Reservoir Petroleum Geochemistry of Sarita (South Texas), Eugene Island 330 and South Marsh Island 128 (Offshore Louisiana) Oil Fields

RAFALSKA, JANINA K., Conoco Inc., Ponca City, OK, PAUL A COMET, MAHLON C. KENNICUTT II, THOMAS J. MCDONALD, and JAMES M. BROOKS, Geochemical and Environmental Research Group, Texas A&M University, College Station, TX

Hydrocarbon fluid compositions within three petroleum fields were shown to be heterogenous in some aspects while appearing to have a common source for each field. However biomarker parameters indicate that three different sources were respectively responsible for each field accumulation. Visual inspection of whole oil chromatograms, gasoline molecular composition, and quantitative concentrations of biomarkers were used to define the range of fluid chemistries observed. Biomarker compositions and content were nearly identical regardless of depth of production, suggesting that all reservoirs within a field were filled with a common fluid. Nonthermal alteration processes occurring after petroleum generation are primarily responsible for the observed variability in hydrocarbon fluid compo itions. These processes include biodegradation, evaporative fractionation, water washing, multiple phases of generation and mixing.

Fluids in reservoirs of the Sarita and Eugene Island 330 fields were biodegraded to varying degrees in the shallowest sand reservoirs. Hydrocarbon fluids in these two fields were also altered by evaporative fractionation, a phase segregation allowing condensate to occur shallower than oil. Alteration due to evaporative fractionation is related to secondary injection of post mature gas and subsequent remigration up faults. South Marsh Island 128 fluids are in a nearly pristine condition.

Whole-oil fingerprints and gasoline composition are specific to a given pay zone or fault block and can be used as a correlation tool. Communication between reservoirs is indicated by a common fluid chemistry, whereas fluid chemistry variations suggest compartmentalized accumulations. The scale examination of fluid chemistry within fields can help define the history of reservoir filling and changes in fluid properties during field development and production.

AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)