Abstract: Regional Implications of Oil Entrapment at Cottonwood Creek, Wyoming

Wilson H. Herrod

Cottonwood Creek field is in the southeastern Big Horn basin, Washakie County, Wyoming, approximately 15 mi (24 km) east of the town of Worland. Production is primarily from the Ervay Carbonate Member of the Permian Goose Egg Formation. Original oil in place was approximately 182 × 10⁶ bbl. Cumulative production through 1974 was more than 42 × 10⁶ bbl, and estimated reserves, based on production-decline rates, are about 4.5 × 10⁶ bbl. The field was discovered in 1953, the first major discovery in Wyoming drilled specifically as a stratigraphic-trap prospect. It is on a southwest-dipping monoclinal surface of the southwest flank of the Hidden Dome anticline. No structural closure is evident in the area, and a remarkable 6,000 ft (1,828 m) oil column is present.

A recent investigation of mechanisms of petroleum entrapment at Cottonwood Creek clarifies and amplifies previously published ideas concerning this field, and provides regional insight in locating similar traps elsewhere in Wyoming and Colorado.

Previous studies popularized the concept of an updip facies change from thin carbonate rock to red supratidal siltstones, shales, and evaporites as the trapping mechanism at Cottonwood Creek. Reservoir development has been related most frequently to laminoid fenestral porosity associated with the sediment-binding activities of blue-green algae. The great length of the oil column has led some to hypothesize a downdip component of hydrodynamic flow as a supplemental barrier to updip petroleum migration.

Detailed studies of numerous cores from the Cottonwood Creek area and satellite fields, as well as surface sections north, east, and south of the area, permit the following conclusions. (1) The Goose Egg Formation has many features indicative of peritidal sedimentation in a sabkha type of depositional setting, analogous to the modern Persian Gulf. Such features include ripple marks (bimodal and flat topped), numerous desiccation and solution features, bedded and nodular evaporites, and intraformational breccias. (2) Porosity near the updip limit of Cottonwood Creek is of the laminoid fenestral type related to algal-mat proliferation in the intertidal zone. (3) Downdip porosity is related to subtidal facies including winnowed pellet packstones and grainstones and moldic fossiliferous p ckstones. (4) Diagenesis has left a heavy overprint on most sediments including dolomitization, solution features, and fracturing, which have enhanced reservoir development, and interstitial and pore-filling evaporite precipitation which has diminished it. (5) The basic trapping mechanism at Cottonwood Creek is the updip transition from porous intertidal carbonate rocks to dense, nonporous carbonate mudstones of the supratidal zone. Hydrodynamic influences are insignificant. (6) Similar stratigraphic traps are to be expected along depositional strike within the Goose Egg Formation where intertidal carbonate sedimentation could develop.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado