Abstract: Jurassic Beach: A Depositional Facies Model for Smackover Stratigraphic Traps in the Ark La Tex
Tim Brown, Lawrence Bruno, Mike Green
State Line field, Union County, Arkansas, produces oil from a five-well stratigraphic trap at 8900 ft. Conventional cores were cut in all wells. Core studies show 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 main 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% and 496 md, respectively. The ooid beach facies is characterized by well-sorted, cross-bedded, and massive ooid grainstones that tend to be extensively calcite cemented. Porosity and permeability values are generally below 2% 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 b ach (grainstone) 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.
AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994