This paper describes recent results from an ongoing geochemical study of the supergiant Greater Burgan field, Kuwait. Oil occurs in a number of vertically separated reservoirs including the Cretaceous Third Burgan, Fourth Burgan, Mauddud, and Wara. The Third and Fourth Burgan sands are the most important producing reservoirs. Over 100 oils representing all major producing reservoirs have been analyzed using oil fingerprinting as the principal method, but also supported by gravity and sulfur measurements.
From a reservoir management perspective, an important feature of the field is the approximately 1,200-ft long hydrocarbon column which extends across the Burgan reservoirs. Oil compositions vary with depth in this thick oil column. For example, oil gravity varies in a nonlinear fashion from about 10 API near the oil-water contact to about 39 API at the shallowest Wara reservoir. This gravity-depth relationship makes identification of reservoir compartments solely from fluid property data difficult. Including oil geochemistry in the traditional mix of PVT and production logging data improves the understanding of compartmentalization and fluid flow in the reservoir, both in a vertical and lateral sense.
The composition of reservoir fluids is controlled by a number of geological and physical processes. We attempted to identify unique sets of geochemical parameters that were sensitive to specific oil alteration processes. One set of geochemical properties correlated strongly with gravity and is therefore related to the gravity-segregation process. A second set of parameters showed essentially no correlation with gravity or depth but established unique oil fingerprints for most of the major producing reservoirs and identified a number of different oil groups within the Burgan and Wara reservoirs. We interpret the presence of these oil groups to indicate reservoir compartments owing to laterally continuous shales and faults, which act as seals on a geologic time frame. Compositional differences between groups of oils arise from the reservoir filling process. A third set of parameters correlate with water washing and/or biodegradation processes, indicating oil alteration during production. We are investigating these parameters to determine if they can identify production-time-frame barriers. The geochemical data were integrated with PVT-data for better understanding of the fluid distribution.
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