Fluid history of the lower Cambrian Longwangmiao Formation in the Anyue gas field (Sichuan Basin, SW China)

The Anyue gas field in China is an important source of natural gas supporting the country energy transition to the goal of carbon-neutrality. However, the fluid P-T history, which will provide a better insight into gas accumulation and probable leakage in the main reservoir (the Lower Cambrian Longwangmiao Formation) has still yet to be fully understood. This study combines detailed petrographic observations with transmitted light, ultraviolet light, reflected light and cold cathodoluminescence, fluid inclusion microthermometry, Raman spectrometry, 1D basin modeling and fluid P-T modeling techniques to reconstruct the fluid-involved basin evolution of the Longwangmiao Formation in the Anyue region. The results showed that the paragenesis of the Longwangmiao Formation includes dolomite matrix, dolomite 1, dolomite 2, dolomite 3 (saddle dolomite), dolomite 4, quartz and calcite. The first generation of bitumen occurs between dolomite 1 and dolomite 2, corresponding to the primary oil generation of the Lower Cambrian Qiongzhusi Formation at the burial maximal in the Silurian. The second generation of bitumen, corresponding to intense oil cracking into gas, has occurred between dolomite 4 and quartz at a geological period between the Middle Jurassic and Early Cretaceous (early Yanshanian movement). Fluid inclusions trapping pressure modeling explains that the trapping pressure increases from dolomite 4 to quartz due to burial and the oil cracking into gas, then stays constant from quartz to calcite due to the sealing of fractures. The open fractures formed after calcite partially reduced the overpressure condition of the Longwangmiao Formation. Collectively, this study will provide necessary information for a better understanding of deeply buried reservoirs to support more efficient development of such reservoirs in higher quality zones and reduce exploration risks and operational costs.