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Stress Memory and Overpressure Retention in the Wufeng-Longmaxi in the Vicinity of a Fault Zone


Understanding the spatial-temporal stress state of organic-rich shale is crucial for the shale-gas exploration and production. Stress memory in Wufeng-Longmaxi shale located in the vicinity of the boundary fault of the eastern Sichuan Basin was extracted by the Kaiser effect analysis; subsequently, shale-gas retention capacity controlled by stress memory was assessed with a simulation. The maximum effective horizontal stress (σH-eff) at 120 Ma (at the onset of first exhumation episode) was approximately 85 MPa at ~6000 m, with a maximum sustainable pressure of 135 MPa. The poro-thermoelastic mechanism caused the steady retention of shale-gas with an exhumation thickness of <2500 m (i.e., burial depth >3500 m), despite the decrease of pressure magnitude during the first exhumation episode. In contrast, the pore-fluid factor (Pfv) increased with exhumation and reached 1 after 2500 m of exhumation (i.e., burial depth <3500 m). Horizontal hydro-fracturing thus induced shale-gas leakage after >2500 m of exhumation. The σH-eff at 20 Ma (at the onset of second exhumation episode) was 73 MPa at ~3500 m and it had a gradient of 38 MPa/km. The reverse faulting at the onset of the second exhumation induced shale-gas leakage and the fluid pressure remarkably dropped. The fluid pressure of Wufeng-Longmaxi shale with burial depth shallower than ~2800 m at the end of first exhumation episode dropped to the hydrostatic pressure. The transition from a reverse to a strike-slip faulting stress regime seems to have also induced a reduction in fluid pressure and shale-gas leakage during the second exhumation episode.