Cottonwood Creek Field, Washakie County, Wyoming, Carbonate Stratigraphic Trap

Abstract

The Cottonwood Creek field is perhaps the first major discovery in Wyoming drilled specifically as a stratigraphic trap prospect. At present, the field is 4 miles long and 2½ miles wide, with 5,000 acres proved productive by 28 Phosphoria wells. The southern and western limits of the field have not yet been defined. During deposition of the Phosphoria, west-central Wyoming was the site of a large arcuate embayment on a shallow platform that sloped westward into the Cordilleran miogeosyncline. Throughout most of Phosphoria time the eastern margin of marine deposition fluctuated within the area of the present Bighorn basin. The Cottonwood Creek field is near the eastern edge of the final marine transgression of the Phosphoria sea. The facies change from marine dolomite to red shale and anhydrite provides the trap for oil accumulation. Development of porosity in the dolomite at Cottonwood Creek may be due to a reef. The producing zone is oolitic, fossiliferous, stylolitic, fractured dolomite with pin-point to coarse vuggy porosity. The thickness of dolomite in the producing wells ranges from 30 to 95 feet; the net porous dolomite varies from a few feet to 74 feet. The Cottonwood Creek field is believed to be a volumetric reservoir with solution gas drive as the producing mechanism. The oil is 30.2° A.P.I. gravity with 2.7 per cent sulphur. The production for all oil wells in the field is 10,000 barrels of oil per day.