Tensleep Reservoir Study, Oregon Basin Field, Wyoming - Reservoir Characteristics
Authors: Morgan J.T. , Cordiner F.S. , Livingston A.R. 
Published: December 2007

Engineers and geologists worked together to define the Tensleep reservoir. Often previously considered a relatively uniform, thick sand, the Tensleep was subdivided into zones that perform as discrete reservoirs. Maps of these zones reveal complex reservoir geometries.

Introduction
This is the first of two papers that describe a study of the Tensleep reservoir, Oregon Basin Field, Wyo. The study was performed as a joint project between geologists and engineers from performed as a joint project between geologists and engineers from Marathon Oil Co.'s Production Technical Service Dept. at Littleton. Colo., and from production offices in Cody and Casper, Wyo. This paper describes the Tensleep reservoir and its characteristics, paper describes the Tensleep reservoir and its characteristics, continuity, and distribution. The second paper reports how this study served as a working tool to guide engineering planning and operations for development of a portion of the field. The Oregon Basin field (Fig. 1), located in Park County on the west side of the Big Horn Basin in northwestern Wyoming, is about 9 miles long and is composed of a North and a South Dome. Major production has been obtained from the Pennsylvanian Tensleep at Oregon Basin and the several fields identified in Fig. 1. The regionally widespread Tensleep, which may be several hundred feet thick, is transitional with varicolored shales, carbonates, and anhydrites of the underlying Amsden formation. It is separated from the overlying Phosphoria formation by a significant erosional unconformity. At Oregon Basin field, the Tensleep reservoir is presumed to have been deposited primarily in a marine and, perhaps, presumed to have been deposited primarily in a marine and, perhaps, partly in an eolian environment. Others have interpreted the partly in an eolian environment. Others have interpreted the Tensleep to have been deposited in a marine or shallow-marine and coastal dune environment. The North and South Domes of the Oregon Basin field are large, faulted anticlines having nearly 1,000 ft of closure (Fig. 2). Faults subdivide the domes into several productive areas. At South Dome. the Tensleep occurs at an average depth of about 3,840 ft and is productive over nearly 900 ft of elevation. At North Dome, it occurs at an average depth of about 3,520 ft and is productive over 700 ft of elevation. By early 1976, more than 122 million bbl of Tensleep oil had been produced from the Oregon Basin field s trice the Tensleep reservoir produced from the Oregon Basin field s trice the Tensleep reservoir was discovered in 1927. Production from the two domes. Tensleep productive areas of which are separated by a saddle, has amounted productive areas of which are separated by a saddle, has amounted to 71.1 million bbl from the North Dome and 51.8 million bbl from the South Dome.

General Characteristics
During the 50 years that oil has been produced from the Tensleep reservoir at Oregon Basin field, there has been a general inclination to consider the Tensleep a relatively uniform, thick reservoir sand. However, a combination of original depositional characteristics and post depositional cementation has caused the Tensleep to become a highly layered reservoir. Nonreservoir dolomite layers. and dolomite- and anhydrite-cemented layers, effectively, separate the Tensleep into zones that perform as discrete reservoirs, rather than as one thick, uniform reservoir sand. Such layering causes the flow of any injected or produced fluid to be mostly horizontal, and almost exclusively confined to a single zone. These zones are sufficiently separated so that little or no crossflow occurs. Individual zones may have their own secondary gas caps. Sandstone distribution also has been affected strongly by post-Tensleep erosion. post-Tensleep erosion. JPT
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