The amalgamated portions show more textural heterogeneity in terms of the amount of dissolution cavities, clay cements, and quartz overgrowths. The reservoir quality and the degree of heterogeneity of the amalgamated reservoirs were controlled by the degree of sediments sorting and the grain size distributions.

The portion of the reservoirs that were less sorted corresponded with areas with better reservoir quality. The overbank zones tend to be more homogeneous, with higher quartz overgrowths, less dissolution cavities and clay cements. Generally, the overbank reservoirs were highly compartmentalized than the amalgamated reservoir reservoirs, as suggested by the well log signatures, and by the lateral discontinuities of individual beds; as observed from multi-well correlation across the study area.

Two types of heterogeneities were observed in the study area. They are the basin and the rock heterogeneity.

Basin heterogeneity occurred in the form of vertical and lateral heterogeneity. Vertical heterogeneities were identified on the well logs by the degree of serrated well log signatures, which reflect the amount of shales that are interspersed between clean sand bodies. On the other hand, lateral heterogeneities reflect the extent of horizontal bed connectivities were made apparent by multi-well correlation across the study area. Basin heterogeneity controls the degree of compartments within each reservoir. Overbank reservoirs have much more compartments than

62 the amalgamated reservoirs. These types of heterogeneity have negative impact on the recovery rate because it creates huge production differences between well-to-well and laterally across well bores.

Rock heterogeneities were identified by the degree of diagenetic changes in the composition and texture of the original rock facies. These changes were driven by the interaction between pore-fluids, organic matters, and the framework grains within the reservoir, through which cementing materials like clay cements and quartz overgrowths precipitated. This kind of heterogeneity may complicate the completion of the horizontal well processes by introducing unpredicted reservoir variability arising from the variation in cementations.

The increases of clay content toward the south and within the amalgamated reservoirs cause these portions to be shalier. This has negative consequences on the reservoir. For instance, wells in these areas may still be productive, but fracturing the formation effectively may become more complicated, which may cause production and recovery rates to be well below those at the northern portion of the area. Better lateral connectivity and thickness of the amalgamated reservoirs may counter balance the negative impact of clay cements, and therefore, act as better performance drivers within these areas all things being equal.

Cementations were placed into two categories based on the impact on the reservoir quality. They include the rim cements and the occluding cements, which is in accordance with the Pittman and Larese classification (1991).

Rim cements were syn-depositional in the area. They include clay coatings that precipitated on the surfaces of the framework grains as interspersed between clean sandstones.

Where clay coatings were enveloped by quartz cementation, the dissolution of the framework grains was nearly absent. Rim cements were more noticeable in overbank reservoirs. According

63 to Houseknecht (1982), even small amount of rim cements have great tendency of reducing permeability without altering the porosity. Occluding cements were post-depositional. They included the pore-filling clays and quartz overgrowths. They occluded cavities left by the dissolution of the framework grains. They exhibit no preferential relationships to the framework grains surfaces unlike the rim-types.

Cementations in the study area are capable of causing significant reduction in hydrocarbon production. The negative impact of the cements will be more felt within the overbank reservoir portions than in the amalgamated units. This is because of the fact that the amalgamated reservoirs possessed more porosity. Therefore, hydrocarbon fluids will be able to navigate around the isolated quartz overgrowths through these pores than in the tighter overbank reservoirs.

CHAPTER 5

CONCLUSIONS

1. Regional and abrupt north-south thickenings of the sedimentary fill of the middle Atoka

Formation were not observed at the study area.

2. The basin dips in a north-south direction. Anticlines and synclines are observed within

the central portion along the east-west direction.

3. The reservoirs include the amalgamated reservoirs and the overbank reservoirs.

4. The amalgamated reservoirs are poor to moderately sorted reservoirs. They contains

much higher sand/shale ratio. The two types of the amalgamated reservoirs include

proximal and medial submarine fans.

5. The overbank reservoirs are moderate to well sorted reservoirs. The ratio of sand/shale

ratio is low. Three classes are recognized - the levee-overbanks, crevasse splays, and the

distal fan complex.

6. The amalgamated reservoir comprises poor to moderately sorted deposits, composed of

lower quartz overgrowths, higher amount of clay cements and dissolution cavities.

7. The overbank reservoir is moderate to well-sorted deposits, composed of higher quartz

overgrowths, lower amount of clay cement and dissolution cavities.

8. Clay cementation increases toward the south and decreases progressively northward.

9. Quartz overgrowths increase toward the north and decrease in the southern direction.

10. The amount of preserved dissolution is higher in the north than in the south due to the

lower amount of clay cements.

11. The middle Atoka Formation is composed of very fine to coarse quartzarenites,

subarkoses, sublitharenites, and litharenites. They were transferred and transported into

the basin from the north, east, and the southern portion of the Arkoma Basin.

12. The reservoirs were deposited in the deep-water environments by the submarine fan

systems.

13. The geometry of the reservoir is elongate and radial depending on the stratigraphic units.

14. The Borum and Nichol sandstone units’ reservoirs are elongate and distributed within the

central portion of the study area.

15. The Turner and Basham sandstone units have both elongate and radial geometries. The

distribution of their reservoirs are isolated and unevenly spread across the area.

16. Wells in the south can still be productive. But the higher clay content will make

fracturing of the reservoir much more complicated than it is in the north. Production and

recovery rate will be well below wells in the northern portion.

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APPENDICES

Sanderson 3-9H

State 2-22

Carter 2-26 Porosity Secondary Hale 5-17 Primary

Holcomb 1-36

Schult-Teeter 3-18H

0 2 4 6 8

Figure 35 showing the percentage abundance of the primary and secondary porosities from modal analysis of the samples obtained from the six wells in the area. Porosity is generally higher in the north as shown in the bar chart. Carter 2-26 well has the highest values among the vertical wells. The values obtained from the Sanderson 3-39H may be biased due to the poor nature of the thin sections.

Hale 5-19

Carter 2-26 Framework Grains State 2-22 Lithic fragments Feldspar Sanderson 3-9H Quartz Holcomb 1-36

Scultz-Teeter 3-18H

0 20 40 60 80 100

Figure 36 showing the composition of the framework grains of the middle Atoka Formation from the six wells. Quartz has the highest composition.

Scultz-Teeter 3-18H Cementing materials Holcomb 1-36 Dolomite Hale 5-19 Calcite cements Feldspar overgrowths Carter 2-26 Quartz overgrowths State 2-22 Clay cements

Sanderson 3-9H

0 20 40 60 80 100

Figure 37 showing percentage abundance of the component of cementing materials. Carter 2-26 and Schultz-Teeter 3-18H are both in the northern portion of the area. Clay cements and quartz overgrowths are generally higher in the south and north respectively (Figure 35).

Scultz-Teeter 3-18H

Holcomb 1-36 Acessory minerals Hale 5-19 Biotite Carter 2-26 Muscovite

State 2-22

Sanderson 3-9H

0 20 40 60 80 100 120

Figure 38 showing the distribution of the accessory minerals with muscovite averaging about

99% of the accessory minerals. Biotite occurs in trace amount.

Secondary Primary

Others

Bitoite Muscovite Depth (ft.) 8010 Dolomite Calcite cement 7890 Felspar Overgrowths 7770 Quarts Overgrowths 7620 Clay Cement 6288

Lithic Fragments

Felspar

Polycrystalline Monocystalline

0 20 40 60 80

Figure 39 showing percentage abundance of the thin sections modal analysis of the samples obtained from the various depths of the Sanderson 3-9H well.

Secondary Primary

Others

Biotite Muscovite Depth (ft.) 7130 Dolomite 7040 Calcite cement 6110 Felspar Overgrowths 5630 Quarts Overgrowths 5990 Clay Cement 5570

Lithic Fragments

Felspar

Polycrystalline Monocystalline

0 10 20 30 40 50 60 70

Figure 40 showing the percentage abundance of the thin sections modal analysis of the thin sections obtained from the various depths of the State 2-22 well.

Secondary Primary

Others Depth (ft.) 4080 Biotite 3910 Muscovite 3870 3830 Calcite cement 3800 Felspar Overgrowths 3680 Quarts Overgrowths 3670 Clay Cement 3660 3640 3630 Lithic Fragments 3620

Felspar

Polycrystalline Monocystalline

0 10 20 30 40 50 60

Figure 41 showing the distribution of the thin sections modal analysis obtained from the various depths of the Carter 2-26 well.

Secondary Primary

Others Depth (ft.) 8110 Biotite 7080 Muscovite 6900 6970 Dolomite 8350 Calcite cement 7180 Felspar Overgrowths 7780 Quarts Overgrowths 7330 Clay Cement 7570 9134

Lithic Fragments 6750 6950 Felspar

Polycrystalline Monocystalline

0 20 40 60 80

Figure 42 showing the distribution of the thin sections modal analysis obtained from the various depths of the Hale 5-19 well.

Secondary Primary

Others

Biotite Depth (ft.) Muscovite 7609 7588 Dolomite 7573 Calcite cement 7518 Felspar Overgrowths 7509 Quarts Overgrowths 7075 7027 Clay Cement 7018

Lithic Fragments

Felspar

Polycrystalline Monocystalline

0 10 20 30 40 50 60 70

Figure 43 showing the percentage abundance of the thin sections modal analysis obtained from the various depths of the Holcomb 1-36 well.

Secondary Primary

Others

Biotite Muscovite Depth (ft.) Dolomite Calcite cement 7470 Felspar Overgrowths 7080 Quarts Overgrowths 6000 Clay Cement

Lithic Fragments

Felspar

Polycrystalline Monocystalline

0 20 40 60 80

Figure 44 showing the percentage abundance of the thin sections modal analysis obtained from the various depths of the Schultz-Teeter 3-18H well.

Thin Section Analysis Schultz teeters 3-18H Sabestian/Logan Counties Depth (ft): 70808 6000 7470 Cuttings Condition (mm): Very Poor Good Poor Grain Size Average (mm): 0.14 0.23 0.32 Grain Size Range (mm): 0.1-0.18 0.03-0.42 0.03-0.60 Sorting: Well Sorted Moderately Moderately Rock Name (Folk): Subarkose Quartzarenite Subarkose

Framework Grains Quartz 56 59 71 Monocystalline 53 52 69 Polycrystalline 3 7 2 Felspar 8 1 2 Lithic Fragments 6 7 5

Cementing Materials 11 Clay Cement 10 5 5 Quarts Overgrowths 4 5 6 Felspar Overgrowths 0 0 0 Calcite cement 0 0 0 Dolomite 0 0 0

Accessory Minerals 4 Muscovite 4 2 4 Biotite 0 0 0 Others 6 12 0 6 9 7 Porosity 6 7 4 Primary 0 2 3 Secondary TOTAL (%): 100 100 100

Table 3. Petrographic Data of Samples of Schultz-Teeters 3-18H

Thin Section Analysis Holcombe 1-36 Sabestian/Logan Counties Depth (ft): 7609 7588 7573 7518 7509 7075 7027 7018 Cuttings Condition (mm): Good Good Good Good Good Good Good Good Grain Size Average (mm): 0.26 0.13 0.17 0.16 0.21 0.2 0.21 0.1 Grain Size Range (mm): 0.04-0.52 0.04-0.24 0.08-0.32 0.08-0.26 0.08-0.32 0.04-0.36 0.04-0.52 0.068-0.14 Rock Name (Folk): Subarkose Quartzarenites Subarkoses

Framework Grains Quartz 57 61 57 53 61 50 50 49 Monocystalline 52 59 55 48 61 50 48 49 Polycrystalline 5 2 2 5 0 0 2 0 Felspar 7 3 4 4 5 3 5 4 Lithic Fragments 3 3 1 2 1 1 1 1

Cementing materials 13 19 27 26 9 26 25 33 Clay Cement 11 9 15 18 5 11 8 29 Quarts Overgrowths 2 11 12 6 4 14 15 4 Felspar Overgrowths 0 0 0 1 0 1 2 0 Calcite cement 0 0 0 1 0 0 0 0 Dolomite 0 0 0 0 0 0 0 0

Accessory minerals Muscovite 2 1 1 2 4 3 3 9 Biotite 0 0 0 0 0 0 0 0 Others 9 1 1 3 2 9 12 2

Porosity 9 11 9 10 18 8 4 2 Primary 3 3 3 3 2 1 1 2 Secondary 6 8 6 7 16 7 3 0

TOTAL (%): 100 100 100 100 100 100 100 100

Table 4. Petrographic Analysis of Thin-Sections obtained from Holcomb 1-39 well

Thin Section Analysis Hale 5-17H Sabestian/Logan Counties Depth (ft): 6950 6750 9134 7570 7330 Cuttings Condition (mm): Poor Poor Very Poor Very Poor Very Poor Grain Size Average (mm): 0.07 0.21 0.17 0.16 0.09 Grain Size Range (mm): 0.036-0.12 0.12-0.32 0.08-0.0.26 0.08-0.24 0.08-0.16 Pore Throat Average (mm): 0 0.016 0.012 0 0.004 Rock Name (Folk): Sublithics Subarkose Sublithics Q/arenite Sublithics

Framework Grains (%): Quartz 70 50 54 44 52 Monocystalline 56 40 49 33 44 Polycrystalline 14 10 5 11 8 Felspar 1 7 5 1 1 Lithic Fragments 13 3 5 1 8

Cementing Materials 6 19 14 35 19 Clay Cement 6 3 13 24 15 Quarts Overgrowths 0 16 1 11 4 Felspar Overgrowths 0 0 0 0 0 Calcite cement 0 0 0 0 0 Dolomite 0 0 0 0 0

Accessory Minerals Muscovite 0 3 1 0 0 Biotite 0 0 0 0 0 Others 10 1 7 2 2

Porosity 0 17 14 17 18 100 100 100 100 100 TOTAL (%):

Table 5. Petrographic Analysis of Thin-Sections obtained from Hale 5-17H Well

Thin Section Analysis Hale 5-17H continues… Sabestian/Logan Counties Depth (ft): 7780 7180 8350 6970 6900 7080 8110 Cuttings Condition (mm): Very Poor Very Poor Very Poor Very Poor Good Poor Poor Grain Size Average (mm): 0.1 0.16 0.1 0.15 0.12 0.16 0.08 Grain Size Range (mm): 0.08-0.12 0.008-0.24 0.08-0.2 0.08-0.22 0.05-0.55 0.03-0.44 0.02-0.062 Pore Throat Average (mm): 0.11 0.01 0.01 0.012 Rock Name (Folk): Q/arenite Sublithics Quartzarenites

Framework Grains (%): Quartz 64 64 37 54 70 66 66 Monocystalline 64 54 32 46 68 60 66 Polycrystalline 0 10 5 8 2 6 0 Felspar 5 1 5 1 2 2 1 Lithic Fragments 1 8 9 1 3 2 2

Cementing Materials 23 9 21 20 15 17 5 Clay Cement 23 9 16 20 5 6 5 Quarts Overgrowths 0 0 5 6 10 11 6 Felspar Overgrowths 0 0 0 0 0 0 0 Calcite cement 0 0 0 0 0 0 0 Dolomite 0 0 0 0 0 0 0

Accessory Minerals Muscovite 0 0 0 0 0 2 0 Biotite 0 0 0 0 0 0 0 Others 2 3 0 0 1 2 16

Porosity 5 15 28 18 9 9 4 100 100 100 100 100 100 100 TOTAL (%):

Table 6. Petrographic Analysis of Thin-Sections obtained from Hale 5-17H well

Thin Section Analysis Carter 2-26 Sabestian/Logan Counties Depth (ft): 3620 3640 3660 3670 3680 3800 Cuttings Condition (mm): Poor Poor Poor Poor Poor Poor Grain Size Average (mm): 0.17 0.18 0.28 0.14 0.13 0.2 Grain Size Range (mm): 0.08-0.28 0.13-0.22 0.06-0.6 0.06-0.2 0.04-0.20 0.08-0.32 Pore Throat Average (mm): 0.001 0.001 0.012 0.01 0.008 0.01 Rock Name (Folk): Sublitharenites Q/arenites Sublitharenites Framework Grains Quartz 48 54 59 53 66 49 Monocystalline 48 54 44 52 31 23 Polycrystalline 0 0 15 1 35 26 Felspar 1 0 0 1 1 2 Lithic Fragments 8 8 11 15 7 14

Cementing Materials 19 25 19 12 9 12 Clay Cement 4 11 15 10 8 6 Quarts Overgrowths 15 14 4 2 1 6 Felspar Overgrowths 0 0 0 0 0 0 Calcite cement 0 0 0 0 0 0

Accessory Minerals Muscovite 6 2 4 0 3 6 Biotite 0 0 0 0 0 0 Others 2 6 3 9 6 3

Porosity 16 5 4 10 8 14 100 100 100 100 100 100 TOTAL (%):

Table 7. Petrographic Analysis of Thin-Sections obtained from Carter 2-26 well

Thin Section Analysis Carter 2-26 continues… Sabestian/Logan Counties Depth (ft): 3830 3870 4080 3630 3910 Cuttings Condition (mm): Good Very Poor Poor Good Poor Grain Size Average (mm): 0.12 0.16 0.21 0.33 0.3 Grain Size Range (mm): 0.04-0.24 0.008-0.32 0.04-0.42 0.03-0.82 0.02-0.73 Pore Throat Average (mm): 0.006 0.006 0.01 0.002 0.05 Rock Name (Folk): Framework Grains Quartz 59 61 63 60 59 Monocystalline 31 39 43 50 52 Polycrystalline 28 22 20 10 7 Felspar 2 0 4 0 1 Lithic Fragments 14 12 6 6 7

Cementing Materials 10 9 19 22 20 Clay Cement 2 3 15 6 5 Quarts Overgrowths 8 9 4 16 15 Felspar Overgrowths 0 0 0 0 0 Calcite cement 0 0 0 0 0

Accessory Minerals Muscovite 0 0 0 3 1 Biotite 0 0 0 0 0 Others 1 1 1 1 3

Porosity 14 13 7 8 9

TOTAL (%): 100 100 100 100 100

Table 8. Petrographic Analysis of Thin-Sections obtained from Carter 2-26 well

Thin Section Analysis State 2-22H Sabestian/Logan Counties Depth (ft): 5990 5630 7040 7130 5570 6110 Cuttings Condition (mm): Very Poor Very Poor Poor Very Poor Poor Poor Grain Size Average (mm): 0.22 0.16 0.16 0.23 0.19 0.14 Grain Size Range (mm): 0.1-0.39 0.08-0.24 0.12-0.21 0.12-0.44 0.01-0.48 0.02-0.55 Pore Throat Average (mm): 0.016 0.016 0.012 0.015 Rock Name (Folk): Sublitharenites Q/arenite Framework Grains Quartz 60 55 64 65 63 66 Monocystalline 60 55 64 65 60 65 Polycrystalline 0 0 0 0 3 1 Felspar 8 5 1 0 0 1 Lithic Fragments 11 15 9 20 5 4

Cementing materials 12 19 6 4 12 13 Clay Cement 6 19 6 4 7 8 Quarts Overgrowths 3 0 0 0 5 5 Felspar Overgrowths 0 0 0 0 0 0 Calcite cement 3 0 0 0 0 0 Dolomite 0 0 0 0 0 0

Accessory minerals Muscovite 0 0 0 0 2 3 Biotite 0 0 0 0 0 0 Others 1 1 2 1 15 9

Porosity 8 5 18 10 3 4

TOTAL (%): 100 100 100 100 100 100

Table 9. Petrographic Data of Samples of State 2-22H Well

Thin Section Analysis Sanderson 3-9H Sabestian/Logan Counties Depth (ft): 8010 7770 7620 6288 7890 Cuttings Condition (mm): Very Poor Very Poor Very Poor Poor Poor Grain Size Average (mm): 0.24 0.01 0.19 0.22 0.19 Grain Size Range (mm): 0.2-0.28 0.008-0.012 0.14-0.24 0.03-0.67 0.02-0.81 Rock Name (Folk): Subarkose Quartzarenites Framework Grains Quartz 50 67 68 68 51 Monocystalline 50 67 66 66 50 Polycrystalline 0 0 0 2 1 Felspar 4 1 1 1 1 Lithic Fragments 4 1 1 1 2

Cement 18 22 27 17 13 Clay Cement 9 15 8 8 6 Quarts Overgrowths 9 7 11 9 5 Felspar Overgrowths 0 0 0 0 0 Calcite cement 0 0 0 0 0 Dolomite 0 0 8 0 2

Accessory minerals Muscovite 4 0 0 1 5 Bitoite 0 0 0 0 0 Others 20 6 1 10 21

Porosity 0 3 4 2 7

TOTAL (%): 100 100 100 100 100

Table 10. Petrographic Data of Samples of Sanderson 3-9H Well

Gross Sands Wells Basham Nichol Turner Borum TANNER 'B' 57 163 61 0 Ft Chaffee 171 131 200 359 GIPSON 0 0 0 0 EVELYN "G" 41 196 245 318 LOWE 69 65 73 0 TATUM 147 118 45 41 James 86 127 53 0 HOWARD 163 98 102 41 FERREL 171 135 86 20 WILD HOG 114 69 90 0 Anderson R 102 102 69 0 Sanatorium 212 37 24 0 Gilker James 65 90 49 0 GASAWAY 106 65 82 0 ROBERTS JACK 65 53 41 0 Wooten 53 16 12 0 JONES 155 37 241 0 LEWIS 216 135 180 0 TEXAS JACK 208 147 151 0 Wooten EJ 135 69 86 0 BUCCELLA 69 143 49 0 HARRELL 73 241 0 0 AINSWORTH 94 78 270 20 HANCOX 155 114 0 0 BARNES 86 102 220 167 YADON 0 90 139 0 HERMIE 131 139 167 0 HOPEWELL 151 253 41 33 PAUL X 286 180 90 147 GIBSON PEARL 106 233 147 274 LOWE 65 245 24 261 TYLER 110 65 269 41 TURNER 110 216 0 0 WHITNEY 225 200 98 294 GREGORY 98 294 61 0 HALE 135 143 135 57 STATE 139 106 131 0 HATTABAUGH1-33 163 196 241 0 HATTABAGAUH 143 114 49 94 HOLCOMB 192 131 98 0 CARTER 188 94 24 0 STATE 122 159 159 0 HALE 151 269 61 0 Sterling 163 94 139 41 Cate B 114 110 33 102

Table 11. Gross sands of the four Stratigraphic Units

Framework Grains Quartz (%) Felspar (%) Lithic Fragments (%) Depth (ft) Wells 80 11 9 70808 81 11 8 6000 Schultz-Teeters 3-18H 91 2 7 7470 85 10 5 7609 91 5 4 7588 93 1 6 7573 89 7 4 7518 Holcombe 1-36 90 7 3 7509 92 5 3 7075 88 9 3 7027 89 8 3 7018 84 8 8 8010 98 1 1 7770 98 1 1 7620 Sanderson 3-9H 97 1 2 6288 95 2 3 7890 75 10 15 5990 73 7 20 5630 86 1 13 7040 State 2-22H 72 0 28 7130 75 0 25 5570 93 0 7 6110 83 4 13 3620 86 0 14 3640 84 0 16 3660 79 2 19 3670 98 1 1 3680 75 4 21 3800 Carter 2-26 78 2 20 3830 83 0 17 3870 86 6 8 4080 90 0 10 3630 88 1 11 3910 84 1 15 6950 83 11 6 6750 82 9 9 9134 97 1 2 7570 84 2 14 7330 91 7 2 7780 Hale 5-17H 87 1 12 7180 72 9 19 8350 97 1 2 6970 93 2 5 6900 95 2 3 7080 96 1 3 8110 87 4 9 Total %

Table 12: Percentages of the framework grain components for all the depths