Fluid migration through fault damage zones in Sichuan Basin, China

Seismically active faults may control fluid flow while affecting regional geohazards. Here, we analyzed fluid migration processes along a large fault in a carbonate and evaporite sequence of the western Sichuan basin, adjacent to the Mw 7.9 Wenchuan earthquake. This study focuses on the architecture of the Pengxian fault (PXF), which is a dominant blind thrust at the frontal zone between the Sichuan basin and the Longmen Shan range. The PXF is ~55 km long with a damage zone thicker than 1 km, which apparently affected the regional migration of hydrocarbons. For the analysis, we used 3D seismic survey data, geochemical data, and core samples from a gas reservoir in the western Sichuan basin. The results indicate: (1) the PXF damage zone is separated into two vertical mechanical blocks by a ductile detachment zone of Triassic anhydrite; (2) bitumen occurrences and carbon isotope analyses indicate that gas and fluid migration across the anhydrite of this detachment zone occurred along the PXF damage zone; (3) the PXF damage zone served as a preferential conduit for subsurface fluid flow and acted as a fault valve in a seismic cycle. We anticipate that fluid communication along fault damage zones could lead to fault weakening and associated hazards in seismically active regions.