HETEROGENEITY IN A “SHALE” FORMATION: LITHOLOGIES IN THE PERMIAN WOLFCAMP SHALE OF THE DELAWARE BASIN, WEST TEXAS

Significant heterogeneity in rock lithologies exists within the Permian Wolfcamp Shale in the subsurface Delaware Basin of West Texas. Detailed XRF measurements on several cores guided sampling of the various lithologies. Petrographic examination, including scanning electron microscope (SEM) study, was used to define and illustrate the variations present in a number of samples. Different lithologies have varying pore systems and differing rock properties that affect hydrocarbon production.

Lithological abundance varies depending on stratigraphic interval. However, siliceous-argillaceous and argillaceous-siliceous mudstones generally make up the majority of the section. Much of the organic matter is in these two lithologies. These mudstones both contain radiolarians recrystallized and replaced by quartz and chlorite. Less abundant calcareous mudstones show textural evidence (calcitized or dolomitized radiolarians) of being diagenetically altered from an originally more siliceous lithology. Debrites containing shelf-derived carbonate clasts up to cm-scale are also common and occur in beds ranging in thickness from centimeters to meters. The coarse carbonate clasts in these debrites float in a matrix with variable compositions including siliceous mudstone, calcareous mudstone, and very fine quartz-rich sandstone. A range of carbonate beds are also present, from variably siliceous dolomitic siltstones (possible fine-grain debris flows) to lime mudstones. Sandstone layers are uncommon and generally confined to the upper part of the Wolfcamp section. Sandstones are typically fine-grained, are subarkosic compositions and contain minor dolomite grains. Chert, thin volcanic ashes, and phosphate concretions and layers make up very minor amounts of the Wolfcamp section.

The above described lithologic heterogeneity results from a complex geologic history and contributes to the Wolfcamp having variable physical properties and a complicated fluid flow system.