Classification of elemental chemofacies as indicators of cement diagenesis in mudrocks of the Permian Spraberry Formation and Wolfcamp formation, western Texas

Mineralogical composition is a primary control on the mechanical strength of tight mudrock reservoirs and is a critical rock property in the identification of intervals for hydraulic fracturing. The x-ray fluorescence (XRF) spectroscopy and x-ray diffraction (XRD) analyses were conducted on mudrocks from the Permian Wolfcamp formation and Spraberry Formation, Midland Basin, Texas, to study their chemical composition and potential impact on mechanical rock properties. Additionally, we use a combination of thin-section petrography, scanning electron microscopy, and mineral segmentation mapping to document rock texture, especially the amount and distribution of quartz and calcite cements. We find that samples dominated by extrabasinal grain components show isolated clusters of intergranular cementation in grain-supported packing arrangements, whereas samples dominated by intrabasinal grain components show pervasive cementation in matrix-supported grain assemblages. We present a novel workflow using correlative relationships between elemental Si, Al, and Ca to classify mudrocks into chemofacies and predict which chemofacies are cement prone. This workflow identifies four XRF-based chemofacies for the Wolfcamp and Spraberry mudrocks: (1) abundant siliciclastic detrital grain components, (2) intergranular calcite cement, (3) abundant quartz and calcite cement, and (4) pervasive microcrystalline quartz cement. Results show that cement-prone facies of the Wolfcamp and Spraberry, particularly chemofacies 2, correlate to high elastic response and represent the strongest core materials. Our workflow can be applied to any mudrock system with available compositional data sets, such as XRF, XRD, or Fourier transform infrared spectroscopy, aiding in the prediction of mechanical mudrock properties and the development of brittle fractures in unconventional reservoirs.