CHEN Zhiyuan^1,2, MENG Xianwu², SONG Xiaobo², HAO Xiaobo³, LONG Ke²

1. Post-doctoral Research Station, Southwest Oil & Gas Company, SINOPEC, Chengdu 610041, Sichuan Province, China;

2. Research Institute of Petroleum Exploration and Production, Southwest Oil & Gas Company, SINOPEC, Chengdu 610041,
Sichuan Province, China;

3. Well Testing Company, CNPC Xibu Drilling Engineering Company Limited, Karamay 834000, Xinjiang, China

Abstract: Through thin-section observation and cathodoluminescence, a comprehensive study of the filling sequence of diagenetic minerals in the fourth member of the Leikoupo Formation (T₂l⁴) is carried out using the geochemical data of fluid inclusions and C, O, and Sr isotopes to clarify the sequences and sources of diagenetic fluids in Leikoupo Formation, Western Sichuan Basin. The results show that the filling minerals of different grain sizes in T₂l⁴ in Western Sichuan Basin can be divided into three stages with filling sequences as follows: micro-crystalline powder dolomite and calcite in stage I, fine crystalline dolomite and calcite in stage II, and medium crystalline dolomite, calcite, and intercalated crystalline dolomite in stage III. The three stages are roughly the early–late Late Triassic (232–214 Ma B.P.), the latest Late Triassic–the latest Late Jurassic (212–145 Ma B.P.), and the Early Cretaceous–the latest Late Cretaceous (142–70 Ma B.P.), respectively. The C, O, and Sr isotopes of matrix dolomite and infill in pores and fractures in T₂l⁴ show that the diagenetic fluid exhibits the geochemical characteristics of normal marine carbonate rocks. Also, it is revealed that the high ⁸⁷Sr/⁸⁶Sr in T₂l⁴ results from the fluid intrusion of capillary concentration, infiltration and backflow in the syngenetic-quasi syngenetic period amid evaporation tidal flat, and the release of sealed sea water during the burial period.