Migration of petroleum into Vermilion 14 field, Gulf Coast, U.S.A.—molecular evidence

Oils and condensates from Vermilion 14 field, offshore Louisiana, U.S.A., are compared using sulfur and nitrogen concentrations, whole oil carbon isotope ratios and hydrocarbon distributions. All parameters suggest large differences in the organic facies of the source rock(s) and in thermal maturity of the source at the time of expulsion. Certain bulk geochemical data, including n-alkane distributions and whole oil carbon isotope ratios, suggest that migration-fractionation resulted in oils/condensates whose molecular distributions are shifted significantly toward lower molecular-weight components. Migration-contamination is supported by the occurrence in the oils of unsaturated and thermally unstable hydrocarbons, including oleanenes, dammarenes and diasterenes, and molecular maturity ratios indicative of immature organic matter. Molecular maturity ratios of the oils increase with increasing reservoir depth, regardless of reservoir lithology or formation. For example, the 20S/(20S + 20R)-5α(H),14α(H),17α(H)-24-ethylcholestane ratio increases from 0.05 to 0.58, over a depth range of 6316 ft to 15,563 ft. The same ratio for cuttings extracts from two wells in the Vermilion 14 field shows lower values, although they also increase with increasing depth. It is proposed that the Vermilion hydrocarbons were generated from deep source rocks (approximately 15,000 ft or more) and possibly subjected to migration-fractionation during movement to shallower reservoirs. During migration and after trapping, migration-contamination added unsaturated hydrocarbons to the oils/condensates, resulting in anomalously low molecular maturity ratios. We suggest further that the combined effects of migration-fractionation and migration-contamination resulted in oil in the Vermilion 14 field that is compositionally different from that which initially left the deep source rock.