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

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