Stress memory extracted from shale in the vicinity of a fault zone: Implications for shale-gas retention

Stress memory in the Wufeng-Longmaxi Shale located in the vicinity of the boundary fault of the Sichuan Basin was extracted by Kaiser effect analysis; subsequently, shale-gas retention capacity as determined by stress memory was assessed with a simulation. Due to gas generation and geothermal heating, the Kaiser effect rarely reflects stress states before the maximum burial depth. The strong deformation within and close to the fault damage zone negated the stress memory from the early exhumation episode; instead, the stress state of the recent exhumation episode was recorded. The maximum effective horizontal stress (σH-eff) at 120 Ma was ∼85 MPa at ∼6000 m, with a maximum sustainable pressure of ∼135 MPa. The porothermoelastic mechanism caused a decrease of the pressure magnitude but an increase of the pore-fluid factor during exhumation. After the Wufeng-Longmaxi Shale was uplifted from ∼6000 m to ∼3500 m, the pore-fluid factor almost reached 1 and horizontal hydrofracturing caused shale-gas leakage. The σH-eff at 20 Ma was ∼73 MPa at ∼3500 m and it had a gradient of 38 MPa/km. Reverse faulting at 20 Ma induced a remarkable pressure drop, which indicates shale-gas leakage at that time. Subsequently, a transition from reverse to strike-slip faulting further induced fluid pressure reduction and shale-gas leakage.