Abstract: Veins in shale fractures record important information about paleofluid and fracture activities, which has crucial indications for the enrichment and preservation conditions of shale gas. This study focuses on the calcite and quartz veins filled in fractures of the Wufeng Formation-Longmaxi Formation shale in the south of Jiaoshiba area, Sichuan Basin. Through thin section observations and cathodoluminescence analyses of rock samples, in combination with the test of in-situ elements, fluid inclusions, and carbon, oxygen and strontium isotopes, this paper analyzes fluid source and formation time of fracture veins. The results show that the fractures of the Wufeng Formation-Longmaxi Formation shale in the south of Jiaoshiba area are successively filled with three stages of vein minerals. Calcite vein is found in the first and third stages, and quartz-calcite vein is in the second stage. For calcite veins of the first stage, the cathodoluminescence color is brown-yellow, the total amount of rare earth elements is low, but the amount of the heavy rare earth elements is slightly rich. For vein minerals of the second stage, the cathodoluminescence color of calcite is brownish yellow, and the total amount of rare earth elements is relatively high. The Eu of calcite in the vein materials of these two stages shows obvious positive anomalies. Further, though analyzing the carbon, oxygen, and strontium isotopes, it is revealed that the vein-forming fluid is derived from the intralayer diagenetic fluid of the Wufeng Formation-Longmaxi Formation shale, and the vein materials are formed in a relative reductive diagenetic environment. For calcite veins of the third stage, the cathodoluminescence color is brown yellow, the total amount of rare earth elements is high, and the heavy rare earth elements are highly enriched, showing obvious negative Eu anomalies. The vein-forming fluid may be sourced from exogenous fluids mixed with terrestrial materials. Based on the formation sequence and petrographic characteristics of the two stages of calcite veins and one stage of quartz-calcite veins and the analysis results of homogenization temperature, burial history and thermal evolution history of the primary aqueous inclusions in calcite and quartz veins, it is determined that the calcites in three stages of vein materials were formed at 177-166 Ma, 156-145 Ma, and 80 Ma, respectively. The quartz in the second-stage vein material was formed at 149-139 Ma, among which the quartz may be derived from the precipitation of silicon-containing fluid in wall rock. The third-stage calcite veins were formed in the uplift stage and its vein materials was formed in an oxidation environment, which may indicate that the preservation conditions of shale gas in the south of Jiaoshiba area have been damaged to a certain extent.

Key words: fracture vein, rare earth element, fluid inclusion, vein-forming fluid, Jiaoshiba area

摘要： 页岩裂缝中的脉体记录了古流体活动和裂缝活动的重要信息，对页岩气的富集和保存条件具有重要的指示作用。针对四川盆地焦石坝南部地区五峰组—龙马溪组页岩中裂缝内充填的方解石脉和石英脉，通过岩石薄片观察和阴极发光分析，结合微区原位元素、流体包裹体和碳、氧、锶同位素测试等技术手段分析了成脉流体的来源和形成时间。研究结果表明，焦石坝南部地区五峰组—龙马溪组页岩的裂缝内依次充填了3期脉体，其中，第1期和第3期为方解石脉，第2期为石英-方解石脉。第1期方解石脉的阴极发光颜色为棕黄色，稀土元素总量低，重稀土轻度富集，第2期脉体中方解石的阴极发光颜色为褐黄色，稀土元素总量较高，这2期脉体中方解石的Eu为正异常；结合碳、氧、锶同位素分析，揭示成脉流体来源于五峰组—龙马溪组页岩的层内成岩流体，脉体均形成于相对还原的成岩环境。第3期方解石脉的阴极发光颜色为棕黄色，稀土元素总量高，重稀土高度富集，表现出明显的Eu负异常，成脉流体可能来自混有陆源物质的外源流体。基于2期方解石脉体和1期石英+方解石脉体的形成顺序和岩相学特征，利用方解石脉和石英脉中原生盐水包裹体的均一温度以及埋藏史和热演化史的分析结果，确定3期脉体方解石的形成时间分别在距今177~166 Ma、156~145 Ma和80 Ma，而第2期脉体中的石英形成于距今149~139 Ma，其中，石英可能来源于围岩内含硅流体的沉淀。第3期方解石脉形成于地层抬升阶段，脉体形成于偏氧化环境，可能指示焦石坝南部地区页岩气的保存条件在一定程度上遭受了破坏。

关键词: 裂缝脉体, 稀土元素, 流体包裹体, 成脉流体, 焦石坝地区