Summary
A post-development study was carried out at Bullwinkle Field in the Gulf of Mexico to develop and test geochemical methods for evaluating reservoir architecture, establish confidence in reservoir geochemistry technology, and provide a framework for surveillance work at Bullwinkle. In line with previous work, this study shows how fluid composition data can be used to obtain improved reservoir architecture interpretations. Geochemistry is a low-cost direct approach for determining reservoir architecture. When data are integrated with results from other tools and techniques, the final result is to improve decision making and production efficiency for both development and redevelopment projects.
Introduction
Throughout the life of an oil or gas field—from the time the count and placement of the first development wells are planned, to the last water-flood or sidetract/recompletion—a critical operational issue is the continuity and compartmentalization of reservoir units. Over the last 9 years, a number of papers have demonstrated that a variety of geochemical tools can be used to better define and constrain reservoir architecture interpretations,1–4 especially when resulting data are integrated with other geological, geophysical, and engineering information.1,5-7
The basic tenant applied in petroleum fingerprinting studies is deceptively simple—fluids in single reservoirs are compositionally homogeneous, while fluids in separate compartments show some degree of compositional heterogeneity. Differences are more important than similarities—much like pressure data, compositional homogeneity supports the idea of a continuous well-connected reservoir, although it usually does not prove it.5 Interpretations are complicated by a number of issues, including compositional grading, reservoir filling history, and insufficient time for mixing, but many published field studies support this general model.1-4,6,7
Geochemical fingerprinting technology has a number of advantages: as opposed to many static tools and methods, geochemical fluid data can be a direct indicator of fluid-flow barriers, much like PVT and production data,6,7,12 geochemical tools/methods are inexpensive, and most provide fast turn-around times, and the ability to work with core samples6–8 provides the opportunity for greater coverage, especially in areas with multiple stacked pay such as the Gulf of Mexico. Despite the applicability and promise of this new technology, its true value is not widely recognized in the industry. Consequently, it is not applied as much as it could be in appraisal, development, and re-development projects. An important reason for this lack of recognition/use is that a variety of issues and problems need to be addressed to increase our level of understanding about this technology. We need to know when it works, when it does not and why. Only then will this technology gain credibility and be used to its full potential.
To help alleviate this situation, a geochemical study of the Bullwinkle Field was carried out to: identify the optimum analytical techniques and interpretation methods for application studies in the deepwater Gulf of Mexico; understand how to interpret fingerprinting data in the presence of compositional grading; increase Shell's confidence in reservoir geochemistry technology through a series of well-calibrated case studies; and participate in surveillance work aimed at planning subsequent field redevelopment work for the field.
Study Area and Methods
Bullwinkle field, in Green Canyon blocks 65 and 109 (Fig. 1), is considered a combination stratigraphic/salt-flank trap.9,10 The lower Pleistocene "J" sand package, which contains over 90% of the reserves at Bullwinkle, was mapped originally as four separate pay intervals (J1 through J4).9 Of these, the J2 sand (Fig. 2) contains the bulk of the recoverable reserves and has an estimated column height of about 1600 ft. Partly because of its size and importance, the J2 pay zone was the primary focus of this study. As discussed in this paper, the J2 pay interval is divided into the J2-RA and J2-RB reservoirs (Fig. 2). A comprehensive set of oils (n=18) and associated (dissolved) gas (n=16) samples, taken from nearly all of the initial oil producing wells, were analyzed in this study.
