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University of Arkansas, Fayetteville ScholarWorks@UARK Theses and Dissertations 8-2012 A Subsurface Stratigraphic Study of the Middle Atoka, Sebastian County, Arkansas Sara Beth Sutton University of Arkansas, Fayetteville Follow this and additional works at: http://scholarworks.uark.edu/etd Part of the Geology Commons, and the Stratigraphy Commons Recommended Citation Sutton, Sara Beth, "A Subsurface Stratigraphic Study of the Middle Atoka, Sebastian County, Arkansas" (2012). Theses and Dissertations. 477. http://scholarworks.uark.edu/etd/477 This Thesis is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of ScholarWorks@UARK. For more information, please contact [email protected], [email protected]. A SUBSURFACE STRATIGRAPHIC STUDY OF THE MIDDLE ATOKA, SEBASTIAN COUNTY, ARKANSAS A SUBSURFACE STRATIGRAPHIC STUDY OF THE MIDDLE ATOKA, SEBASTIAN COUNTY, ARKANSAS A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology By Sara B. Sutton University of Arkansas Bachelor of Science in Geology, 2009 August 2012 University of Arkansas ABSTRACT Raster log images were analyzed to study the subsurface stratigraphy of the Middle Atoka in the Arkoma Basin of the western portion (Sebastian County) of Arkansas. The most significant sandstone units of the Middle Atoka in this area were determined to be two generally coarsening-upward sequences in the lower portion of the formation. Based on lithologic correlations, these two units were determined to be the informally named Casey and Freiburg members of the Middle Atoka. Thickness information based on picked tops was used to create isopach maps of the units of interest. The log signatures and geometry of the sand bodies indicate that they were deposited in a deltaic system that prograded across this portion of this basin. Distributary-mouth bar deposits dominate the Casey member, and are occasionally overlain by localized channel deposits. Avulsion of the delta lobe possibly occurred, and a new lobe then began to prograde, depositing the sediments that formed the Freiburg. This unit is also dominated by distributary-mouth bar deposits. This thesis is approved for recommendation to the Graduate Council. Thesis Director: _______________________________________ Dr. Doy Zachry Thesis Committee: _______________________________________ Mr. Douglas C. Melton, Jr. _______________________________________ Dr. Xiangyang Xie THESIS DUPLICATION RELEASE I hereby authorize the University of Arkansas Libraries to duplicate this Thesis when needed for research and/or scholarship. Agreed __________________________________________ Sara Sutton Refused __________________________________________ Sara Sutton ACKNOWLEDGEMENTS I would like to begin by thanking Dr. Doy Zachry, who has been both my undergraduate advisor, and thesis advisor as a graduate student. His guidance throughout the years has played an important role in my accomplishments as a geology student. He deserves special thanks for his patience and willingness to help, in spite of his busy schedule. I would also like to extend a special thank you to Doug Melton of Southwestern Energy. Not only has he so kindly served on my thesis committee, but also allowed me my first internship opportunity with Southwestern Energy. His guidance as a professional geologist has been invaluable to my development as a geologist and a professional. Thank you, also, to Dr. Xiangyang “Cheyenne” Xie, who has served as a thesis committee member, and to all the geology professors at the U of A who have shared their knowledge with me over the last few years. My family has also been a great source of support throughout my time in school. I would like to thank my father, Frank Butler. Without his encouragement and support, I might not have made it through my undergraduate years. Thanks go, as well, to my mother and my in-laws for always being there for me when I needed an ear. Finally, thank you to my lovely husband, Peter. He has always believed in me even when times seemed too difficult for me to believe in myself. TABLE OF CONTENTS List of Figures 1.0 INTRODUCTION....................................................................................................... 1 1.1 Methods .................................................................................................................... 5 1.2 Previous Investigations ........................................................................................... 8 2.0 GEOLOGIC SETTING.............................................................................................. 9 2.1 Ozark Uplift ............................................................................................................. 9 2.2 Arkoma Basin ........................................................................................................ 10 2.3 Ouachita Fold Belt ................................................................................................ 12 2.4 Tectonic and Structural History .......................................................................... 12 2.5 Paleogeography and Depositional History .......................................................... 16 2.51 Atokan............................................................................................................... 16 2.52 Desmoinesian .................................................................................................... 20 2.6 General Regional Stratigraphy ............................................................................ 24 2.7 Deltaic Sedimentation ........................................................................................... 25 3.0 LOCAL STRATIGRAPHY ..................................................................................... 25 4.0 STRATIGRAPHIC ANALYSIS.............................................................................. 41 5.0 CONCLUSIONS ....................................................................................................... 50 REFERENCES ................................................................................................................ 51 APPENDIX A …………………………………………………………………………. 53 LIST OF FIGURES FIGURE TITLE PAGE Figure 1 General location of study area in relation to geologic provinces 4 of Arkansas and Oklahoma (from Zachry and Sutherland, 1984). Figure 2 Location of study area (in blue) in Sebastian County Arkansas. 6 Figure 3 Base map of study area showing well locations highlighted in 7 red. Figure 4 North-south cross section across the Arkoma Basin illustrating 11 large syndepositional normal faults during the Middle Atokan time (modified after Zachry and Sutherland, 1984). Figure 5 Tectonic evolution of the Arkoma Basin (from Houseknecht, 14 1986). Figure 6 Graphic illustration of subsidence rates in the Arkoma Basin 15 (Houseknecht, 1985). Figure 7 Early Atokan paleogeography (from Sutherland, 1988). 17 Figure 8 Middle Atokan paleogeography (from Sutherland, 1988). 19 Figure 9 Regional paleogeography of the Early to Middle Atokan (from 19 Houseknecht and Kacena, 1983). Figure 10 Explanation to the Houseknecht and Kacena (1983) 20 paleogeographic diagrams. Figure 11 Earliest Desmoinesian paleogeography (from Sutherland, 21 1988). Figure 12 Regional paleogeography of the Desmoinesian (from 22 Houseknecht and Kacena, 1983). Figure 13 Stratigraphic column of the Carboniferous Period (from 23 McFarland, 1998). Figure 14 Location map of the growth fault zone that lies south of the 26 study area (from Zachry, 1983). Figure 15 North-south stratigraphic cross section of the Middle Atoka 28 showing sand content diminishing to the south (see Figure 16 for location). Figure 16 Figure 16. Index map of cross sections. 29 Figure 17 Isopach map of the Casey sandstone unit of the Middle Atoka. 30 Figure 18 Stratigraphic divisions (Units A, B and C) of the lower Casey 31 sand package in the central portion of the study area. Figure 19 Stratigraphic divisions (Units A, B, C and D) of the Casey sand 32 package in the central portion of the study area. Figure 20 Isopach map of Unit A of the Casey. 35 . Figure 21 Isopach map of Unit B of the Casey. 36 Figure 22 Isopach map of Unit C of the Casey. 37 Figure 23 Isopach map of Unit D of the Casey. 38 Figure 24 Log showing the stratigraphic divisions of the interval of 39 interest. Figure 25 Isopach map of the Freiburg sandstone unit of the Middle 40 Atoka Figure 26 West – East cross section showing intervals of interest and 42 lateral continuity of Shale 2 (see Figure 16 for location of cross section). Figure 27 Geometry and log signatures of distributary-mouth bar deposits 43 (Coleman and Prior, 1980). Figure 28 Idealized coarsening-upward signature formed from a deltaic 44 environment. Modified from Brown, 1979. Figure 29 Sedimentation zones in a delta front environment. From 45 Coleman and Prior, 1981. Figure 30 North-South cross section showing diminishing sand content 46 towards the south (see Figure 16 for cross section location). Figure 31 Log showing depositional environments of a prograding delta. 49 1.0 INTRODUCTION In the western and northern portions of the Arkoma Basin of Arkansas and Oklahoma several sandstone units of the Middle Atoka are substantial natural gas producing reservoirs. This makes understanding of the Middle Atoka of significance. The Middle Atoka is informally divided into sandstone units, which are separated by shales. These Middle Atoka sands

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