Regional source rock thermal stress modeling and map-based charge access modeling of the Port Isabel passive margin foldbelt, northwestern Gulf of Mexico

Abstract

The Port Isabel passive margin foldbelt covers 17,000 km2 of the northwestern, deepwater U.S. Gulf of Mexico (GOM). Seven oil exploration wells were drilled in the area from 1996 to 2007, yielding a single, uncommercial gas discovery. The 5-7 km thick Oligo-Miocene section prevented drilling from penetrating the underlying Paleogene and Mesozoic source rocks. Accommodation space for the Oligo-Miocene section was created by the collapse of a paleo-salt wall, leading to linked fault systems in the upper decollement to the west. We used 13 exploration wells to construct 1D and map-based 2D basin models to investigate the burial and thermal history of three inferred source rock horizons (Paleogene, Turonian, and Tithonian). We interpreted a 2D seismic data grid tied to four wells to constrain stratigraphic depths and thicknesses, younger and shallower Wilcox source rock horizons, and interpreted Jurassic and Cretaceous source rock horizons. Our results show vitrinite reflectance as a proxy for the thermal stress levels reached by the source rocks combined with maps of hydrocarbon charge access. We conclude that all three source rock intervals reached varying degrees of maturity and expelled hydrocarbons in late Paleogene to mid-Neogene and likely continue expelling hydrocarbons to the present-day at a reduced rate. The deposition of the Oligocene and Middle Miocene sedimentary section buried the underlying source intervals and likely brought them into the gas/condensate window at present-day. Our mapping of the seismic grid revealed four-way structural closures, three-way stratigraphic traps, and salt truncation structures associated with amplitude anomalies which may support our predictions for maturity in the underlying source rocks. Thermal stress maps predict source rocks have matured. There arises a need to investigate the hydrocarbon migration model, including assessing charge access for each well. The timing of late trap formation and early hydrocarbon charge remains a risk factor.