Fault damage zone and its effect on deep shale gas: Insights from 3D seismic interpretation in the southern Sichuan Basin, China

Fault damage zones affect the preservation and enrichment of shale gas. Most fault damage zone studies are based on directly observed outcrops. In this study, we provide an example of fault damage zone analysis using 3D seismic data of deep shales buried more than 3500 m in the Sichuan Basin, China. The 3D seismic variance attribute and the cumulative frequency method were used to define the background value and then estimate the damage zone widths of different sizes of faults. We found that the damage zone widths range from 80 m to 1570 m, and trend to stabilize with the increase in displacements. The scattering caused by the relay zone can be interpreted as segment linkage mode and damage superposition related to subsidiary faults. The damage intensity decays following a power law with distance from the fault core, in agreement with field observations and the distribution of seismic variance. Damage zone widths display a significant asymmetry across the fault, and the hanging wall damage zone is generally wider than the footwall damage zone. We discuss the impact of the fault damage zone on shale gas preservation, which is related to the sealing of the direct caprock. It appears that the seismic variance attribute is an effective tool for subsurface characterization of fault damage zones and can help identify the location of deep unconventional energy.