鄂尔多斯盆地大牛地气田奥陶系马家沟组裂缝充填特征及成岩流体性质OA

Research on hydrocarbon-bearing fluids is one of the key and difficult aspects of hydrocarbon accumu-lation studies,and fracture fillings record important information about hydrocarbon-bearing fluids.Petrological characteristics of cores,thin sections,and cathodoluminescence images from the Ordovician Majiagou Formation in the Daniudi Gas Field,Ordos Basin,were analyzed to determine the formation stages of fractures and the types of fillings.Laser Raman spectroscopy and microthermometry of inclusions in fracture-vug calcite veins were per-formed.Carbon and oxygen isotopes and rare earth elements(REEs)were also analyzed to identify the types,properties,and sources of hydrocarbon-bearing fluids.As indicated by the research results,multistage tectonic fractures are developed in the Majiagou Formation,and these fractures cut and offset each other.Fracture fillings differ among different intervals.In the weathering crust interval,fractures are filled with calcite,dolomite,mud-stone,limestone breccia,and other materials.Fractures in the inter-salt and subsalt intervals are mainly filled with calcite,and minor quartz filling occurs in the subsalt interval.Three stages of oil-gas inclusions were trapped in the calcite veins,and their carbon isotopic compositions show a marked negative shift due to the influence of hydrocarbon-bearing fluids.This indicates that fractures served as important pathways for hydrocarbon migration and reveals a favorable coupling between tectonic activity and source rock hydrocarbon generation.The calcite veins in the Majiagou Formation exhibit negative oxygen isotopic shifts and positive δCe anomalies.The contents of trace elements such as Mn and Sr are comparable to those of the host rocks,and the REE distribution patterns are generally consistent with those of the host rocks,characterized by enrichment in light rare earth elements and depletion in heavy rare earth elements.These features suggest that the calcite veins formed during the burial stage,with diagenetic fluids characterized by high temperatures and reducing conditions.Calcite veins in the study area generally show negative δEu anomalies and lack typical magmatic hydrothermal minerals,indicating that the diagenetic fluids in fractures were dominated by formation water.However,minor local input of magmatic hydrothermal fluids occurred,as indicated by positive δEu anomalies in some samples and minor quartz filling in the subsalt interval.