Multiscale fracture and damage zone characterization in a tight sandstone reservoir, Sichuan Basin, China

Abstract

Fractures within fault damage zones are crucial for the migration of subsurface fluids, which is challenging the characterization of the fractures and damage zones in the subsurface due to the lack of subsurface data. We have investigated the fractures and fault damage zones in the tight sandstone gas-bearing Xujiahe Formation in the northeast Sichuan Basin, China, based on a comprehensive study of seismic data, well log, core, as well as field observations. We have demonstrated the structure and distribution of damage zones from the basin-scale down to the microscale. The core samples find mostly opening-mode microfractures that can be subcritically generated under low tectonic stress fields since the Late Triassic-Middle Jurassic. These opening-mode microfractures in such ultra-low-permeability sandstone are likely to provide a storage volume for coal gas to accumulate. Macrofractures from image logging and outcrop display well-developed joint networks within the sandstone. The basin-wide distribution of such macrofractures implies potential conduits for gas migration during basin uplift in the Cretaceous. The damage zones are formed and controlled by a system of reverse faults, with thicknesses ranging 100–1500 m. The multiscale analysis of fractures implies that, for the tight sandstone within the Xujiahe Formation, the fractures and damage zones are likely to control the gas migration and accumulation during the tectonic transformation from the Late Cretaceous to the Quaternary Period. The overpressure history, coupled with the fracture system, enhances the redistribution of the gas reservoir. This approach and these data lead to a more in-depth understanding of the fractures and damage zones on a regional scale, which could extend to hydraulic and mechanical characterization of damage zones in the upper crust.