A Suggested Genetic Classification of Gulf of Mexico Offshore Oils

Mahlon C. Kennicutt, II, Keith F. M. Thompson

Petroleum fluids produced in the Gulf Coast are the result of a number of complex processes. These processes include maturation, migration effects (including fractionation, adsorptive losses and hydrocarbon contamination from carrier or reservoir strata), microbial degradation (and water washing), evaporative fractionation, and mixing. Correlation of fluids having such complex histories must be based on parameters primarily controlled by the organic matter of the source rock (i.e., genetic parameters) and those that are least altered during postgeneration modification. Carbon and sulfur isotopic ratios, as well as the relative abundances of nickel and vanadium, were determined to be particularly useful correlation parameters. Sulfur content was occasionally valuable.

Whole oil (C₁₅₊) carbon isotopic values for 357 offshore Gulf of Mexico oils range from -23.9 to -27.9 ^pmil. This range in values is larger than is typical for the products of most single source rocks. The largest proportion of Gulf oils (90.8%) have ^dgr¹³C values between -25.8 to -27.3 ^pmil, and are interpreted as being intimately related. Based on carbon isotopes, three subgroups are distinguishable from the main group: (1) isotopically heavy offshore Texas Miocene/Pliocene condensates, > -25.7 ^pmil; (2) a group of South Marsh Island oils, > -25.8 ^pmil; and (3) a group of Grand Isle oils, <= -27.2 ^pmil.

Sulfur isotope data are available for the majority of the oils and are negative in all but three cases. ^dgr³⁴S values exhibit a large range (-14.0 to 7.1). High-sulfur oils from western Florida have ^dgr³⁴S values similar to Jurassic Smackover oils and are recognized as a separate group. Further subdivisions of the major oil group can be recognized based on relative abundances of nickel and vanadium and sulfur isotopes. The vast majority of the oils are nickel-dominated. A subgroup of vanadium-dominated sulfur-rich oils is located along the slope-shelf break offshore Louisiana (Green Canyon area) and evidently includes the Challenger Knoll and IXTOC oils. The presence of high sulfur, dominant vanadium, and low pristane/phytane ratios in these oils are characteri tic of extreme anoxia. Interestingly, this subgroup is not distinguishable from the main group by means of carbon isotopes.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.