ABSTRACT

State Line field, Union County, Arkansas, produces oil from a five-well stratigraphic trap at 8,900 ft. Conventional cores were cut in all wells. Core studies show that the trapping porosity pinch-out is a facies change from lower foreshore to ooid beach. Sedimentation occurred along a high-energy coastline. Thus, the depositional setting at State Line field differs from the commonly accepted "oolite bar" model used for many other fields in the trend.

Four facies were delineated: (1) siliciclastic lagoon (Buckner Formation), (2) ooid beach, (3) oncoid-ooid lower foreshore, and (4) patch reef. Intergranular porosity is facies selective, found mainly in the poorly sorted lower foreshore facies. Cross-stratification and the absence of lime mud indicate high-energy conditions. Porosity and permeability in the lower foreshore facies average 10.9 percent and 496 md, respectively. The ooid beach facies is characterized by well-sorted, crossbedded, and massive ooid grainstones that tend to be extensively calcite cemented. Porosity and permeability values are generally below 2 percent and 1 md, respectively, although they can be higher adjacent to porous lower foreshore strata. The top of the Smackover is a transition from high-energy, sandy ooid beach (grain-stone) to low-permeability, lagoonal siliciclastics, which seal the reservoir. Depositional features suggesting tidal channels at the east and west ends of the field support a beach and/or barrier island interpretation. Coral-algal boundstones of the patch reef facies are thin, local, and not of reservoir quality.

The value of predicting reservoir trends from cores is shown by a successful 400-ft sidetrack away from a borehole with no reservoir facies or oil shows. A slabbed "piece of the rock" can pay off in Smackover development.

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