Structure- and lithofacies-controlled natural fracture developments in shale: Implications for shale gas accumulation in the Wufeng-Longmaxi Formations, Fuling Field, Sichuan Basin, China

Descriptions, field emission-scanning electron microscopy and geochemical analyses on shale core samples taken from the fractured intervals of the Wufeng-Longmaxi Formations in three wells from the Fuling Field were used to understand the developmental properties of natural fractures and elucidate their controlling factors. Furthermore, the effects of geological fractures on shale gas accumulation were systematically analyzed. The macro-fractures observed in the shale cores are mainly non-tectonic bedding fractures; to a lesser extent, tectonic fractures, including oblique shear, vertical tension and tension-shear fractures, as well as bed-parallel slip fractures are visible. In contrast, micro-fractures are mainly interparticulate, intraparticulate, and grain marginal in nature. The extent of natural fracture development is controlled by external structural and internal lithofacies factors. For shales with similar lithofacies, the development degree of tectonic fractures increases with decreasing distance from the tectonically deformed and faulted zones. Within the same structural zone, the fractures are well developed in shales of organic matter-rich S-3 (i.e., argillaceous-rich siliceous) lithofacies. The study of fracture properties and their controlling factors has resulted in a model for the development of natural fractures in Wufeng-Longmaxi Shales generated by both structural and lithofacies effects. Overall, shale reser-voirs in a "fractured but not broken" state are the most favorable ones for gas accumulation.