Direct Observation of Clay Fabrics and Clay Pores in Naturally Faulted Shales

Abstract

Clay fabrics and clay-hosted porosity, as the most essential components in shale gas reservoirs, are the result of a long succession of geological processes that range from sediment deposition and later diagenesis to tectonic activity. A high-resolution electron microscopy study on naturally faulted Longmaxi shales from the Southeast Sichuan Basin was conducted to understand the development of clay fabrics and their hosted pore network and place them in a tectonic context. The investigation demonstrates a significant uniformity between brittle deformation detected in the shale matrix with optical microscopy and slip deformation observed related to a strong degree of preferred alignment for clay particles documented with the scanning electron microscopy (SEM) fabric measurements. SEM and transmission electron microscopy (TEM) images reveal that clay pores are intimately linked to clay fabrics and are associated with grain slip deformation. Two major clay pore types can be identified and classified by their origin, which we named primary pores and secondary pores (SEP, tectonic origin). The majority of the primary pores are shelter pores (SHP), which are preserved due to the presence of pressure shadows that resist tectonic compaction and deformation, preventing pore structure collapse. SEP are linear nanometer-size openings that commonly have wider and elongated shapes and preferred orientations parallel to the clay foliation, and they constitute the bulk of available porosity. These findings offer new insight into the nature of clay-hosted porosity down to the nanoscale and explain microstructural deformation and porosity preservation of clay fabrics during faulting.