Multiscale Fracture Within Fault Damage Zones

In this chapter, we investigate the fractures and fault damage zones in a tight sandstone coal gas-bearing Xujiahe Formation in northeast Sichuan Basin, China, based on comprehensive study of seismic data, well log, core as well as field observations. We demonstrate the structure and distribution of damage zones are demonstrated from basin-scale down to micro-scale. The core samples show 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. Macro-fractures from image logging and outcrop display well-developed joint networks within the sandstone. The basin-wide distribution of such macro-fractures 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 multi-scale 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 in the upper crust.