GEOCHEMISTRY OF THE WOLFCAMP AND LOWER LEONARD FORMATIONS IN THE MIDLAND BASIN OF WEST TEXAS

The southern Midland Basin, a sub-basin of the larger Permian Basin, is host to a number of hydrocarbon-bearing horizons within the “Wolfberry” play, including the Leonard and Wolfcamp formations. These formations are of particular interest for their ability to produce both natural gas and oil as well as for their exceptional thickness. The Leonard and Wolfcamp have high reported TOC values, as much as 6.3 wt%, and are generally comprised of siliceous and calcareous mudstones.

Here, we present a geochemical analysis of two cores from northern Reagan County, TX, to better understand the effects of paleoredox conditions on source rock deposition and production. Located near the paleogeographic center of the basin, the Greer-1 and Greer-2 cores span the lower Leonard and Wolfcamp and the lower Wolfcamp, respectively. 745 ft. of core was sampled and analyzed for TOC, iron speciation, pyrite sulfur contents, major and minor elemental analyses, and redox-sensitive trace metal abundances.

It has been suggested that across Wolfcampian time and into the lower Leonard, conditions were trending towards a more stratified, more restricted bottom water environment, allowing for greater organic matter accumulation and preservation. Our preliminary results from Greer-2 confirm an anoxic bottom water environment. Sediments were mostly deposited in ferruginous conditions during early Wolfcampian time with Fe_HR/Fe_T > 0.38 and Fe_py/Fe_HR < 0.7. Occasionally, Fe_py/Fe_HR vales are > 0.7, indicating intermittent transitions to euxinic conditions. In addition, uranium and molybdenum enrichments indicate reducing water column conditions, consistent with previous work. Ultimately, through comparison with data from other Paleozoic shales, we aim to understand what drives changes in redox conditions and quality of source rock reservoirs.