As a three-dimensional geological body, a fault zone has a complex internal structure. Disputes remain over flow pathways of fluids within fault zones. Well and seismic data cannot be used to effectively identify the internal structures of a fault zone. Furthermore, continuous core sampling in fault zones is commonly limited. Fewer studies of flow pathways along reverse faults are done in a sedimentary basin. Through extensive outcrop observations, sampling, and measurements in the northwestern Sichuan Basin of China, this study enhances our understanding of fluid evolution and the main pathway of vertical fluid flow along a reverse fault. In the studied carbonates, deep hot brine initially entered the fault zone and migrated upward along the fault core, then moved to shallow strata, mixed with meteoric water, and cooled in the fault zone. In the studied sandstone and shale, a paleo-oil pool formed in the fault damage zone. After that, forced by uplift and reactivation, oil migrated into the fault core along fractures and was cooled, washed, biodegraded, and oxidized by meteoric water. In the sandstone–sandstone juxtaposition faults, the oil shows are distinctly different between hanging wall and footwall. Fault rocks (sand and shale gouges) that developed along the principal slip surface seem to have prevented fluid flow across the fault. This evidence suggests that fault core and inner damaged zone are the main pathways of vertical fluid flow along the investigated reverse fault zone.

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