The 2017 USGS Assessment of Undiscovered Oil and Gas Resources in the Eagle Ford Shale and Associated Cenomanian Strata, Texas

Abstract

The U.S. Geological Survey (USGS) previously assessed undiscovered, technically recoverable oil and gas resources in the Cenomanian – Turonian Eagle Ford Shale across Texas in 2010. Since then, there has been a huge increase in drilling activity, hydrocarbon production, and geologic research. The availability of significant new geologic and production data has necessitated an updated USGS assessment of undiscovered continuous oil and gas resources in the Eagle Ford Shale and associated Cenomanian strata.

The USGS defined four continuous assessment units (AU) within the Eagle Ford Shale: (1) Eagle Ford Continuous Oil AU; (2) Eagle Ford Continuous Gas AU; (3) Cenomanian Continuous Oil AU; and (4) Cenomanian Continuous Gas AU. The boundaries of these AUs are defined by lithologic variations, thermal maturity boundaries, regional geologic features, and the spatial distribution of productive fairways. In order to better capture the resource heterogeneity of the Cenomanian interval, strata in the carbonate-dominated Eagle Ford Shale strata mainly south of the San Marcos arch were separated from siliciclastic-dominated Cenomanian - Turonian strata north of the San Marcos arch. In and around the Brazos basin the stratigraphic section is referred to as Cenomanian because of uncertainty about the age(s) of organic-rich mudstone in this part of the assessment area.

Literature and USGS research data were used to define the geologic models for the assessment units. The Eagle Ford Shale self-sourced reservoir interval is comprised of organic-rich marl with mainly Type II kerogen, and interbedded brittle limestone intervals. The Cenomanian self-sourced reservoir interval is comprised of organic-rich siliciclastic mudstone, with mixed Type II and Type III kerogen, and interbedded siltstone and sandstone intervals.

The spatial extent of the Eagle Ford Shale oil and gas AUs is defined by: (1) the Texas-Mexico border; (2) an interpretation of increasing clay content north of the San Marcos arch; and (3) the outboard expression of the shelf margin of the Lower Cretaceous Sligo Formation. The oil AU is distinguished from the gas AU by the thermally modeled downdip limit of oil generation. The spatial extent of the Cenomanian oil and gas AUs is defined by: (1) the updip limits of oil and gas generation, respectively; (2) increased clay content; (3) the outboard expression of the Upper Cretaceous shelf margin; and (4) the eastern extent of productive fairways.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018