The complexity of the fault zone structure is a direct consequence of the superposition of tectonic movements rather than of a single dramatic deformation. In general, the fault damage zone is mainly composed of two parts, the fault core and the damage zones. However, in a large-scale fault or faults with severe tectonic deformation, the fault may evolve from a single fault core into a composite fault structure with multiple fault cores. It is challenging to obtain the structural features of such fault damage zones to understand the subsurface oil and gas migration.

Based on the three-dimensional seismic variance properties, we provide a new interpretation approach and quantify the widths of fault damage zones of the second member of the Xujiahe tight sandstone in the Malubei region of northeastern Sichuan. The results include: (1) the seismic attribute of variance presents faults with a single fault core and composite fault structures with multiple fault cores. According to the turning point of accumulated values of seismic variance statistically, 0.2 is selected as a critical value of variance to define the width of damage zones. The width range is 700~1200 m. (2) numerical simulation shows that the deformation of the fault zone decays exponentially with increasing distance away from the fault core, which is consistent with the understanding of the faults in the outcrops. (3) The quantitative analysis of the width of the fault damage zones shows strong inhomogeneity of the fault zones as the width varies between the fault damage zones. There is also a significant variation in the depth of the same fault, which indicates an obvious "lithologically-controlled effect". This study is limited by selecting empirical values of seismic variance in determination of fault widths.  

The result validates this new approach and tools for the structural analysis of fault zones in tight sandstones. It provides a basis for further establishing a fault-fracture system in the Northeast Sichuan basin, and the structural features of the fault zones are of great significance for the analysis of petroleum accumulation and distribution in the tight sandstones.