Structural Geology of the Circle Ridge Oil Field, Fremont County, Wyoming

Abstract

Abstract The Circle Ridge Oilfield is located about 60 miles northwest of Riverton, Wyoming, in the Wind River Basin. It was discovered in 1923, and has produced over 28 MMBO from five reservoirs—the Phosphoria, Tensleep, Amsden, Darwin, and Madison. The average surface elevation is 7200 feet above sea level, and the depth of oil production ranges from 300 feet to 1500 feet below the surface. There are over 200 wells in the field. Some of the reservoirs have been on waterflood since 1979; there is an active water drive in others. Waterflood modifications resulting from the present study have so far increased production from about 1475 BOPD to over 1700 BOPD. Circle Ridge is a well expressed doubly plunging surface anticline with exposed section from the Upper Cretaceous Mowry to the Permian Phosphoria Formations. This provides a rare opportunity to integrate surface and subsurface geology in an attempt to obtain a 3D view of a Rocky Mountain foreland structure. As the only pre-existing surface geologic map of the area was published in 1944 at a scale of 1:48000, the author re-mapped the surface geology at the more detailed scale of 1:6000. Much more extensive faulting was recognized, as well as several areas of small scale accomodation folding. In addition, the presence of an erosional remnant of the Jurassic Nugget Formation was discovered. Twenty-two stratigraphic cross sections tie together the extensive well control, and three balanced structural cross sections were constructed and restored to the pre-deformation state. The southwest flank of the structure is overturned, resulting in a complex structural setting. Seismic data are nearly unusable, since the steep to vertical dips are not imaged. Five thrust faults can be recognized in the subsurface, which compartmentalize the reservoirs into six distinct blocks. These blocks are isolated for fluid communication. Understanding the subsurface structural complexity has identified numerous examples of injection into reservoir blocks with no open producers, producing wells in a block with no injectors, and even isolated blocks with reserves which have never been tapped by a producing well. Ongoing efforts to remedy these problems are resulting in increased production rates (by 15%) and reduced injection (by 75%).