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Controls of Lithology Stacking Patterns on Variations in Oil Saturation, Wolfcamp A, Delaware and Midland Basins


Typical meter-scale cycles observed in cores from the Wolfcamp A unit comprise basal carbonate facies overlain by calcareous or siliceous mudrocks. These cycles are the fundamental macroscopic units of deposition in the basinal mudrocks from the subsurface Delaware and Midland Basins. TOC values changes abruptly over meter-scale vertical intervals associated with these lithology variations. Siliceous mudstones are the most TOC-rich facies (TOC > 3 wt.%) whereas fine-grained, thin carbonate beds are the TOC-leanest (TOC < 1 wt.%). Do the thin carbonate-rich beds serve as reservoir for storage of migrated oil or act as a seal? Three 10-foot (3.3 m) intervals of meter-scale lithofacies cycles in the Wolfcamp A unit were selected for sampling and analyzed using X-ray diffraction (XRD) analysis, TOC and HAWK analysis, crushed-rock gas analysis and organic petrography in order to investigate the controls to the variations in oil saturation.

Oil saturation index, which is the normalized free oil per gram TOC (S1*100/TOC from rock pyrolysis), is used as an indicator of free oil enrichment or depletion. The thin carbonate beds contain the lowest TOC content, but have a similar oil saturation index values compared to

TOC-rich siliceous mudstone and calcareous mudstone. Both TOC-lean fine-grained carbonate and TOC-rich siliceous mudstone lithofacies have high oil saturation in these meter-scale cycles. These observations indicate that migrated oil is present, leading to high oil saturation in the fine-grained thin carbonate beds, which have void-filling migrated solid bitumen by petrographic observation.

Gas released from rock crushing displays lower gas dryness values (C1/C1-5) but higher oil indicator values (C4+5/C1-5) in TOC-lean thin carbonate beds compared to the TOC-rich mudstones. Abnormal low gas dryness values but high oil indicator values in TOC-lean thin carbonate beds likely indicates 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, and charged into adjacent OM-lean thin carbonate beds. Multiple-stages charging of migrated oil into overlain thin carbonate beds has resulted in a mixture of heavy crude, normal oil and light oil that is generated at different thermal maturity levels. Short-distance vertical oil migration from TOC-rich siliceous/calcareous mudstones into carbonate rich lithologies is a key factor in variations of oil saturation in the Wolfcamp A unit.