CHARACTERIZATION OF ORGANIC FACIES, PALEOGEOGRAPHY, AND THERMAL MATURITY OF EARLY LEONARDIAN AGED STRATA, MARTIN COUNTY, TEXAS

The Permian Basin has been a prolific hydrocarbon producer for almost a century. The Permian’s largest sub-basin, the Midland Basin, has seen a resurgence of activity as horizontal drilling techniques have established the Wolfcamp shale as a preeminent unconventional resource. The Wolfcamp is a Pennsylvanian–lower Permian aged formation that is comprised of highly organic rich siliceous and calcareous shales with interbedded detrital carbonate facies. This study focuses on the early Leonardian aged Wolfcamp A (operational name) in northern Martin County, Texas. Cuttings samples were acquired from eight wells within the study area and used for organic facies and bulk geochemical analyses. A total of 85 samples were used for pyrolysis and kinetics analysis, and 40 microscope slides were prepared for visual kerogen analysis under transmitted light.

Hierarchical clustering analysis was used to delineate three organic facies. The organic facies model supported paleogeography interpretation and source rock evaluation. The kerogen types and organic facies in the study area were found to be dictated primarily by proximity to the Horseshoe Atoll, an isolated carbonate platform that had been mostly buried by the early Leonardian time. All three organic facies identified fell within the peak oil window and appeared to constitute an excellent unconventional petroleum system.

Correlations were established between bulk geochemical measurements and kerogen analysis. Degraded phytoclast abundances had a strong positive correlation with T_max values whereas terrestrial palynomorphs had a positive spatial correlation with hydrogen index (HI). A nonlinear relationship between thermal maturity and depth was identified in the study area. The presence of a Type II/III mixed kerogen system that has undergone less conversion of terrestrial palynomorph material was able to explain lower T_max values in the central portion of the study area. Increased abundances of degraded phytoclast material that experienced a higher degree of degradation prior to burial likely from shallower water depositional environment was able to explain the higher T_max values observed on eastern side of the study area.