Depositional Processes and Environments in Wolfcampian-leonardian Strata, Southern Midland Basin, Texas

Abstract

The Early Permian (Wolfcampian-Leonardian) Wolfcamp interval of the Permian Basin in West Texas is a mixed siliciclastic-carbonate succession that hosts one of the most important unconventional oil and gas plays in the world. Wolfcamp strata comprise stacked, cyclic gravity flow deposits separated by hemipelagic mudstone and siltstone. Interest in the geologic heterogeneity of these deposits and its effect on well performance has sparked research on the sedimentological, geophysical, and geochemical signatures and their correlation to depositional environments and reservoir properties.This investigation analyzed a stratigraphic test core and associated laboratory data from the Wolfcamp play to help refine interpretations of the late Wolfcampian to early Leonardian succession in the southern Midland Basin. Detailed core descriptions and petrographic analyses were integrated with geochemical data to define facies, evaluate vertical stratigraphic successions, and relate reservoir properties with specific facies and depositional sequences. Interpretation of stratal stacking patterns and gravity flow transport processes were used to determine how vertical variations in rock fabric may be related to base level fluctuations and the morphology of platform margins and slopes. Sequence stratigraphic interpretations were made to help develop a predictive depositional framework.Based on the characteristics and distribution of gravity flow deposits and interbedded hemipelagic sediment, a distal toe-of-slope to basin plain setting is inferred. In these settings, quartz and organic matter accumulated as background sedimentation interrupted by episodic deposition of gravity flows derived from the continental platform. Cyclicity is observed in individual, upward-fining trends of relatively coarse-grained skeletal material overlain by calcareous and/or siliceous mudstone. It is interpreted that the majority of gravity flows occurred during lowstands of relative sea level. Widespread pyrite and phosphatic nodules and TOC content as high as 8% indicates that low oxygen levels prevailed during accumulation of these sediments.