Biostratigraphic and Geochemical Constraints on the Stratigraphy and Depositional Environments of the Eagle Ford and Woodbine Groups of Texas
Richard A. Denne, John A. Breyer, Tobi H. Kosanke, Joan M. Spaw, Arden D. Callender, Russell E. Hinote, Mohsen Kariminia, Nataliya Tur, Zachary Kita, Jackie A. Lees, Harry Rowe, 2016. "Biostratigraphic and Geochemical Constraints on the Stratigraphy and Depositional Environments of the Eagle Ford and Woodbine Groups of Texas", The Eagle Ford Shale: A Renaissance in U.S. Oil Production, John Breyer
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The 130-year history of study of the Cenomanian–Turonian Eagle Ford and Woodbine Groups of Texas has created a complicated and often confusing nomenclature system. Deciphering these nomenclatures has frequently been hindered by outdated biostratigraphic studies with inaccurate age interpretations. To resolve these issues, a comprehensive compilation and vetting of available biostratigraphic, geochemical, and lithologic data from Eagle Ford and Woodbine outcrops and subsurface penetrations was undertaken, which was then tied to a large network of wells in both south and east Texas. Composite sections were built for four outcrop areas of central and north Texas (Dallas, Red River, Waco, Austin), five outcrop areas from west Texas (Langtry, Del Rio, Big Bend, Chispa Summit, Quitman Mountains), four subsurface areas from south Texas (Webb County, Atascosa County, Karnes County, DeWitt/Gonzales Counties), and two cross sections from the east Texas subsurface (basin center and eastern margin). The resulting datasets were utilized to construct age models and characterize depositional environments, including paleoceanography. In agreement with previous studies, the total organic carbon (TOC)-rich Lower Eagle Ford was interpreted to have been deposited under anoxic to euxinic conditions and the Upper Eagle Ford under dysoxic to anoxic conditions. The Oceanic Anoxic Event 2 (OAE2) interval is missing at all locations north of Atascosa County; when present it is characterized as having been deposited under oxic to suboxic conditions. High abundances of radiolaria and calcispheres identified within recrystallized medial to distal limestones of the Lower Eagle Ford indicated limestone formation during periods of enhanced water-column mixing and increased primary productivity, in contrast to proximal limestones composed of planktonic foraminifera and inoceramid prisms concentrated by bottom currents.
Standardized nomenclature systems and age models are proposed for each of the outcrop and subsurface areas. Proposed changes to existing nomenclatures include reassignment of the Tarrant Formation of the Eagle Ford to the Lewisville Formation of the Woodbine in the Dallas area and the Templeton Member of the Lewisville Formation to the Britton Formation of the Eagle Ford in the Red River area. The proposed term “Waller Member” of Fairbanks (2012) for the former Cloice Member of the Lake Waco Formation in the Austin area is recognized with a new stratotype proposed and described, although the Waller Member is transferred to the Pepper Shale Formation of the Woodbine. The Terrell Member is proposed for the carbonate-rich section at the base of the Boquillas Formation in the Langtry and Del Rio areas, restricting the Lozier Canyon Member to the organic-rich rocks underlying the Antonio Creek Member. The south Texas subsurface is divided into the Upper Eagle Ford and Lower Eagle Ford Formations, with the clay-rich Maness Shale Member at the base of the Lower Eagle Ford and the foraminifera grainstone dominated Langtry Member at the top of the Upper Eagle Ford. Use of the term “middle Eagle Ford” for the clay-rich facies south of the San Marcos arch is not recommended.
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Known as a world-class source rock for years, the Eagle Ford Shale became a world-class oil reservoir early in the second decade of the 21st century. Oil production from the Eagle Ford grew from 352 barrels of oil per day (BOPD) in 2007 to over 1.7 million BOPD in March 2015. Since then, the play has been a victim of its own success. Production from shale oil in the United States has helped contribute to a glut in world oil supply that led to a precipitous drop in oil prices beginning in the summer of 2014. As prices fell from over $100 per barrel in July 2014, to less than $30 per barrel in January 2016, production from the Eagle Ford declined over 500,000 BOPD. Anyone interested in the geology behind this remarkable play and the new ideas that reshaped the global energy supply should read AAPG Memoir 110.