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Silica Diagenesis in Mudstones and the Impact on Consolidation and Brittle Deformation

Abstract

Here, we present preliminary characterization of silica diagenesis occurring in mudstones of the Permian Spraberry and Wolfcamp Formations. We aim to understand the mechanisms controlling silica diagenesis in mudstones to allow better subsurface reservoir quality predication and hydrocarbon recovery. Powder X-ray diffraction (XRD) and other core analyses show higher silica content in the Wolfcamp Formation with respect to the Spraberry Formation, which has significantly higher carbonate content. Scanning electron microscopy (SEM) and thin section petrography indicate the presence of silica in the form of authigenic pore-filling quartz cement. This cement occurs in both formations, but to a lesser degree in the Spraberry Formation, where carbonate cement is more prevalent. We find a negative covariation in the normalized ratio, with carbonate removed, of clay mineral content to quartz content. This relationship is identical in both formations up to a threshold value where the quartz to clay ratio equals 1.2:1. Above this concentration, the Wolfcamp Formation trend deviates from the Spraberry Formation trend to favor higher quartz content. We propose the presence of this increased quartz cementation in the Wolfcamp Formation occurs at the expense of carbonate cementation, and that the quartz to clay mineral ratio can be used to predict the presence of macropore-preserving quartz cement as revealed through SEM imaging. Additional observations on the alignment of micas parallel to more-cemented areas within the quartz-rich samples suggest that the quartz-cemented fabric may be the result of early cementation leading to more indurated mudstones resistant to normal consolidation behavior. Through experimental rock deformation, we will test the efficiency of brittle deformation in linking these larger preserved pore throats for increased permeability and better recovery potential.