Formation and preservation of extremely high-pressure fluid in sedimentary basins is critical to understanding petroleum accumulation and basin evolution; however, this issue remains poorly understood due to a lack of key evidence. We present fluid inclusion evidence from the Paleozoic black shales in the eastern Sichuan Basin (South China block) that suggest that gas generation can form methane-saturated fluids with pressures significantly exceeding the lithostatic pressure. The fluid inclusion internal pressure is so high (e.g., ~77 MPa) that gas hydrate formed at ambient temperature, aqueous-vapor homogenization could not be achieved under ambient pressure, and the homogenization pressure was beyond the scope of the current equation of state for the NaCl-H2O-CH4 system. We infer that the formation of high-pressure (e.g., >230 MPa), methane-saturated fluids induces hydrofracture under weak compressional tectonic regimes, where the excess lithostatic fluids can be accumulated and sustained. Fluid inclusion records with an abnormally high magnitude of overpressure indicate small differential stress and thus can serve as a microscale indicator for tectonic quiescence.

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