Some Apparent Effects of Vertical Migration on Physical and Chemical Properties of Crude Oils Produced in Offshore Gulf of Mexico

Robert K. Olson

The chemical and physical properties of crude oils reflect not only genetic characteristics inherited from their sources but also the cumulative effects of secondary alteration processes that occur during migration and in the reservoir. Four types of secondary alteration are believed to have affected Gulf of Mexico oils: biodegradation, mixing, light-end loss, and heavy-end loss.

Of the four processes, biodegradation has been discussed most often in the literature. Biodegradation causes selective and sequential loss of n-alkanes, branched compounds, and cyclic compounds; decreases API gravity; and increases both sulfur content and optical activity. Mixing of oils simply produces the average of the properties of the individual oils. Light-end loss and heavy-end loss describe the apparent loss of low or high boiling-point fractions of crude oils. With light-end loss, the residual heavy fraction becomes enriched in sulfur, optically active compounds, and biomarkers. The opposite is true for heavy-end loss. Carbon isotope ratios do not appear to change significantly with either process.

In 1965, Silverman suggested that moving crude oils from a high-pressure single-phase regime to a lower pressure two-phase regime results in differential partitioning between the two phases. If one phase were preferentially lost, the composition of the remaining fluid could be significantly altered. The depositional setting and structural style of Gulf Coast Tertiary rocks are ideally suited for fractionating hydrocarbons according to Silverman's model.

Data gathered on oils from several fields in the Gulf of Mexico suggests that the four processes of biodegradation, mixing, light-end loss, and heavy-end loss produce nearly as great a variation in the chemistry of Gulf of Mexico oils as occurs between unaltered oils from different fields. We must fully understand how these alteration processes effect the chemistries of oils before we are able to establish a meaningful genetic classification.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.