Tensleep Reservoir Study, Oregon Basin Field, Wyoming - Engineering Plans for Development And Operation, South Dome
Authors: Cordiner F.S. , Livingston A.R. 
Published: December 2007

Combination of geologic and engineering expertise has contributed greatly to a better understanding of characteristics of the Tensleep reservoir. Knowledge of reservoir zonation and distribution has resulted in improved performance of the Tensleep water-injection program. Modification of performance of the Tensleep water-injection program. Modification of water-injection plans will result in significant increases in oil recovery.

Introduction
The Oregon Basin field, as described by Morgan et al. is located on the western side of the Big Horn Basin in Park County, Wyo. Oil was discovered in the Tensleep Park County, Wyo. Oil was discovered in the Tensleep reservoir on the North Dome of Oregon Basin in 1927, and on the South Dome in 1928. The field produces gas from formations of Cretaceous, Triassic, and Cambrian ages. It produces oil and associated gas from formations of Permian, Pennsylvanian, and Mississippian ages. This paper deals principally with the Tensleep reservoir of Pennsylvanian age on the South Dome of the Oregon Pennsylvanian age on the South Dome of the Oregon Basin field.
About 52 million bbl of oil have been recovered from the Tensleep reservoir at South Dome. As shown in Table 1, oil produced from the Tensleep is 21 degrees API gravity and has an average viscosity of 6.5 cp. The average depth of the reservoir is about 3,840 ft, and the productive area encompasses about 5,000 acres. Properties of the black, asphaltic crude oil vary with structure, with the entire reservoir on the South Dome being undersaturated initially.
Saturation pressures and solution GOR's varied from 1,565 psi and 254 cu ft/bbl, respectively, at the crest to as low as 380 psi and 70 cu ft/bbl at the water-oil contact. Vertical closure above the water-oil contact is about 900 ft.
Principal sources of primary energy have been both solution gas expansion and a limited edge-water drive.
Previous reservoir studies of the Tensleep reservoir on the South Dome indicated a lateral transition of sand character from a clean, porous sandstone into hard, dense dolomitic sand, or dolomite. This was in contrast to earlier beliefs that the Tensleep was relatively thick, uniform reservoir sand. The area of this transition was confined mainly to the crestal portions of the structure. Recent geologic studies, described by Morgan et al., indicated that the Tensleep reservoir is divided into discrete zones, separated by persistent correlatable layers of dolomite cemented sand, or dolomite. Fig. 1 is a typical log from a well on South Dome showing the zonation of the Tensleep. Zones A through I are present in the North Dome. On the South Dome, only Zones C through I are present. Erosion by post-Tensleep paleo-streams also has present. Erosion by post-Tensleep paleo-streams also has removed portions of Zones C, D, and EF on the South Dome. On the North Dome, erosion affected only Zones A and B.
Better knowledge of these variations in reservoir character has led to additional development and improvements in the operation of the south Tensleep reservoir. The project described here is a part of Marathon Oil Co.'s ongoing reservoir management program. Ultimately, the program will result in significant increases in oil recovery from the Tensleep reservoir.

Development History
Development of Oregon Basin field was sporadic from the time of its discovery until WW II created a ready market for the black, asphaltic oil. By 1947, 17 wells were producing Tensleep oil from the South Dome. Prior oil production was mainly from the shallower Embar formation.
JPT
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