Abstract

West Smyer field, Hockley County, Texas, produces from the upper part of the Lower Clear Fork (Leonardian) in the northern part of the Midland basin. The field produces from a resedimented dolomitized grainstone at the base of the slope separating the Northwestern shelf from the deeper parts Midland basin. Since its discovery in 1984 the field has produced over 350,000 barrels of oil from just two wells. Shelf facies such as reefs or oolite shoals have been the primary exploration targets in the northern Midland basin. Areas of basinal facies remain relatively unexplored, but resedimented grainstones along the base of the slope are proven reservoirs in West Smyer and Levelland NE fields and potential reservoirs elsewhere.

Laminated lime mudstones representing normal basinal sedimentation and fossiliferous grainstones composed of shallow water sediments carried into the basin by turbidity currents alternate in core from West Smyer field. Fossils in the grainstones underwent micritization, silicification, and glauconitization while still residing on the shelf and were then resedimented into deeper water. During burial the grainstones were cemented by calcite and then partially dolomitized. Next the remaining calcite was dissolved creating intraparticle, interparticle and moidic pores. These secondary pores were then partially occluded by dolomite followed in turn by pyrite and anhydrite, and later filled by hydrocarbons.