Biomarkers and carbon isotopes of oils in the Jurassic Smackover Trend of the Gulf Coast States, U.S.A.

Abstract

The geochemistry of 25 oils from 23 fields in Alabama and the Mississippi and Louisiana-Arkansas Jurassic salt basins was evaluated. Results show that the oils were generated by a carbonate source rock which was deposited under highly anoxic conditions, and which contains mainly marine derived organic matter. The Mississippi and Louisiana-Arkansas oils are geochemically similar, indicating similar depositional environments for the source. Although the Alabama oils were also derived from a carbonate source rock, they are dissimilar to the Mississippi and Louisiana-Arkansas oils. Terpane biomarkers suggest that in addition to marine derived organic matter, the source for the Alabama oils had an organic input from a more near shore (paralic) environment, i.e. with a component of terrestrially-derived kerogen. Within each area the oils are similar. Therefore, the Norphlet and Upper Smackover oils in Alabama share a common source and the Upper Smackover, Cotton Valley and some of the Lower Tuscaloosa oils (where production is from faulted structures) in Mississippi also share a common source. Maturities of the oils in the three areas vary from low in the updip Mississippi salt basin, high in the Louisiana-Arkansas salt basin, to very high in portions of Alabama. Based on the maturity of oils in Mississippi, oil generation and migration commenced during the Cretaceous when the source was at modest levels of thermal maturity. Oils migrated relatively short distances into nearby reservoir rocks. Some oils reached high maturities in the reservoirs, resulting in abundant late-forming bitumen and pyrobitumen deposition in pore spaces.