Origin of authigenic quartz in organic-rich shales of the Wufeng and Longmaxi Formations in the Sichuan Basin, South China: Implications for pore evolution

Primary sedimentary composition of shales and subsequent diagenetic modifications are key factors in controlling the pore development and preservation in shale gas reservoirs. Thin section examination, X-ray diffraction analysis, scanning electron microscopy (SEM) determination with cathodoluminescence and geochemical analyses were combined to investigate Wufeng and Longmaxi shales of the Sichuan Basin. These experimental methods allowed us to focus on the origin of authigenic quartz in organic-rich shales and evaluate its importance in the evolution of shale gas reservoirs. The siliceous shales in Wufeng and Longmaxi Formations contain up to 60% microcrystalline aggregates of quartz, which are more abundant than silt-size detrital quartz to suggest an authigenic origin. Most authigenic quartz originates from biogenic siliceous dissolution and re-precipitate. Therefore, high excess silica concentration indicates a higher surface productivity with more organic matter (OM) deposition. The abundance of OM pores is positively correlated with the excess-Si content (R2 = 0.77). We suggest that the migrated OM filled in authigenic microcrystalline quartz aggregates is the principal matrix for OM pore development. Authigenic microcrystalline quartz aggregates filled in primary pore space and destroyed primary interparticle porosity. However these authigenic quartz can restrain compaction and preserve the internal pore structure as a rigid framework. Pore space is abundant enough in microcrystalline quartz aggregates for oil and bitumen filling during the oil window. Migrated OM filled the associated pore network in authigenic microcrystalline quartz aggregates would more likely to generate a better three-dimensional OM pore network through secondary cracking and provides favorable reservoir spaces for shale gas.