diagenesis and reservoir characterization of the
TRANSCRIPT
University of Arkansas, FayettevilleScholarWorks@UARK
Theses and Dissertations
12-2012
Diagenesis and Reservoir Characterization of thePennsylvanian Middle Atoka Formation, Sebastianand Logan Counties, West-Central ArkansasElvis Chekwube BelloUniversity of Arkansas, Fayetteville
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Recommended CitationBello, Elvis Chekwube, "Diagenesis and Reservoir Characterization of the Pennsylvanian Middle Atoka Formation, Sebastian andLogan Counties, West-Central Arkansas" (2012). Theses and Dissertations. 583.http://scholarworks.uark.edu/etd/583
DIAGENESIS AND RESERVOIR CHARACTERIZATION OF THE PENNSYLVANIAN
MIDDLE ATOKA FORMATION, SEBASTIAN AND LOGAN COUNTIES,
WEST-CENTRAL ARKANSAS
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
By
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
1
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.
2
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).
3
Figure 1. Regional maps showing geological setting of Arkoma Basin (modified from
Encyclopedia of Arkansas).
Arkoma Basin
Ozark Plateau
Ouachita Mountains
West Gulf Coastal Plain
4
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.
T6N R31W T6N R30W T6N R29W T6N R28W
T5N R31W T5N R30W T5N R29W T5N R28W
•West central Arkansas
•Sebastian & Logan
Counties
•Townships 5 N – 6 N
•Ranges 28 W – 31 W
•288 square miles
SebastianLogan
Crawford Franklin
Scott
Polk
Yell
Johnson
MontgomeryGarland
Pope
Perry
Study area
5
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.
6
System Series Formation Sandstone Unit
Desmoinessian
Carpenter A
Alma
Carpenter B
Morris
Areci
Bynum
Freiburg
Casey
Sells
Jenkins
Dunn B
Dunn C
Paul Barton
Cecil Spiro
Pennsylvanian
MorrowanBloyd
Hale
Middle
Upper
Lower
Hartshorne
AtokaAtokan
Table 1. Stratigraphic column of the Atoka Formation, Arkoma Basin, Arkansas (after
Zachry 1983).
Sandstone unit
(This study)
Morris
Areci
Bynum Nichol
Freiburg Turner
Casey Borum
Morrowan
Formation Member Sandstone unit
Pennsylvanian
System
Basham
Upper Atoka Formation
Middle Atoka
Lower Atoka Formation
Bloyd
Atoka
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
7
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,
8
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.
9
Figure 3. Type log used for the study
Resistivity Log
Conductivity Log
Gamma-Ray
Holcomb 1-36
10
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
Top of middle
Atoka marker:
Top of low
blocky
conductivity
motif toward the
track
Gregory
11
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).
12
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.
13
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.
North Arkoma Basin Ouachita Mountains
Sabine
uplift
South
?
14
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.
15
Most of these basins produce petroleum, particularly the shale resource plays of the
Arkoma Basin, and the Fort Worth Basins.
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).
Ouachita Mountains
Ozark Plateau
Ouachita Salient
?
16
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
Ozark Plateau
A
Arkoma Basin
Ouachita Mountains
Metamorphic belt
Frontal
belt
17
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
18
(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.
19
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
N
N
OklahomaArkansas
Texas
Mississippi
Louisiana
Distal fan Medial fan Proximal fan
Several kilometers
20
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).
21
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).
22
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).
23
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.
24
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
Blocky &
serrated
Blocky & massive
Fining upward
successions
Thin &
symmetrical
Crescent shape
Thin beds
PROMINENT LOG SIGNATURES
Tracks
Holcomb 1-36 Carter 2
25
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
26
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.
Figure 10 showing the conceptual submarine fan depositional model of the study area
(Kim et al, 2012).
Levee-overbank
Crevasse splay
Feeder
Channel
Non-channelized
(Medial)
Distal lobe
Channelized
(Proximal)
100’s
of km
27
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).
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
Shelfal margin Continental slope Deep-water
Several kilometers
28
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
29
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
30
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
31
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).
32
Figure 12. Well log signatures depicting the depositional environments (after Miall,
2008).
Amalgamated
deposits
Overbank
deposits
Overbank deposits
Amalgamated
deposits
Holcomb Hales
Two type of sandstone reservoirs
33
Figure 13: Showing clean sands (API 75) for the four stratigraphic units
TMA
Hattabaugh Holcomb
Basham sands
Nichol sands
Turner sands
Borum sands
34
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’
Carter
Schultz-Teeter - H
Sanderson - H
Hale
Holcomb
State
Subsea (SS)
35
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
36
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.
37
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
38
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
39
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
40
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
41
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.
42
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.
43
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.
44
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).
45
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.
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
Q
LF
10
25
SublithareniteSubarkose
Quartzarenite
Arkose Lithicarkose LithareniteFeldspathic
litharenite
46
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.
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
Lithic Fragements0 10 20 30 40 50 60 70 80 90 100
Quartz Arenites
0
10
20
30
40
50
60
70
80
90
100
Feldspar
0
10
20
30
40
50
60
70
80
90
100
Schult-teeter
Holcomb
Sanderson
State
Carter
Hale
F
FL
47
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.
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.
Polycrystalline Quartz
Undulose Extinction
Chert Fragments
Undulose extinction
Straight Extinction
Advance quartz overgrowth
Grain dissolution
Grain Fractures
Grain edge dissolution
A B
C D
48
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.
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.
Quartz cementation, well-compacted Grain replacement, alteration, & dissolution
Polycrystalline, Lithic fragments, matrix
Grains alteration to clay
A B
C D
49
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.
50
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.
Moderately sorted
Muscovite
Mudstone
MatrixMono-quartz
Fractured grain Advance quartz overgrowth
Undulose & straight extinction
Quartz edge
dissolution
Fracture healed by quartz cementation Clay coatings before quartz overgrowth
A
C
B
D
51
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
Clay-cemented grains, no quartz overgrowth Clay-filled voids
Partial Dissolution
Grain to grain contacts
Isolated dissolution structures
Poorly interconnected pores
Clay mineral occluded porosity
Effective grain coatings
Leached remnant
A
C
B
D
52
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).
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
Deformed muscovite
Poorly sorted
Plant fragments
Poor plate
Mudstone Broken muscovite
Angular edge
A B
CD
53
structural clays (selectively replaced grains). Laminar and dispersed clays are both pre- and syn-
depositional. 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.
54
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).
Polycrystalline grains Well-sorted, well rounded &
compacted grains
Poorly sorted, angular to rounded grains Poorly sorted
Grain edge dissolution
A
C
B
D
55
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.
56
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).
Figure 30: Clay minerals distribution in the reservoir rock (modified after Boyd et al,
1995).
Laminar clays Structural clays
Dispersed clays as pore coatingsDispersed Clays as pore fillings
57
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).
58
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).
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 =
a
Qm
F Lt
Quartzose
recycled
Craton
Interior
Basement
uplift
Lithic recycled
Transitional
recycled
Transitional
continental
Transitional
arc
Undissected
Dissected
arc
Mixed
Q
FL
Undissected arcTransitional arc
Dissected arc
Recycled orogeny
Basement
uplift
Transitional
continental
Craton
interior
59
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.
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.
Lithic Fragements0 10 20 30 40 50 60 70 80 90 100
Quartz Arenites
0
10
20
30
40
50
60
70
80
90
100
Feldspar
0
10
20
30
40
50
60
70
80
90
100
Schult-teeter
Holcomb
Sanderson
State
Carter
Hale
F
F
L
60
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
Q
Lithic Fragments0 10 20 30 40 50 60 70 80 90 100
Monocrystalline Quartz
0
10
20
30
40
50
60
70
80
90
100
Feldspar
0
10
20
30
40
50
60
70
80
90
100
Sanderson
State
Carter
Hales
Holcomb
Schult-teeter
Qm
F Lt
Qm
61
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
62
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
63
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
64
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.
65
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.
66
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.
67
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APPENDICES
79
0 2 4 6 8
Schult-Teeter 3-18H
Holcomb 1-36
Hale 5-17
Carter 2-26
State 2-22
Sanderson 3-9H
Secondary
Primary
Porosity
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.
0 20 40 60 80 100
Scultz-Teeter 3-18H
Holcomb 1-36
Sanderson 3-9H
State 2-22
Carter 2-26
Hale 5-19
Lithic fragments
Feldspar
Quartz
Framework Grains
Figure 36 showing the composition of the framework grains of the middle Atoka Formation from
the six wells. Quartz has the highest composition.
80
0 20 40 60 80 100
Sanderson 3-9H
State 2-22
Carter 2-26
Hale 5-19
Holcomb 1-36
Scultz-Teeter 3-18H
Dolomite
Calcite cements
Feldspar overgrowths
Quartz overgrowths
Clay cements
Cementing materials
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).
0 20 40 60 80 100 120
Sanderson 3-9H
State 2-22
Carter 2-26
Hale 5-19
Holcomb 1-36
Scultz-Teeter 3-18H
Biotite
Muscovite
Acessoryminerals
Figure 38 showing the distribution of the accessory minerals with muscovite averaging about
99% of the accessory minerals. Biotite occurs in trace amount.
81
0 20 40 60 80
Monocystalline
Polycrystalline
Felspar
Lithic Fragments
Clay Cement
Quarts Overgrowths
Felspar Overgrowths
Calcite cement
Dolomite
Muscovite
Bitoite
Others
Primary
Secondary
8010
7890
7770
7620
6288
Depth (ft.)
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.
82
0 10 20 30 40 50 60 70
Monocystalline
Polycrystalline
Felspar
Lithic Fragments
Clay Cement
Quarts Overgrowths
Felspar Overgrowths
Calcite cement
Dolomite
Muscovite
Biotite
Others
Primary
Secondary
7130
7040
6110
5630
5990
5570
Depth (ft.)
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.
83
0 10 20 30 40 50 60
Monocystalline
Polycrystalline
Felspar
Lithic Fragments
Clay Cement
Quarts Overgrowths
Felspar Overgrowths
Calcite cement
Muscovite
Biotite
Others
Primary
Secondary
4080
3910
3870
3830
3800
3680
3670
3660
3640
3630
3620
Depth (ft.)
Figure 41 showing the distribution of the thin sections modal analysis obtained from the various
depths of the Carter 2-26 well.
84
0 20 40 60 80
Monocystalline
Polycrystalline
Felspar
Lithic Fragments
Clay Cement
Quarts Overgrowths
Felspar Overgrowths
Calcite cement
Dolomite
Muscovite
Biotite
Others
Primary
Secondary
8110
7080
6900
6970
8350
7180
7780
7330
7570
9134
6750
6950
Depth (ft.)
Figure 42 showing the distribution of the thin sections modal analysis obtained from the various
depths of the Hale 5-19 well.
85
0 10 20 30 40 50 60 70
Monocystalline
Polycrystalline
Felspar
Lithic Fragments
Clay Cement
Quarts Overgrowths
Felspar Overgrowths
Calcite cement
Dolomite
Muscovite
Biotite
Others
Primary
Secondary
7609
7588
7573
7518
7509
7075
7027
7018
Depth (ft.)
Figure 43 showing the percentage abundance of the thin sections modal analysis obtained from
the various depths of the Holcomb 1-36 well.
86
0 20 40 60 80
Monocystalline
Polycrystalline
Felspar
Lithic Fragments
Clay Cement
Quarts Overgrowths
Felspar Overgrowths
Calcite cement
Dolomite
Muscovite
Biotite
Others
Primary
Secondary
7470
7080
6000
Depth (ft.)
Figure 44 showing the percentage abundance of the thin sections modal analysis obtained from
the various depths of the Schultz-Teeter 3-18H well.
87
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
Thin Section Analysis
Schultz teeters 3-18H
Sabestian/Logan Counties
Table 3. Petrographic Data of Samples of Schultz-Teeters 3-18H
88
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
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
Quartzarenites Subarkoses
Thin Section Analysis
Holcombe 1-36
Sabestian/Logan Counties
Table 4. Petrographic Analysis of Thin-Sections obtained from Holcomb 1-39 well
89
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
TOTAL (%): 100 100 100 100 100
Thin Section Analysis
Hale 5-17H
Sabestian/Logan Counties
90
Table 5. Petrographic Analysis of Thin-Sections obtained from Hale 5-17H Well
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
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
TOTAL (%): 100 100 100 100 100 100 100
Thin Section Analysis
Hale 5-17H continues…
Sabestian/Logan Counties
Sublithics Quartzarenites
Table 6. Petrographic Analysis of Thin-Sections obtained from Hale 5-17H well
91
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): 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
TOTAL (%): 100 100 100 100 100 100
Thin Section Analysis
Carter 2-26
Sabestian/Logan Counties
Sublitharenites
Table 7. Petrographic Analysis of Thin-Sections obtained from Carter 2-26 well
92
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
Thin Section Analysis
Carter 2-26 continues…
Sabestian/Logan Counties
Table 8. Petrographic Analysis of Thin-Sections obtained from Carter 2-26 well
93
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): 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
Thin Section Analysis
State 2-22H
Sabestian/Logan Counties
Sublitharenites
Table 9. Petrographic Data of Samples of State 2-22H Well
94
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):
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
Thin Section Analysis
Sanderson 3-9H
Sabestian/Logan Counties
Subarkose Quartzarenites
Table 10. Petrographic Data of Samples of Sanderson 3-9H Well
95
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
Gross Sands
Table 11. Gross sands of the four Stratigraphic Units
96
Quartz (%) Felspar (%) Lithic Fragments (%) Depth (ft)
80 11 9 70808
81 11 8 6000
91 2 7 7470
85 10 5 7609
91 5 4 7588
93 1 6 7573
89 7 4 7518
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
97 1 2 6288
95 2 3 7890
75 10 15 5990
73 7 20 5630
86 1 13 7040
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
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
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
Framework Grains
Sanderson 3-9H
State 2-22H
Carter 2-26
Wells
Hale 5-17H
Total %
Holcombe 1-36
Schultz-Teeters 3-18H
Table 12: Percentages of the framework grain components for all the depths
97