塔里木盆地旗探1井寒武系−震旦系源储系统流体演化与油气成藏机制OA

[Objective]Deep-to-ultra-deep hydrocarbon reservoirs are characterized by long-term fluid evolution histories and complex hydrocarbon accumulation mechanisms.The Tarim Basin is a typical exploration target for deep-to-ultra-deep hydrocarbons in China.However,only a small number of boreholes have penetrated the Lower Paleozoic Sinian-Cambrian source rocks and reservoirs.At present,direct isotopic chronological constraints on the complete source-to-reservoir fluid evolution and hydrocarbon charging processes remain scarce,which restricts the in-depth understanding of hydrocarbon accumulation mechanisms at depths of 10 000 m within the Tarim Basin.[Methods]This study focused on fracture-and vug-filling vein minerals in core samples from the Cambrian Yuertusi Formation(source rock)and the Sinian Qigebulake Formation(reservoir)of Well Qitan-1(QT1).The fluid sources and genesis of multiple generations of veins were identified.Multiple analytical approaches were integrated,including petrographic observation of fluid inclusions,micro-in-situ U-Pb dating of carbonate veins,Re-Os isotopic dating of solid bitumen,REE-Sr isotopic geochemistry,fluid inclusion microthermometry,and basin burial-thermal-history modelling,to reconstruct the dynamic hydrocarbon accumulation sequence of deep source-reservoir systems.[Results]Two generations of fracture-filling calcite veins were developed in the Cambrian Yuertusi Formation source rocks of northern Tarim.REE and Sr-isotopic signatures indicated that both calcite veins were of hydrothermal origin,precipitated from deep Sr-rich fluid systems.Correspondingly,two generations of pore-filling dolomite veins occurred in the Sinian Qigebulake Formation reservoir.Geochemical proxies indicated that these dolomite veins precipitated from formation diagenetic fluids,and the Sr-isotopic compositions of the second-generation dolomite veins recorded fluid mixing with contemporaneous Cambrian seawater.The first-generation calcite vein in the source rocks((466±5) Ma)and the first-generation dolomite vein in the reservoirs((460±10) Ma)crystallized during the Middle Ordovician.These early-formed veins were later modified by hydrocarbon charging and trapped oil inclusions,which recorded the primary oil-charge event during the Permian(Hercynian),indicating good source-reservoir matching.Burial-thermal modelling revealed that the Yuertusi Formation source rocks reached the oil-generation threshold in the Carboniferous.Both the second-generation calcite vein in the source rocks((263±69) Ma)and second-generation dolomite vein in the reservoirs((55±15) Ma)contained abundant oil inclusions.Constrained by burial-history simulation,the large-scale secondary hydrocarbon charging event took place in the Miocene,showing good spatiotemporal coupling between hydrocarbon generation and accumulation.A large amount of solid residual bitumen occurring after the second-generation dolomite veins in the reservoirs yielded a Re-Os age of(30±14) Ma,documenting the destruction and readjustment of Sinian paleo-hydrocarbon reservoirs in the early Oligocene driven by Himalayan tectonic uplift.[Conclusion]This study conducts systematic isotopic-chronology and fluid evolution analyses for deep source-reservoir intervals in the Tarim Basin.It provides direct experimental evidence for reconstructing the whole process of hydrocarbon generation,expulsion,accumulation,and preservation within the Lower Paleozoic Sinian-Cambrian source-reservoir system.