Diagenesis and pressure evolution in Ediacaran carbonate rocks in the North Slope (Penglai area) of the central Sichuan Basin (SW China)

The Penglai area is an emerging giant gas field in the Sichuan Basin supporting the carbon neutrality goal. To accelerate the exploration, this study combines petrographic observations using optical and cold cathodoluminescence microscopy, fluid inclusion microthermometry, Raman spectroscopy, 1D basin modeling and fluid inclusion PVTX modeling techniques to reveal the paleo-fluid P-T evolution in the second member of the Dengying Formation (Ediacaran age). The results firstly showed a diagenetic sequence which includes three phases of dolomite (1–3), bitumen, quartz (core and overgrowth) and fluorite. Then, ten paleo-fluid episodes were identified based on microthermometric histograms of different mineral phases. Finally, by integrating the Raman results, the hydrostatic and lithostatic gradients calculated by the constructed 1D basin model, and the existing equation of states, the homogenization pressure of fluid inclusions from each fluid episode was determined. By assuming the system was saturated by CH4 as suggested by Raman results, the homogenization pressure equals to the trapping pressure. The P-T trajectory indicates that the second member of the Dengying Formation experienced two sever pressure accumulation stages. One is between fluid episode #3 and #4 recorded by dolomite 3, due to oil generation, and the other is from fluid episode #5 in dolomite 3 to episode #7 in quartz overgrowth to episode #8 in fluorite due to oil cracking to gas. Also, two pressure releasing periods were recorded by fluid episode #5 in dolomite 3 and #9 and #10 in fluorite, caused by both basin uplifting and fluid pressure reaching the lithostatic pressure. This study provides insights into fluid pressure evolution in deeply buried carbonate reservoirs. The understandings can support further geoenergy utilization and reduce exploration risks.