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Petroleum properties and thermal history of Taylor-Navarro Group constrain petroleum migration in Maverick basin, south Texas


Maverick basin lies at the southwestern end of an area of prolific historical petroleum production from Mesozoic strata in south Texas. Around Maverick basin, the shallowest productive formations are the Campanian San Miguel Formation (Taylor Group) and Maastrichtian Olmos and Escondido Formations (Navarro Group), with a significant portion of modern production from tight reservoirs. To characterize these resources, we compiled API gravity measurements and calculated cumulative gas to oil ratios (GOR) for hundreds of wells in the three formations. The compilation is for a region defined by the Upper Cretaceous outcrop belt to the north, the San Marcos Arch to the east, the Lower Cretaceous Sligo shelf margin to the south, and the U.S.-Mexico border to the west. Plots of the compiled data indicate a clear trend, with progressively heavier oils updip to the north grading to dry gases downdip to the south. An exception to the trend is seen in Maverick basin in the western part of the study area where GOR values indicative of wet and dry gases are interspersed with values that correspond to black and heavy oils and API gravity values vary randomly.

To the east of Maverick basin, product types defined by API gravity and GOR closely correlate to oil (vitrinite reflectance, or VR, < 1.2%), wet gas (VR = 1.2% to 1.5%), and dry gas (VR > 1.5%) windows of the underlying Eagle Ford. The correlation suggests an Eagle Ford petroleum source and approximately vertical migration. However, in Maverick basin, the randomly varying API gravity and GOR measurements imply a more complex scenario.

Additional insight about Maverick basin comes from our work on anomalous high surface thermal maturity using analysis of sonic logs in nine wells and in-progress apatite fission track analysis for three Taylor-Navarro Group outcrop samples. Our analysis indicates km-scale exhumation of Chittim anticline, which extends across much of Maverick basin, beginning in the Eocene. Considering the magnitude of exhumation, the varying API gravity and GOR values may be accounted for by depressurization, gas exsolution, gas expansion, and possibly remigration in pre-Eocene petroleum accumulations that were advected towards the erosion surface. An additional complication is that the area of Maverick basin which lies west of the Frio River hingeline may have multiple petroleum sources, possibly including strata in the Mexican cordilleran foreland to the west.