Evaluating Reservoir Architecture in the Northern Gulf of Mexico Using Oil and Gas Chemistry: Insights from a Study at Bullwinkle Field

Joseph T. Westrich, Pat O. Knigge, Anthony N. Fuex, Henry I. Halpern

A study was carried out at Bullwinkle Field in the Gulf of Mexico to 1) develop and test geochemical methods for evaluating reservoir compartmentalization, 2) establish confidence in reservoir geochemistry technology, and 3) provide a framework for surveillance work at Bullwinkle. Costs and strategies for new development and redevelopment projects are highly dependent on reservoir architecture models. Previous studies have shown how fluid geochemistry can be used to obtain better architecture interpretations, but more work is needed before this technology can be applied reliably and cost-effectively.

At Bullwinkle Field, over 90 percent of the reserves are contained in the J1-J4 reservoirs (lower Pleistocene). Petroleum samples collected over the last ten years from the J sands were analyzed using standard geochemical techniques.

Based on seismic, well, and PVT data, the northern part of the J2 reservoir is considered to be a separate accumulation (the J2-RA), even though the RA and RB reservoirs share a common oil/water contact, and pressure data indicate connectivity downdip in the aquifer. Interpretations are complicated further by the fact that significant gradients in bulk fluid properties exist in the J2-RB reservoir. Despite this complication, geochemical data support the notion of a flow barrier separating the J2-RA and J2-RB reservoirs. The petroleum samples from the J2-RB reservoir have nearly identical geochemical signatures, while the petroleum in the J2-RA reservoir has distinctly different source and maturity characteristics. Similar data indicate that the underlying J4 sand also is split into tw compartments. The applicability of these results to other Gulf of Mexico fields will be discussed.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995