Differential exhumation, stress evolution and overpressure limits in Lower Cambrian shale of the Sichuan Basin (South China)

Abstract

Exhumation-induced stress triggers natural hydraulic fracturing, governing fluid overpressure limits in deeply buried strata. In the Sichuan Basin (South China), the geomechanical response of Lower Cambrian shale to multi-phase differential exhumation remains poorly quantified. We integrated apatite and zircon thermochronology, vitrinite reflectance, residual stratal thickness, and Skempton’s poroelastic model to reconstruct the exhumation and pore pressure history of the Qiongzhusi Formation. Results reveal a four-stage, northwestward-migrating exhumation since the Cretaceous, featuring severe uplift (3000–4000 m) along basin margins and the Weiyuan anticline, versus moderate uplift (2000–2500 m) in the Ziyang intra-basin area. This differential exhumation halted thermal evolution, establishing a spatial thermal maturity distribution (VR = 3.2%–4.0%) and a lower boundary at ∼5500 m, below which graphitization degrades rock properties. Crucially, differential exhumation magnitude and varying initial pore pressures at maximum burial (∼7000 m) governed stress trajectories during uplift. Driven by higher initial pressure, the moderately exhumed Ziyang area reached the natural fracturing threshold at 4100 m, significantly deeper than the heavily exhumed Weiyuan area (3250 m). These critical failure depths mechanistically define the upper limits for overpressure preservation, demonstrating that differential exhumation and poroelastic relaxation fundamentally control the vertical boundaries of overpressured shale compartments.