Abstract
Controls on deposition and reservoir quality of mixed unconventional carbonates and siliciclastics are not adequately understood. This project explores the Wolfcamp ‘A’ (early Leonardian) in Howard County, TX to determine what stratigraphic and sedimentologic controls lead to areas with the best reservoirs.
Core, thin section, XRD, TRA, and rock-eval pyrolysis data were used to analyze sedimentary facies and rock properties. Core observations were used to calibrate facies to 1122 well logs, which were used to correlate the Wolfcamp A internal stratigraphy across an area of 3637 km2. Facies distribution and thickness were mapped in each stratigraphic package to analyze controls on distribution of high and low reservoir-quality sediment gravity flow (SGF) facies.
Out of 11 lithofacies, the dominant facies assemblages are coarse-grained packstone-floatstone-rudstone (CGC), fine-grained calcareous mudstone-wackestone (FGMW), and siliceous mudstone-siltstone (SMS). CGC facies have sharp, locally erosive surfaces, rip-up clasts, are massive or have internal grading, suggesting deposition from SGFs. The dominance of detrital quartz, lack of radiolarians, rarity of shallow-water skeletal fragments, and massive or normal graded laminations suggest SMS and FGMW were deposited as SGFs with a separate siliciclastic (SMS) or carbonate slope (FGMW) source. SMS facies have the best unconventional reservoir potential, with total porosity ranging from ~6-10%, TOC of 2-3.2 wt%, and low clay content (< 50%).
Six regionally identifiable major units show progradational and compensational geometries, and each pair of major units has wedge-on-wedge relationships. The lowest two major units are CGC-rich, the middle two are characterized by SMS facies, and the upper two contain all three facies assemblages. The three major units that are thickest proximally are lobe-shaped and sourced from between the Eastern Shelf and Glasscock “nose” with internal units downlapping basinwards. The other three major units are thickest distally and laterally with internal units onlapping proximally and, with some exceptions, are mostly sourced from the Eastern Shelf and Glasscock “nose.”
The results suggest CGC facies were commonly deposited during high relative sea level whereas most SMS and FGMW facies were deposited during low relative sea level. Notch-like features in the slope acted as foci for SGFs. Promising sweet spots of greatest thickness and SMS prevalence are in the middle pair of major units and located near a northeast-southwest trend of thick SMS deposition situated medially. The distribution of those sweet spots is predictable by mapping paleotopographic funneling mechanisms, and understanding how relative sea level controls facies distribution and how paleotopography controls sediment dispersal and geometries. These controls are broadly applicable across areas of the Permian Basin.
