重庆南川三汇场寒武系碳酸盐岩中不同期次大气淡水作用的证据

多期构造叠加及多种成岩环境下的早古生代碳酸盐岩储集层形成机理十分复杂。重庆南川三汇场剖面寒武系碳酸盐岩出露完整,是研究沉积与成岩史的理想解剖点之一。研究表明,三汇场剖面寒武系白云岩分别经历了早期海水胶结、大气淡水淋滤、准同生白云岩化、中浅层埋藏、中期构造大气水作用、中晚期埋藏及热液作用和构造晚期表生大气水成岩作用,不同期次的大气淡水作用特征不同。在早成岩期,沿向上变浅的高频层序界面附近发育了鲕模孔、窗格溶孔等组构性溶孔,胶结物和充填物的δ¹⁸O、δ¹³C值均比同期海水负偏,⁸⁷Sr/⁸⁶Sr值与同期海水相近,不发光(CL),含有较小的液相包裹体;加里东晚期—海西期,发育了非组构扩溶孔洞缝,伴有氧化沥青,不发光充填物的δ¹⁸O、δ¹³C负偏、⁸⁷Sr/⁸⁶Sr值正偏,发育含烃的低温不混溶包裹体,并叠加后期的深埋藏及热液成岩作用;燕山期和喜马拉雅期, 发育孤立的非组构溶蚀孔洞、缝洞,多被巨晶方解石或黏土充填,方解石中δ¹⁸O、δ¹³C强烈负偏($\delta^{13}C_{PDB}$=-4.6‰~-23.4‰,$\delta^{18}C_{PDB}$=-8.6‰~-17.8‰)、含有低温不混溶包裹体(<28.5℃),是有机质或生物甲烷(细)菌参于下的表生大气水成岩作用产物。

Evidence of multiepisode dissolution of meteoric fluids of the Middle and Upper Cambrian carbonate rocks of Sanhuichang outcrop,Nanchuan area,Chongqing

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.