Joint Meeting: University of Wyoming Department of Geology and Geophysics Wyoming Geological Association Geological Survey of Wyoming: April 2-4, 1982 Laramie, Wyoming: Subsurface Practices in Geology and Geophysics Abstracts of Papers - Compiled by James R. Steidtmann

Cottonwood Creek oil field is formed by a carbonate stratigraphic trap which occurs in Washakie County, southeastern Bighorn Basin, north-central Wyoming. Production is from the Ervay Member of the Permian Goose Egg Formation.

The Goose Egg Formation is intrepreted as representing a peritidal to very shallow subtidal, arid to semi-arid, depositional environment. Evidence for this includes: (1) bedded and nodular evaporites; (2) numerous desiccation features such as mudcracks, tabular intraclasts, and flatpebble conglomerates; (3) early lithification features such as tepee structures and intraclast pavements; (4) pervasive aphano-crystalline dolomite; (5) thin-bedded carbonates within wide-spread redbeds; (6) stromatolites; (7) a sparse, impoverished fauna or salinity-tolerant species; (8) a general lack of bioturbation; and (9) ripple lamination.

Cottonwood Creek Field is a facies-controlled porosity pod contained within the peritidal carbonates of the Ervay Member. There is no structural closure involved in the entrapment; nor is hydrodynamic flow a consideration. The updip seal to the north and east of the field is provided by dense dolomitic mudstones and evaporites of the supratidal environment. Anhydrite-plugged fenestral fabrics of the intertidal zone comprise the southern boundary. Western field limits are arbitrary, and there is probably a limited continuity between Cottonwood Creek and satellite fields downdip to the west.

There are three principal lithofacies within the Ervay Member in the field area: (1) dense dolomitic mudstones, wackestones, and evaporites of the supratidal zone; (2) poorly sorted fenestral grainstones to packstones of the intertidal zone (which may be subdivided into lower intertidal rocks characterized by fine-laminoid fenestral fabric, and middle to upper intertidal rocks exhibiting irregular fenestral fabrics); and (3) bioturbated skeletal wackestones to grainstones of the subtidal environment. Fenestral fabrics are a diagenetic product related to the decay of sediment-binding, blue-green algae.

The intertidal, fenestral carbonates are the principal reservoir of the eastern and central parts of Cottonwood Creek Field. Production from the western part of the field comes from subtidal rocks. Shelter and moldic porosity associated with phylloid algae, and additional moldic and intergranular porosity in crinoid-bryozoan-molluscangrain-stones/packstones, are an important factor in reservoir development.

Two opposing diagenetic features provide the key to the productivity of the fenestral carbonates: (1) the distribution of pore-occluding anhydrite, and (2) the superimposed vertical fracture pattern. The extent of this fracture pattern is revealed by a third-order structural residual map.

The vertical sequence of the above lithofacies within cores reveals a transgressive-regressive cycle within the Ervay Member. A strong correlation between organic-rich subtidal units and high gamma-ray response provides a means for delineating the maximum extent of this transgression in the subsurface. The subtidal wedge is sandwiched by a lower (usually thinner) and upper package of intertidal rocks. The maximum width of this potentially productive fairway of fenestral rocks is approximately 15 mi (24 km). A preferred target for future exploration would be the relatively thick accumulation of coalescing intertidal zones just east of the maximum subtidal advance. The average width of this zone should be about 2.0 to 2.5 mi (3.2 to 4.0 km).