ABSTRACT: Depositional Environments, Petrology, and Fractures of the Lower Atoka "Davis" Sandstone, A Low-Permeability Gas-Bearing Sandstone of the Fort Worth Basin, North-Central Texas
COLLINS, EDWARD W., STEPHEN E. LAUBACH, and SHIRLEY P. DUTTON, Bureau of Economic Geology, University of Texas at Austin, Austin, TX, and ROBIN E. HILL, CER Corporation, Las Vegas, NV
The gas-bearing Pennsylvanian "Davis" sandstone, also called the "Pregnant" shale, consists of several stacked coarsening-upward depositional packages that formed in wave-dominated deltaic environments. In northwestern Parker County, the Davis consists of an upper 100-ft-thick (30-m-thick) sandstone-rich unit deposited in prodelta, delta-front, and channel-mouth-bar environments. This upper unit overlies about 150 ft (45 m) of older Davis shale and siltstone deposited as prodelta and distal delta-front facies of older deltas that did not prograde as far west-northwestward as did the upper deltaic package. Cores of the upper sandstone-rich Davis indicate that it is composed of multiple shoaling-upward depositional cycles that are between 10 and 40 ft (3 and 12 m) thick and typically co sist of shale, sandstone interbedded with thin (centimeter-size) shale layers, and relatively clean sandstone. Sandstones exhibit mostly ripple cross-laminations and some planar cross-bedding.
Quartz, plagioclase, and metamorphic rock fragments are the most abundant detrital grains. Quartz is the most abundant cement, ranging from 8 to 16% of the rock volume in clean, well-sorted sandstones. Authigenic chlorite, illite, kaolinite, calcite, and ankerite also occur. Thin-section porosity ranges up to 6%; porosimeter porosity is generally less than 8%. Matrix permeability is mostly less than 0.1 md, although natural fractures may contribute to higher local permeability. Natural fractures ranging in height from a few inches to 1 ft (30 cm) occur in core from wells located in areas that lack mappable folds and faults. Fractures are most common in sandstone beds, although some cut shale layers interbedded in sandstone. Calcite commonly partly fills the fractures. Most of the natu al fractures strike north-northeastward between N1 degree W and N40 degree E, approximately parallel to the direction of current maximum horizontal stress.
Understanding the variability in depositional facies, diagenesis, and natural fracture distribution can aid in predicting zones having the best permeability and in determining appropriate production methods in this low-permeability sandstone.
AAPG Search and Discovery Article #91018©1992 AAPG Southwest Section Meeting, Midland, Texas, April 21-24, 1992 (2009)