Geological and Geochemical Investigation of the Petroleum Systems of the Permian Basin of West Texas and Southeast New Mexico



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Fifty (50) crude oils and nineteen (19) source rock samples from strata ranging in age from Ordovician to Middle Permian, and distributed widely over the Permian Basin, were studied geochemically and chemometrically to identify the petroleum systems within the greater Permian Basin. Variations in the stable carbon isotope and biomarker compositions of the oils, together with chemometric correlations of oils and source rocks, revealed the existence of at least five petroleum systems, involving organic-rich, type II-II/III source rocks of the Simpson Group Shale, the Woodford Shale, the Barnett Shale, the Wolfcamp Shale, and the Bone Spring Marls. The Spraberry source rock is mostly immature with respect to petroleum generation. Geochemical evidence strongly suggests source-rock deposition under suboxic and anoxic-suboxic conditions for all but the Bone Spring and Simpson source beds, in which case the geochemical evidence suggests deposition under anoxic conditions.

Geochemical thermal modeling indicates adequate to advanced maturity for all the source rocks, except for the Spraberry, which is barely entering into the optimum maturation window near the southern end of the Midland Basin. Generation and expulsion from the Wolfcamp Shale begun in Early Triassic, with hydrocarbons migrating laterally, and getting trapped within carbonate aprons encased within the shales of the same stratigraphic age in slope and basinal areas in both the Midland and Delaware basins. Expulsion from the Barnett Shale occurred during Early to Mid-Permian. Migration occurred radially updip from the two depocenters in the Delaware and Midland basins to Pennsylvanian sandstone reservoirs in the Eastern Shelf and Permian carbonate reservoirs in the Central Basin Platform and the Northwest Shelf. The Woodford shale began expelling petroleum during Early to Late Permian. Migration occurred along vertical pathways to underlying Devonian through Ordovician reservoirs. Expulsion from the Simpson occurred during Early Triassic. Migration followed vertical pathways downwards to Simpson sandstones and Ellenberger carbonates. For all the petroleum systems, traps formed well in advance of generation and expulsion. The petroleum systems are segregated with very limited cross-stratigraphic migration because of alternating reservoir-seal (source rock) facies. This study has established a detailed understanding of the petroleum systems in the Permian Basin and documented the geographic and stratigraphic framework for the inter-relationships between the crude oils and their most likely source rocks in the Permian.



Petroleum systems, Permian Basin, Geochemical inversion, Geochemical correlations