Abstract: A Stratigraphic and Sedimentologic Analysis of a Lower Atoka Sandstone, Frontal Ouachita Thrustbelt, Western Arkansas
Debra E. Vader
Prolific lower Atoka (Pennsylvanian) gas reservoirs have been exploited in the frontal Ouachita thrustbelt of eastern Oklahoma. Although the thrustbelt extends into western Arkansas, Pennsylvanian reservoirs have not been recognized. A lower Atoka sandstone unit, exposed behind the Ti Valley thrust fault, exhibits characteristics that indicate favorable reservoir quality. The outcrop is a succession of very fine to medium grained litharenite. Three zones are distinguished by grain size, mineral composition and sedimentary structures. The stratigraphically lowest zone is very fine grained and is composed mostly of monocrystalline quartz with a clay matrix. Sedimentary structures include scours, horizontal burrows and horizontal laminations. Porosity and permeability are no existent in this zone. Alternating sets of horizontally laminated fine to medium grained sandstone and bioclastic sandstone characterize the second zone. Secondary porosity has developed due to dissolution of skeletal grains. The stratigraphically highest zone exhibits porosity and permeability comparable to reservoirs within the Arkoma basin. Porosity values range from 6% to 24% with permeability as high as 248 millidarcies. The zone is a massively bedded, fine to medium grained sandstone. Monocrystalline and polycrystalline quartz are the dominant mineral components along with minor amounts of skeletal fragments. Porosity is secondary and has developed where pervasive chlorite grain coatings preclude silica cementation. The primary sedimentary structure is hummocky cross stratification implying deposition on a storm dominated shelf. The outcrop is significant in that it suggests: (1) the Arkoma "shelf" extended south of the present Ouachita thrustbelt trend and (2) reservoir quality rocks of Atokan age may be present in the subsurface behind the major thrust faults of the trend in western Arkansas.
AAPG Search and Discovery Article #90957©1995 AAPG Mid-Continent Section Meeting, Tulsa, Oklahoma