RESERVOIR GEOCHEMISTRY OF SOUTH PASS-61 FIELD, GULF-OF-MEXICO - COMPOSITIONAL HETEROGENEITIES REFLECTING FILLING HISTORY AND BIODEGRADATION

Cluster analysis of Ge data for gasoline and mid-range hydrocarbon rat ios from fifty-one South Pass 61 Field oils reveals geochemically simi lar oil clusters corresponding to geographically coherent groups. Insi ght into the groupings is obtained from examination of indicators of g eochemical processes, e.g., anaerobic biodegradation, aerobic biodegra dation and extensive remigration of light ends. Six duplicate samples collected in 1986 and 1992 as well as replicate runs on a single sampl e showed excellent analytical reproducibility. Subtle but consistent d ifferences in both gasoline and oil-range hydrocarbon maturity indicat ors are observed between the east, west, and far west flanks of the fi eld, suggesting filling of different segments of the reservoir from di fferent kitchens at slightly different stages of thermal maturity or w ith slightly different chemical character. The west flank of the salt dome was charged with slightly more thermally mature petroleum than th e east flank. The stratigraphically oldest and deepest sand on the far west flank has received the most mature petroleum. Different fluid co ntacts and GORs are observed in different sands and different fault bl ocks. The stacked pay geometry of the field and widespread faulting ha ve led to considerable remigration of gas and condensate as observed i n other fields in the Gulf of Mexico (Thompson, 1987). Biodegradation varies in severity with depth and reservoir zone, but is frequently ov erprinted by remigration of light end hydrocarbons. Anaerobic biodegra dation by sulfate-reducing bacteria is probably the cause of light to moderate alteration in intermediate depth Pliocene sands which are hyd raulically connected to the salt dome (with dissolution of anhydrite f rom the salt dome providing the sulfate). Widespread late pyrite forma tion in reservoir sandstones is inferred to represent the ultimate sin k for reduced sulfur from sulfate reduction during oil biodegradation. Co-produced water compositions suggest no oxygenated freshwater infus ion. Copyright (C) 1996. Published by Elsevier Science Ltd.