Effect of tectonic reworking on shale fracturing and gas preservation in the upper Yangtze block, south China

Lower Silurian shale, with a variable gas enrichment, in the Upper Yangtze Block is used to evaluate the response of shale fracturing and gas loss to differential tectonic reworking. The Meso-Cenozoic uplift process was reconstructed by apatite fission track, zircon fission track, apatite (U–Th)/He and zircon (U–Th)/He dating. During the Mesozoic, due to Pacific-Eurasian convergence the Upper Yangtze Block underwent northwestward progressive deformation, with a periodic uplift process characterized by rapid uplift during the Early-Late Cretaceous, followed by subsequent slow uplift. The Cenozoic uplift process was characterized by rapid uplift in the Sichuan Basin as a result of Indian-Asian collision and inherited slow uplift in the Hubei–Hunan–Guizhou fold belt in response to Pacific-Eurasia convergence. The pressure evolution of the Lower Silurian shale was reconstructed using fluid inclusions and variation in shale gas content was quantified. By comparing the differences in pressure reduction, fracturing, and shale gas loss we explore the effect of tectonic reworking on shale fracturing and gas preservation in relation to deformation, uplift and the maximum burial depth. Deformation increases fracture development and reduces the sealing capacity of the shale. Uplift amplitude is a major controlling factor. Large amounts of uplift resulted in clear fracturing and massive gas loss. Triaxial fracture tests show that the maximum burial depth is associated with shear fractures in the shale and thereby affects shale gas preservation during uplift. The differences of slip deformation, tectonic uplift amplitude and the maximum burial depth of shale resulted in the differential enrichment of the Lower Silurian shale gas.