For this study, a multi-scale evaluation of the reservoir quality of the oil-prone Wolfcamp A was investigated over Reeves, Pecos and Ward counties in the Texas Delaware Basin. A highly detailed core analysis was accomplished on 1,370 ft. from 7 cores with variable stratigraphic interval coverage of the Wolfcamp A. One supplementary well outside of the focal study area was included from Culberson county to gain a further perspective of sedimentological changes in the Delaware Basin.
Facies analysis were supplemented with XRD, thin-section petrography and XRF analyses. Nine mudstone and siltstone facies and four carbonate facies have been identified over the study area. The range of TOC content for each mudstone-siltstone facies is a unique situation where eight of the mudstone and siltstone facies have both the potential to be organic-rich reservoir quality facies (≤2 wt.%) and organic-lean facies (≥2wt.%), the result of heterogeneity both vertically within a single core and laterally between cores. A reservoir quality hierarchy containing primary, secondary and tertiary reservoir quality facies that were defined on two critical parameters: average TOC content and mineralogical composition and consistency. These parameters were chosen based on the implications that these compositional controls have on the reservoir quality properties; hydrocarbons in place, mechanical properties and fracture stimulation.
The stratigraphic record of the Wolfcamp A is dominated by gravity-driven event beds that entered the basin from numerous sediment entry points. The multiple depositional processes have resulted in the complex vertical arrangement of interbedded carbonate debris flow deposits, high and low-density carbonate and siliciclastic turbidites, hybrid event beds, dilute turbulent wake and hemipelagic facies. The variability observed in core of the different facies is a result of the depositional processes.
The most significant challenge to reservoir characterization of the Wolfcamp A in the Delaware Basin, is the inability to take the sedimentological, stratigraphic and reservoir quality variabilities that are documented at the vertical scale and extrapolate into the lateral dimension. Capturing the different level of complexity of the Wolfcamp A in the Delaware Basin, and predicting lateral changes in reservoir quality requires a novel reservoir characterization approach.
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