Cottonwood Creek field is on the east side of the Big Horn basin in northwestern Wyoming. It is on a west-southwest-dipping monoclinal surface on the southwest flank of the Hidden Dome anticline, with about 5,500 ft of structural relief through the productive interval. This field is an excellent example of a stratigraphic trap resulting from an updip facies change. The field was discovered in 1953 and has produced more than 22 million bbl of oil from 14,200 productive acres. Most of the field development was complete by 1958.

The clastic reservoir zones interfinger with a red-shale-anhydrite facies on the north, east, and southeast, which formed a stratigraphic trap for hydrocarbons where the impervious strata of the redbed sequence grades into the carbonate facies. Within the productive area outlined by the Cottonwood Creek field, shoaling and bioclastic thickening in the upper Phosphoria occurred together, with a concomitant porosity increase within what generally is a fine-grained dolomite facies. The producing interval contains oölites, lithoclasts, pellets, and residual fossil bioclasts. Vugs are numerous; some appear to be fossil molds.

In 1958, a gas-injection program was begun to maintain reservoir pressure and increase ultimate oil recovery. This program resulted in rapid movement of gas to producing wells and decline in oil-production rates. Water-injection programs were begun in 1959 and 1962, and resulted in the channeling of injection water to producing wells and rapid decrease in oil production. To account for the reservoir performance, all available geological, engineering, and production data were reviewed. This study indicated that reservoir performance is dependent on both the primary (matrix) rock characteristics and a superimposed fracture system. The fracture system was the primary reason for poor injection performance.

The geologic concept of the reservoir was found to correlate well with field performance and resulted in a rational explanation of the poor secondary-recovery performance. The results obtained from this study are being used to change field-operating practices extensively to improve reservoir performance. Initial results have been quite favorable.

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