Hydrocarbon Migration And Accumulation And Its Enrichment Characteristics In Southwest Depression Of The Tarim Basin,NW China

Southwest Depression of the Tarim Basin has great exploration potential.However,the overall exploration level is relatively low due to the complex structure of foreland thrust belt and the harsh surface environment.The research of the petroleum geological conditions and hydrocarbon migration and accumulation characteristics should be strengthened,and it is necessary to adopt comprehensive research methods to clarify its potential and exploration direction.Accordingly,this paper analyzed the petroleum geological characteristics in Southwest Depression of the Tarim Basin.Three technologies,including source rocks evaluation,basin modeling and 2D modeling of hydrocarbon migration and accumulation,were used to carry out simulation research on the key period of hydrocarbon accumulation,the process of hydrocarbon migration and accumulation,and resource potential in the depression,analyze matching conditions and enrichment characteristics of hydrocarbon accumulation,define favorable exploration zones of Southwest Depression of the Tarim Basin.Source rocks evaluation shows that four sets of effective source rocks in the depression,including the lower Cambrian Yuertusi Formation,Carboniferous,Permian and middle-lower Jurassic,are of high abundance,with medium or above quality,and the source rocks are generally at the stage of maturity-high maturity.The characteristics of eight sets of reservoirs,caprocks,reservoir-cap rock assemblages and main petroleum reservoirs in the depression were analyzed systematically.Basin modeling shows that Southwest Depression of the Tarim Basin experiences three stages of subsidence.Since the Early Paleozoic,the paleotemperature generally shows a slow downward trend.The thermal evolution of each set of source rocks is closely related to the subsidence period and subsidence thickness.The subsidence rate becomes fastest since Neogene.Thick strata are deposited,and the source rocks are deeply buried at the late stage,generally characterized by multi-stage hydrocarbon generation,early-stage oil and late-stage gas generation.The generated oil is 399×10⁸t,the generated gas is 95×10¹²m³,the oil resource is(5.95?6.25)×10⁸t,and the natural gas resource is(1.36?1.41)×10¹²m³.The plane streamline modeling indicates that oil and gas mainly accumulate around the hydrocarbon generation center.Based on the migration and accumulation modeling results of four key sections,the whole process of hydrocarbon migration and accumulation in Southwest Tarim depression was reconstructed.It is shown that hydrocarbon mainly migrates vertically.The faults that connects source rocks with reservoirs are the main channels for the long-distance vertical hydrocarbon migration,and the regional unconformities provide good channels for the lateral hydrocarbon migration.Long-term inherited tectonic paleo-uplifts and regional slope-break zones are favorable directions for hydrocarbon migration and accumulation.Accumulated oil and gas also continue to adjust with the migration of structural highs due to late tectonic movements.The static hydrocarbon geological conditions are generally controlled by the spatial-temporal matching of regional tectonic evolution,which results in four hydrocarbon enrichment characteristics in Southwest Depression of the Tarim Basin:(1)near-source accumulation in the piedmont belt and slope area,(2)effective faults that connects source rocks with reservoirs and regional unconformities controlling accumulation,(3)accumulation in high-quality reservoir-cap rock assemblages and stable matching traps,(4)multi-stage accumulation,late-stage adjustment and enrichment under tectonic setting.Finally,several favorable exploration zones are chosen out:Kekeya-Kedong Structural Belt(K-N),Wuqia-Kelatuo Structural Belt and its periphery area(K),the two flanks of Qimugen Arc Protrusion(K-E),Qunguqiake Structural Belt((?)-O,D-C),Luonan-Niaoshan Structural Belt((?)-O)and Mazhatage Structural Belt((?)-O,C).