Renewed Petroleum Generation Related to Tertiary Intrusions and Increased Heat Flow, Western Permian Basin, Texas and New Mexico

Charles E. Barker, Mark J. Pawlewicz

Higher paleogeothermal gradients, commencing in the Tertiary after maximum burial, have caused renewed petroleum generation in the western Permian basin. Evidence for this reheating is two distinct trends in the mean random vitrinite reflectance (R_m) and depth data compiled from over 40 wells. One group, with a 0.7% R_m/km gradient, is from the western edge of the basin; the other, with a 0.5 R_m/km gradient, is from the central and eastern portions.

Post-Mississippian tilting produced greater subsidence and a thicker, mostly uneroded sedimentary section in the eastern portion of the Permian basin. Continued tilting prior to the Cretaceous caused uplift and erosion that exposed the Upper Permian section in the western part. Potassium-argon ages of igneous intrusions along the western edge of the basin show they were emplaced about 35 Ma, followed by Miocene to Holocene basin-and-range-type block faulting and associated high heat flow. Isopach-reflectance contours confirm this renewed heating is post-tectonic--that is, it occurred after eastward tilting and erosion had reduced burial depth. Maximum temperatures computed from R_m-depth relationships infer that paleogeothermal gradients exceeded 40°C/km (2.2°F/100 ft) in the Tertiary. This reheating thermally matured rocks as young as Guadalupian in the western Permian basin and apparently caused a second episode of petroleum generation. By this time, however, the potential reservoir rocks and evaporite seals had been deeply eroded, resulting in poor conditions for trapping the renewed pulse of petroleum.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.