Little Sand Draw field produces hydrocarbons from an asymmetric anticline located on the southwestern margin of the Big Horn basin. This 479-ac, 32-well field has produced 10.4 million bbl of oil. Most production is from the Ervay Member of the Permian Phosphoria Formation. On the basis of gamma-ray logs and textural evidence, the dolomitized, 90-100 ft (27.5-30.5 m) thick Ervay Member can be subdivided into six reservoir zones. Zones are bounded by marine-flooding surfaces, and each zone is laterally consistent throughout the field. Lower Ervay zones, which were deposited under low-energy open-marine conditions, are predominantly mud-supported rocks. Higher energy, yet more restricted marine conditions occurred during deposition of the upper Ervay and resulted in a domin nce of grain-supported rocks.

Moldic and micro-intercrystalline porosity occurs in four zones. Moldic pores dominate total porosity and have significant hydrocarbon storage capacity. Intercrystalline pores control matrix permeability, which is generally less than 20 md. Such permeabilities cannot account for observed production rates of several hundreds to thousands of barrels of fluid per day. Open fractures are present in all zones and contribute significantly to reservoir drainage. Fracture intensity is controlled by lithology and structural position. Silicified strata are the most highly fractured. Structurally, wells with the greatest fracture intensity are aligned along the northwest-trending hingeline of the fold. However, in much of the reservoir, flow behavior is dominated by east-northeast-trending fract res which are recognized on borehole-imaging logs and as directional permeabilities from pressure-interference tests.

Most, but not all, wells are at or near virgin reservoir pressure. This, along with high water-production rates, suggests strong aquifer support for reservoir pressure. A comparison of pressure and water-cut data shows that those wells with depleted reservoir pressure also had water cuts that rose relatively slowly. Geographically, these wells define dual-porosity "compartments" in which matrix porosities and permeabilities, as well as fractures, affect flow. Elsewhere in the field, fractures are the dominant control on flow in what appears to be single-porosity behavior.