Abstract

The gas-bearing Pennsylvanian Davis sandstone, also called the Pregnant shale, consists of several stacked upward-coarsening 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 that was 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 as 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 upward-shoaling depositional cycles that are between 10 and 40 ft (3 and 12 m) thick and that typically consist of shale, sandstone interbedded with thin (centimeter-sized) shale layers, and relatively clean sandstone. These sandstones exhibit mostly ripple cross-laminations and some planar crossbedding.

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 from 0% 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 partially fills the fractures. Most of the natural fractures strike north-northeastward between N1° and N40°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.