Multiscale reservoir characterization and analysis of gravity-flow slope deposits in the early Permian Leonardian Third Bone Spring Sandstone and Guadalupian Upper Avalon Shale of the eastern Delaware Basin, U.S.A.: Insights into unconventional resource reservoirs

The Leonardian/Guadalupian Bone Spring Formation in the Delaware Basin of West Texas contains two intervals that are unconventional siliceous mudstone reservoirs. These siliceous mudstone intervals are in the Third Sandstone Member and the Upper Avalon Shale in the First Carbonate Member. While previous regional studies have provided stratigraphic correlations and general lithofacies description, this study provides detailed micropetrographic characterization, ranging from thin-section to scanning electron microscopy, enabling a more detailed understanding of rock fabrics, textures, and pore networks.

The predominant lithofacies consist of alternating beds of laminated and burrowed siliceous mudstones, punctuated by thin carbonate debrites and turbidites. Key characteristics of the siliceous mudstones include: (1) Total organic carbon content ranging from 1 to 7 wt% and composed of Type II kerogen, (2) Strata is within the oil window, indicating favorable conditions for hydrocarbon generation, and (3) Relatively good reservoir quality for siliceous mudstones, with a mean porosity of 7.6 % and a geometric-mean permeability of 0.0045 millidarcies (md). The interbedded carbonate units are notably tight, with minimal reservoir potential. Slumping and fragmentation of these carbonate layers prevent the development of continuous lateral permeability barriers, which favors vertical transmissibility. Therefore, without notable permeability barriers, the horizontal and vertical permeabilities are interpreted as good, enhancing the reservoir potential of the siliceous mudstone system. Good reservoir heterogeneity, combined with favorable organic content, porosity, and permeability, underscores the reservoir potential of the Third Bone Spring and Upper Avalon siliceous mudstones. Overall, the siliceous mudstones of the Leonardian/Guadalupian Bone Spring Formation represent a significant hydrocarbon resource play within the Delaware Basin.