Elisabeth L. Rowan1, Peter D. Warwick1, and Janet K. Pitman2
1U.S. Geological Survey, Reston, VA
2U.S. Geological Survey, Denver, CO
The thermal maturation history of the Paleocene-Eocene Wilcox Group in the Texas coastal plain has been reconstructed in conjunction with the 2007 USGS assessment of oil and gas resources in Cenozoic strata of the onshore U.S. Gulf of Mexico. The timing and degree of maturation in the Wilcox are estimated from 1D burial history models for 53 wells located across the Texas coastal plain. The models simulate sedimentation, compaction, and temperature evolution through time, based on heat flow and thermal conductivity assumptions, and were calibrated with bottom-hole temperature and vitrinite reflectance (Ro) data for the Wilcox. Coals and organic-rich shales of predominantly terrigenous origin (Type III kerogen) in the Wilcox are considered the primary source of oil, as well as a source of gas. Oil generation is modeled using new hydrous pyrolysis kinetic parameters (Lewan, M.D., written comm., 2006), and gas generation from Type III kerogen is represented using calculated Ro values.
Model results for the study area, the Texas coastal plain, indicate that the downdip portions of the basal Wilcox had reached sufficient thermal maturity to generate hydrocarbons by early Eocene (~50 Ma). The Wilcox Group strikes approximately parallel to the Texas coast; “downdip” thus indicates a direction approximately perpendicular to the shoreline, as well as the direction of greatest increase in Wilcox thickness. Relatively early maturation is explained by rapid sediment accumulation during the early Tertiary, combined with Wilcox-specific kinetic parameters reflecting labile organic matter. The thermal maturation front moved gradually updip through time, as burial depth and temperature increased. At present day in the Texas coastal plain, the downdip Wilcox is supermature, but portions of the updip Wilcox remain thermally immature and potentially capable of generating hydrocarbon.
AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas