ABSTRACT: Fractures or Fenestrae? Some Sources of Reservoir Heterogeneity in the Ervay Member of the "Phosphoria" Formation, Cottonwood Creek Field, Eastern Big Horn Basin, Wyoming

Edward B. Coalson, Richard F. Inden

Peritidal dolomites of the Ervay Member of the Park City ("Phosphoria") Formation in the eastern Big Horn basin of Wyoming contain large (0.25-4.0 mm) fenestral pores sandwiched between microporous dolomicrite laminae. The fenestrae are well connected by microfractures and relatively large pore throats. This pore geometry helps explain why parts of the reservoir display: (1) remarkably high permeabilities (100-150 md) at low values of porosity (4-6%), (2) high rates of decline during early production, and (3) rapid breakthrough of fluids injected during secondary recovery efforts.

These characteristics could easily be attributed to natural macrofracturing of the reservoir. However, these fenestral rocks have greater total porosity and better connectivity than do purely macrofractured reservoirs, with characteristic effects on well performance, logs, and core data. We suggest that fenestrae not only contribute significantly to reservoir heterogeneity in Cottonwood Creek field, but that they are, in places, the sole cause for anomalous well behavior and the only source of reservoir permeability, with fractures playing no role. This newly perceived significance of fenestral porosity in the Ervay member has important implications for exploration and production. For instance, exploration wells should be located where they have the best chance to encounter fenestral eritidal island deposits, rather than in areas of maximum flexure. Also, secondary recovery efforts must be designed to overcome not only whatever natural fracturing is present, but also the preferential pathways for fluid flow created by connected fenestral porosity.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990