MODELING RESERVOIR HETEROGENEITY IN THE CUT BANK OIL FIELD, MONTANA

Potential for additional production exists in many mature oil reservoirs (stripper well fields) that have undrained compartments owing to reservoir heterogeneities. At the South Central Cut Bank Sand Unit, primary production and waterflood projects have resulted in production of approximately 30 million bbls of the original 150 million bbls of oil in place in complex, heterogeneous reservoirs. The main producing zone is the Lower Cretaceous Cut Bank Sandstone reservoir that is interpreted as a braided-to-meandering fluvial deposit. The channel-fill sands are stratigraphic traps. The key challenge in this reservoir analysis was to predict the vertical and spatial distribution of petrophysical properties to improve reservoir characterization and oil recovery. To define and map reservoir properties, we integrated well log, seismic, and production data to identify reservoir heterogeneities responsible for intra-reservoir entrapment and the presence of the bypassed oil. We used seismic-guided log-property mapping techniques to model the heterogeneous porosity distribution. Next, we integrated geologic, engineering, and production data to identify heterogeneities and their causes. This approach involved several specific steps, including (1) high-resolution mapping of sandstone reservoir architecture using various lithofacies maps, (2) study of diagenetic modifications of the Cut Bank Sandstone, (3) investigation of the reservoir fluid flow trends, and finally, (4) integration of fluid flow-trends with reservoir architecture. This research improved our understanding of geologic controls on the reservoir performance. It should prolong the life and increase the cumulative oil recovery from Cut Bank Field. Moreover, the technical approach should be transferable and lead to increased recovery from other stripper well fields.