The Regional Patterns of Hydrocarbon Alteration in the Gulf of Mexico Area

RAFALSKA, JANINA K., PAUL A. COMET, MAHLON C. KENNICUTT II, and JAMES M. BROOKS, Texas A&M University, College Station, TX

More than 600 oils from Texas, Alabama, Louisiana, Mississippi, Florida, and adjacent offshore areas were analyzed for gasoline and C10+ compounds. The following parameters were evaluated geographically: heptane and isoheptane values (paraffinicity indices), toluene/n-heptane (aromaticity index) and n-heptadecane/pristane. Isoheptane value is defined as the ratio of methylhexanes to dimethylcyclopentanes; heptane value is defined as the percentage of n-heptane in the chromatographic suite between, and including, cyclohexane through methylcyclohexane.

Regional patterns of nonthermal alterations, especially evaporative fractionation and biodegradation, are revealed by contouring gasoline and n-C(17)/i-C(18) ratios. The tectonic style of the area exerts primary control on the dominant alteration processes. Tertiary reservoirs located in tectonically active regions contain extensively altered oils. The primary alteration process is evaporative fractionation. In contrast, Mesozoic reservoirs located in the more stable northern area of the Gulf province, contain mostly intact, unfractionated fluids. Similar patterns were observed for all biodegradation-affected indices that were evaluated [i.e., heptane and n-C(17)/i-C(18) values].

The maturity trends defined by heptane and isoheptane values form coherent groupings that mimic the regional geologic structure. Thermal maturity is consistently higher when determined from isoheptane ratios than from heptane ratios. The heptane ratio, which is more sensitive to biodegradation then the isoheptane ratio, often spuriously indicates suppressed maturity levels due to alteration.

Nevertheless, maturity levels defined by gasoline-range compositions appear to be higher than those defined by higher molecular weight biomarkers (i.e., steranes, triterpanes). A possible explanation is that hydrocarbon fluids in the Gulf Coast are often a combination of fractions derived from organic matter at different stages of thermal evolution, implying progressive expulsion of hydrocarbons of increasing maturity from one or more sources, mixing, and multiple episodes of reservoir filling.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)