Effects of lithofacies on pore system evolution of storm-wave silt-rich fine-grained sediments. Early Silurian Qusaiba Member (Qaliba Formation), NW Saudi Arabia

Abstract

Storm-wave sediments are considered a potential unconventional resource in the Middle East region, i.e., the early Silurian Qusaiba Member of the Qaliba Formation in Saudi Arabia. Extremely few studies have examined the controls of lithofacies heterogeneity on pore types, sizes, distribution, and connectivity. Therefore, this study intended to illustrate lithofacies controls on pore systems using various integrated approaches. Based on lithofacies analysis, seven lithofacies were identified and interpreted as storm-wave-dominated distal, medial, and proximal shelf deposits. The results of this study reveal two main types of pore systems: interparticle and intraparticle pores, which have been thoroughly recongized across all recognized lithofacies. Nuclear magnetic resonance (NMR) data showed positively and negatively skewed bimodal porosity distribution curves in the proximal and distal shelf lithofacies, respectively; however, the NMR data of medial shelf lithofacies were found to be normally distributed with unimodal distribution curves. Phyllosilicate minerals associated with interparticle pores (≤1 μm) were observed in the distal lithofacies; however, larger-scale interparticle pores were observed in both medial and proximal lithofacies. Such variation was also reflected in the NMR curves, resulting in negatively and positively skewed distribution curves of T2. Diagenetic facies (i.e., calcite, phosphate, pyrite, iron oxides, and clay minerals cementations) occurred during syndepositional, early, and intermediate burial stages before oil generation window. This study facilitates the understanding of the impacts of lithofacies and diagenetic facies on the pore system, and, the petrophysical characteristics of mudrocks in similar depostional settings.