Geochemistry of carbonate source rocks and crude oils in Jurassic salt basins of the Gulf Coast

Abstract

Abstract The lower member of the Jurassic Smackover Formation is a source rock for crude oil across the northern Gulf Rim from east Texas to Florida. Type-I crude oils occur in Smackover reservoirs of east Texas, north Louisiana, south Arkansas, and Mississippi, and were generated by algal-derived kerogen preserved in an anoxic environment. Type-II crude oils in Smackover reservoirs of Alabama and Florida reflect an origin from kerogen with a terrestrial component preserved in a less anoxic environment. Thermal maturity differences across the Gulf Rim caused generation and destruction of crude oil to occur at much shallower depths in east Texas, north Louisiana, and south Arkansas than in Mississippi, Alabama and Florida. Jurassic through Cretaceous timing of migration and faulting permitted vertical migration of Type-I Smackover crude oils to many Upper Jurassic, Lower Cretaceous and Upper Cretaceous reservoirs along the Gulf Rim. Vertical migration of Type-II crude oils was less significant, especially in Alabama, because of evaporite barriers to migration. The high contents of hydrogen sulphide and carbon dioxide in deep, hot Smackover reservoirs are related to thermochemical sulphate reduction that has driven early destruction of methane. Compositionally distinct crude oils from Upper Cretaceous and Lower Tertiary siliciclastic source rocks are found in reservoirs south of the Gulf Rim.