The Occurrence and Meaning of Unsaturated Hydrocarbons in Gulf Coast (US) Oils

Joseph A. Curiale

Reservoirs in the Tertiary section of the Gulf Coast of the United States contain oils, condensates and gases that have been subjected to distinctive physical and chemical processes. These processes have affected compositional and isotopic characteristics of the fluids. This study discusses the processes of (a) original source control, (b) migration-fractionation, and (c) migration-contamination, with emphasis on the ability of migrating petroleums to extract biological markers from the surrounding section.

Numerous Tertiary-reservoired oils of the Gulf Coast contain unsaturated biomarkers, including diasterenes, oleanenes and hopenes. Molecular maturity ratios for these oils are unusually low. For example, 20S/(20S+20R)-5^agr(H), 14^agr(H), 175^agr(H)-24-ethylcholestane ratios are often less than 0.25 -- values as low as 0.05 have been determined in oils with high (relative) concentrations of diasterenes. These values suggest the occurrence of either migration- or post-migration-contamination. Such natural 'contamination' overprints the effects of migration-fractionation which, for this data set, modifies n-C₁₅₊ n-alkane content, pristane/phytane ratios, ^dgr¹³c values, sulfur and nitrogen concentrations, and S/N ratios.

The effects of the migration-contamination and migration-fractionation processes serve to mask the original source signal in Gulf coast oils. Reservoir geochemical studies involving single producing fields (and possibly single-source input) can assist in isolating the compositional and isotopic effects associated with these processes. In this manner, the source signals for a single field may be deduced, and predictions of source rock character can be made.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995