Typical meter-scale lithofacies cycles from the Wolfcamp A in the Delaware and Midland Basins comprise basal carbonate facies overlain by calcareous or siliceous mudrocks. Siliceous mudstones are the most organic-rich facies with high total organic carbon (TOC > 3 wt. %), whereas thin carbonate beds have the lowest organic matter (OM) content among the lithofacies present (TOC < 1 wt. %). To determine if the thin carbonate-rich beds serve as either reservoir storage for migrated oil or act as a seal, three core intervals of meter-scale lithofacies cycles in the Wolfcamp A were selected for sampling and were analyzed using x-ray diffraction analysis, TOC, programmed pyrolysis analysis, and residual gas analysis from rock crushing.
Oil saturation index (OSI) (the amount of free oil normalized by TOC; OSI = S1 × 100/TOC) is used as an indicator of oil enrichment or depletion in the reservoir, where S1 is volatile oil in programmed pyrolysis (temperature = 300°C). Both TOC-lean carbonate and TOC-rich mudstone lithofacies have high OSI in these meter-scale cycles (average OSI is 124.5 mg HC/g TOC for carbonate beds), indicating that migrated oil is present. Residual gas analyses show lower dryness values (C1/C1–5) and higher oil indicator values (100 × C4+5/C1–5) in TOC-lean carbonate beds compared to the TOC-rich mudstones, likely indicating a cumulative oil and gas charging effect through source rock maturation. Oil and gas generated at different stages of thermal maturation were partially expelled from OM-rich siliceous/calcareous mudstones into adjacent OM-lean carbonate beds. This study shows oil expulsion from source to adjacent carbonate beds is a key factor in variations of oil saturation in the Wolfcamp A.