Evidence of multiepisode dissolution of meteoric fluids of the Middle and Upper Cambrian carbonate rocks of Sanhuichang outcrop,Nanchuan area,Chongqing
Qian Yi-Xiong1, He Zhi-Liang2, Li Guo-Rong3, Dong Shao-Feng1, Peng Shou-Tao2, Wo Yu-Jin2, Zhang Jun-Tao2, Jiao Cun-Li2, Zhang Wen-Tao1
1 Wuxi Institute,Exploration & Production Research Institute,SINOPEC,Jiangsu Wuxi 214151,China; 2 Exploration & Development Research Institute,SINOPEC,Beijing 100083,China; 3 Chengdu University of Technology,Chengdu 610059,China
Abstract：How to identify and evaluate the influences of multiepisodes of tectonic activities and diagenesis on Paleozoic carbonate rocks is a fundamental and challenging issue for reservoirs formation mechanism study. It would be valuable to unravel the important information of diagenesis fluid evolution and its relationship with formation of oil and gas pools. The well-preserved outcrop of Cambrian carbonate rocks in SanHuiChang,Nanchuan,Chongqing is an idea natural laboratory to study the depositional and diagenesis history of Paleozoic carbonate. The petrographic and geochemical analyses have demonstrated that the Cambrian carbonate rocks have experienced multiepisodes of diagenesis,including the early penecontemporaneous diagenesis in fresh and marine water,the shallow buried-diagenesis in mixed freshwater and seawater or formation water,karstification in meteoric weathering related to the later Caledonian and Hercynian tectonic activities,the middle and deep buried-diagenesis in formation water or hydrothermal fluid and karstification in meteoric weathering in Yanshan and Himalayan period. In different episodes of diagenesis,the meteoric fresh water has played different roles. In the early stage of meteoric diagenesis,the shallowing-upward high-frequency sequence boundary is characterized by molds,dissolved-pores related to fenestral and fabric-selective pores,less negative excursion of δ13C & δ18O and similar87Sr/86Sr of cement and matrix when compared to the coeval sea water,non-luminescent cements(CL)and a few small liquor phase inclusions in cements. In the middle stage of diagenesis,the karstification has been developed in subaerial exposure period in Caledonian and Hercynian,which is characterized by non-fabric-selective pores and vugs,a more pervasive non-luminescent of middle and coarse calcites or dull orange dolomites with δ18O and δ13C depletion and positive values of87Sr/86Sr,oxygenated hydrocarbons(asphalt)and hydrocarbon-bearing,and the low-temperature immiscible inclusions in cements and filling minerals. In addition,this stage of diagenesis has experienced post-meteoric weathering,overprinted and modified by later deep burial diagenesis. In the late stage of diagenesis,the last karstification happened in meteoric weathering environment in Yanshan and Himalayan period,which is documented by isolate-distribution of pores and vugs and fractures with mostly filled by calcites and clay,precipitation of the coarse and giant calcites crystal with δ18O and δ13C strongly negative excursion($\delta^{13}C_{PDB}$=-4.6‰~-23.4‰,$\delta^{18}C_{PDB}$=-8.6‰~-17.8‰),and the low-temperature immiscible inclusions(homogenization temperature <28.5℃),indicating a diagenesis triggered by meteoric fluid interacted with soil-related(carbon)organic acids or biological methane bacteria activities.
Qian Yi-Xiong,He Zhi-Liang,Li Guo-Rong et al. Evidence of multiepisode dissolution of meteoric fluids of the Middle and Upper Cambrian carbonate rocks of Sanhuichang outcrop,Nanchuan area,Chongqing[J]. JOPC, 2019, 21(2): 278-292.
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