Atoka Group (Lower-Middle Pennsylvanian), Northern Fort Worth Basin, Texas: Terrigenous Depositional Systems, Diagenesis, and Reservoir Distribution and Quality

The Fort Worth Basin, in North-Central Texas, is a late Paleozoic foreland basin that was downwarped during the Early to Middle Pennsylvanian Period in response to tectonic stresses that also produced the Ouachita Thrust Belt. The Atoka Group was deposited during the initial westward progradation of chert-rich terrigenous clastics derived both from the Ouachita Thrust Belt and locally from the Muenster Arch across the northern part of the basin. At the northern end of the basin, the Atoka Group interfingers with arkosic conglomerates (granite wash) derived from the Red River-Electra Arch. The granite wash is time equivalent but constitutes a separate stratigraphic sequence. The Atoka Group contains three distinct packages of terrigenous deposits: (1) the lower Atoka lithogenetic unit, interpreted to be a fluvially dominated fan-delta system, (2) the upper Atoka "Davis" lithogenetic subunit, interpreted to be a system of coalesced wave-dominated deltas, and (3) the upper Atoka "post-Davis" lithogenetic subunit, interpreted to be a thin, poorly integrated, fluvially dominated fan-delta system. Atoka Group sandstones are quartz-rich feldspathic (chert) litharenites. The most significant diagenetic events were silica dissolution and cementation. Net porosities of 10 to 15 percent are the result of the preservation of original porosity in between quartz overgrowths and the creation of secondary porosity by chert grain dissolution. Highest porosities occur in channel-fill and coarse-grained fan-delta plain facies. The Atoka Group has a cumulative production history of more than 160 million barrels (oil plus gas equivalent). Production and reservoir distribution and quality are facies controlled. Most oil and gas fields coincide with the distribution of lower Atoka fan-delta lobe complexes. Minor production is located along the axes of upper Atoka "post-Davis" fan-delta complexes.