Diagenetic fluid evolution and hydrocarbon accumulation history of second Member of the Sinian Dengying Formation in the Taihe gas area, central Sichuan basin:Evidence from petrology, in situ geochemistry, fluid inclusions and chronology

Great exploration breakthrough has been achieved in the fourth Member of the Sinian Dengying Formation in central Sichuan.The 2nd Member of the Sinian Dengying Formation in Taihe gas- bearing area has also shown great exploration potential. However,the reservoir of the 2nd Member of the Dengying Formation has the characteristics of old age, large burialdepth, long diagenetic evolution history, and has experienced multiple tectonic movements; the mineral cementation filling periods in the reservoir are complicated, and the characteristics of diagenetic fluids are unclear. All of these limit oil and gas exploration in the 2nd Member of the Dengying Formation reservoir to a certain extent. Based on the rock texture analysis, this paper uses cathode- luminescence, in situ microzone analysis, fluid inclusion and dolomite U- Pb chronology to determine the diagenetic sequence of the 2nd Member of the Dengying Formation in Taihe gas area, its diagenetic environment and diagenetic fluid characteristics. The formation time and representative oil- gas charging events of dolomite cemented backfill at different times,as well as its geological significance and hydrocarbon accumulation history are clarified. The results show that the diagenetic sequence of bitumen and fillings in the reservoir pores of the 2nd Member of the Dengying Formation in the study area are as follows: fibrous dolomite of the first generation in the submarine environment→atmospheric freshwater dolomite of the second generation in the epidiagenetic environment→silty- fine grained dolomite of the third generation in the shallow- burial environment→oxidized degradation bitumen of the first stage→medium crystal dolomite of the fourth generation in the moderate burial environment→coarse crystal dolomite of the fifth generation in the deep- burial environment→thermally cracked bitumen of the second stage→giant- saddle crystal dolomite of the sixth generation in the deep- burial environment→quartz and fluorite of the seventh generation in the uplifting burial environment. Thesemainly reflect the characteristics of four different types of diagenetic fluids: ① high salinity source fluids with high Na, K, low Fe, Mn content, negative Ce anomaly and positive Y anomaly; ② the mixed fluids of fresh water and residual seawater have the characteristics of low Fe and Mn content, and gentle REE pattern; ③ the mixed fluids of high- temperature hydrothermal fluids and early seawater have high Fe, Mn content and Eu positive anomalies; ④ deep acidic and reducing fluids associated with high- temperature hydrothermal fluids are characterized by high Mn content, Fe/Mn<1, high HREE enrichment, positive Eu anomaly, and authigenic quartz and fluorite mineral precipitation. The filling relationship, fluid inclusion phase, homogenization temperature and U- Pb chronology of dolomite showed that the formation and destruction of Caledonian paleo- oil reservoir were recorded respectively by siltyine grained dolomite of the third generation which began to develop hydrocarbon inclusions and the first stage of bitumen which was formed in the Caledonian tectonic uplift period; medium crystal dolomite of the fourth generation and coarse crystal dolomite of the fifth generation recorded the formation of ancient oil reservoir in Indo- China period; part of the coarse crystal dolomite of the fifth generation and bitumen of the second stage recorded the cracking event of ancient oil reservoir in Indo- China period; the giant- saddle crystal dolomite of the sixth- generation and the quartz, fluorite and other minerals of the seventh- generation record the accumulation events of paleo- dry gas reservoir formation→gas reservoir formation, adjustment and finalization. © 2023 Geological Society of China. All rights reserved.