川北地区寒武系筇竹寺组钙质结核裂缝充填物特征及指示意义

摘要

川北地区寒武系筇竹寺组钙质结核泥页岩中早成岩期钙质结核较发育,结核裂缝中见多期次矿物充填.为揭示钙质结核裂缝形成机理,本次结合岩石学特征及地球化学特征等开展系统研究.川北筇竹寺组钙质结核裂缝中见三~四期矿物充填.第一期微晶方解石表面较脏,阴极发光暗淡,δ13CPDB、δ18OPDB平均值分别为-4.26‰及-7.69‰;第二期为细—中晶粒状方解石充填,阴极发光为橘黄色,流体包裹体均一温度为平均值为86.4℃, δ13CPDB、δ18OPDB平均值分别为-3.15‰及-7.41‰;第三期为粗晶方解石充填阴极发光暗淡,流体包裹体均一温度平均值为96.6℃,δ13CPDB、δ18OPDB平均值分别为-1.73‰及-10.28‰;第四期为重晶石充填.结合区域构造史—热演化史分析表明,钙质结核裂缝中四期充填物分别指示早期脱水、烃源岩早成熟期生烃、加里东构造运动抬升释压及深埋藏期硫酸盐热还原作用事件.%Objectives:Elucidating the mechanism and determining the time of fracture-filling in carbonate concretions in the Lower Cambrian Qiongzhusi Formation in the northern Sichuan of Southwestern China. Methods:Thin sections that were doubly polished and blue epoxy-impregnated were produced to determine lithological components,diagenetic phases and prepared for fluid-inclusion work. The fluid-inclusion measurement was employed for experimentation at Southwest Petroleum University, which involved the use of a THMSG600. Electron probe analysis was employed for experimentation at Southwest Petroleum University, which involved the use of a JXA-8230. Samples for isotopic analysis were microdrilled from the cores. Stable isotopic(δ18O, δ13C) analyses of different periods fracture-filling were carried out at Southwest Petroleum University using the carbonate reaction method. Samples were reacted by the application of the Phosphoric acid bath method at 90℃,and the CO2 generated was examined through use of Elementar IsoPrime GC5. Results:The firstly micritic calcites filled in fractures which have dirty crystal-surface are null luminescent under CL, and the calcite filling fractures also exhibit negative values of δ13C and δ18O, respectively with an average of -4.26‰ and -7.69‰;hints that calcites may have been formed in early dehydration. The secondly fine to medium calcites filled in fractures which are orange luminescent under CL, and fluid inclusion analyses of two-phase aqueous inclusions in calcites suggest that second fracture-filling occurred at temperatures with an average of 86.4℃. The negative values of δ13C and δ18O(respectively with an average of -3.15‰ and -7.41‰)indicates that calcites may form due to hydrocarbon generation by early mature source rocks. The thirdly coarse, blocky calcites filled which are null luminescent under CL, and fluid inclusion analyses suggest that calcites occurred at temperatures with an average of 96.6℃. The negative values of δ13C and δ18O(respectively with an average of-1.73‰ and -10.28‰)inferred that calcites may form due to uplift and depressurization in Caledonian tectonic movement. The barites are formed by sulfate reduction reaction in deep buried stage.