2003 U.S. Geological Survey Assessment of Undiscovered Oil and Gas Resources in Upper Cretaceous Navarro and Taylor Groups, Western Gulf Province, Texas

T. S. Dyman, S. M. Condon, R. R. Charpentier, T. A. Cook, T. R. Klett, M. D. Lewan, M. J. Pawlewicz, R. M. Pollastro, and C. J. Schenk
U.S. Geological Survey, Denver, Colorado 80225

The U.S. Geological Survey (USGS) recently completed an assessment of the undiscovered oil and gas potential of the Upper Cretaceous Navarro and Taylor Groups in the Western Gulf Petroleum Province of the northern Gulf Coast region (Dyman et al., 2004). The rocks are included in the Smackover-Austin-Eagle Ford Composite Total Petroleum System (TPS) and consist of, from youngest to oldest, the Escondido and Olmos formations of the Navarro Group, and the San Miguel Formation and Anacacho Limestone of the Taylor Group. Parts of the underlying Austin Group were included in the assessment in some areas.

Within the TPS, primary source rocks include laminated carbonate mudstones and marine shales of the Upper Jurassic Smackover Formation, mixed carbonates and organic clastics of the Upper Cretaceous Eagle Ford Group, and marine shelf carbonates of the Upper Cretaceous Austin Group. Possible secondary source rocks include the Upper Jurassic Bossier Shale and overlying shales within the Upper Jurassic to Lower Cretaceous Cotton Valley Group, Lower Cretaceous marine rocks, and the Upper Cretaceous Taylor Group.

Oil and gas were generated in parts of the study area at different times, relating to variations in depth of burial, geothermal gradient, lithology, and organic matter-composition. A burial history reconstruction, based on data from one well in the eastern part of the study area, indicated that (1) the Smackover generated oil from about 117 to 103 million years ago (Ma) and generated gas from about 52 to 41 Ma; and (2) the Eagle Ford and Austin generated oil from about 42 to 28 Ma and generated gas from about 14 Ma to the present.

Oil and gas migrated from the source rocks up-section and updip through a system of faults and fractures that are pervasive throughout the area, in addition to movement updip along bedding planes and within sandstone units. Trapping mechanisms include stratigraphic pinchouts, folds, faulted folds, and combination stratigraphic-structural traps. Seals consist of interbedded shales and mudstones and diagenetic cementation.

The TPS is subdivided into five assessment units (AU): (1) Travis Volcanic Mounds Oil AU, (2) Uvalde Volcanic Mounds Gas and Oil AU, (3) Navarro-Taylor Updip Oil and Gas AU, (4) Navarro-Taylor Downdip Gas and Oil AU, and (5) Navarro-Taylor Slope-Basin Gas AU. These AUs resulted in estimated mean volumes of 1,682.8 billion cubic feet of undiscovered nonassociated gas and associated gas in oil fields, 33.22 million barrels of oil, and 34.26 million barrels of natural gas liquids. All of the undiscovered oil and gas is in conventional accumulations. The Navarro-Taylor Slope-Basin Gas AU is hypothetical and contains an estimated mean of 924.96 billion cubic feet of gas, representing about 55 percent of the total mean undiscovered gas resource for the Navarro-Taylor Groups.

AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana