AN ORGANIC PROVENANCE ANALYSIS OF LATE PENNSYLVANIAN BLACK SHALES FROM THE MIDLAND BASIN

An organic matter provenance study was performed on samples of late Pennsylvanian black shales and carbonates from the Midland Basin (west Texas). These shales are being explored for unconventional petroleum and they contain a rich archive of paleoceanographic information. The continental arrangement during the Late Pennsylvanian created an icehouse climate, resulting in high frequency, high amplitude changes in sea level. West Texas was situated in the tropics at this time, and monsoon circulation patterns likely affected the region. The Midland Basin was also proximal to Panthalassa, and a connection between the two is thought have influenced water chemistry and depositional processes.

The core samples used in this study came from different locations within the Midland Basin. One core was collected from Midland County, near the basin center, whereas a second core was retrieved from Martin County, along the basin’s northern axis. The third core, located near the southern boundary of the basin, was extracted from Upton County. The final core in the dataset was from Irion County. Total carbon and percent carbonate were determined using a LECO C/S analyzer, and a total carbon coulometer. Total nitrogen, total organic carbon (TOC), ¹³C/¹²C and ¹⁵N/¹⁴N were measured on bulk organic matter using an isotope ratio mass spectrometer coupled to an elemental analyzer. A petrographic reflecting light microscope was used to complete maceral point counts.

The TOC content of the shales is variable but often exceeds 2.0 wt. %, which indicates petroleum source rock and unconventional reservoir potential. The provenance of the organic matter appears to derive from both marine and terrestrial sources. Terrestrial macerals (vitrinite and inertinite) were most abundant in the basin’s northern axis, which may reflect proximity to the shoreline and the influence of seasonal runoff. Evidence for a stronger open marine signal is apparent in samples from the basin’s central depocenter, and includes low C/N values and high amorphinite and bituminite content. Liptinite abundance in the Upton and Irion County samples also signal a marine depositional environment. Future work will focus on integrating stratigraphic and inorganic geochemical data with these results to generate a holistic paleoenvironmental interpretation for the basin.