The Oregon Basin field in northwestern Wyoming is about 9 mi (14.5 km) long and is composed of a north dome and south dome. Since its discovery in 1927, over 122 million bbl of oil have been produced from the Pennsylvanian Tensleep Sandstone at Oregon Basin. Geologists and engineers worked together to describe the reservoir and accumulate data that would aid in determining best methods for recovering oil from this 50-year-old reservoir.

Major reservoir variations are caused by erosion and the presence of nonreservoir dolomite. Layers of nonreservoir material separate the Tensleep into zones that perform as discrete reservoirs. In parts of the field, some of these zones have been completely eroded by post-Tensleep streams. Local anomalies and low-magnitude variations are caused by sedimentary structures, small-scale forms of cement, solution vugs, fracturing, and faulting.

On the basis of thicker, persistent, nonreservoir dolomite and dolomite- and anhydrite-cemented sandstone zones that can be located on the logs, the Tensleep reservoir was subdivided into zones. Maps of these individual zones reveal a complex reservoir distribution which was caused by original deposition characteristics, erosion, and cementation.

The Tensleep, which has an overall thickness of up to almost 200 ft (60 m), has had tens of feet eroded on the north dome by a southeast-flowing stream. On the south dome, erosion by a northeast-southwest-trending stream has been even more extreme, reducing gross thickness to less than 40 ft (12 m). In that area, the combination of erosion, dolomitization, and cementation by anhydrite has reduced reservoir-sandstone thickness to less than 10 ft (3 m).

Such reservoir variation was not recognized previously. The Tensleep generally had been considered as a thick, relatively uniform reservoir. However, maps of individual zones show that injection wells located on the edge of the field did not benefit all zones.

Recognition of reservoir zonation and distribution has led to modification of existing water-injection programs, and has provided the basis for planning further development. Incorporation into the overall reservoir management program of the concepts developed has resulted in a significant increase in oil production rates and will result in increased oil recovery from the Tensleep reservoir.