Structural Characteristics And Prospect Of Oil And Gas In Deep Marine Carbonate Strata In South Part Of Western Sichuan Depression

This paper is based on previous study,combining with field investigations,drilling datas and 2-D seismic profiles to analyze surface-buried structures and basic petroleum geological conditions of south segment of West Sichuan Depression,and to summarize the tectonic evolution stages and how it controls basic petroleum geological conditions.At the same time,using structural sand box experiment to simulate the late structural movement in this area,and to discuss the transformation of petroleum geologic conditions under lateral movements and petroleum geological prospects,results show that:?1?Triassic-Quaternary strata outcrop in surface of south segment of West Sichuan Depression,drilling datas and seismic profiles reveal that the Paleozoic-Cenozoic strata in this area are relatively complete,but the Middle-Upper Cambrian-Carboniferous strata are missing.In present plane,the area can be divided into four structural belts:Area?extended frontal deformation zone of Longmenshan,Area?Mingshan syncline concealed structural belt,Area?Xiongpo-Longquan fault-fold belt and Area?Emei-Washan fault-fold belt.?2?The structural evolution process of south segment of West Sichuan Depression are divided into four stages:?1?Formation of Basement Before Sinian?AnZ?:the formation and decomposition of paleoplatform;?2?stable craton stage?Z-T₂?:the cycle of stable sedimentation-weak compression-weak extension under the background of overall uplift-subsidence of passive continental margin;?3?the development stage of continental basin?T₃-E?:continues to accept continental clastic deposits,with weak structural activitivities;?4?Late Himalayan uplift?N-Q?:faults and deformations occur under the influence of anisotropic stress,which is the main finalization period of the present structure.?3?There are three sets of marine oil and gas source-reservoir-cap assemblages in the study area:?1?Jiuluodong Formation?hydrocarbon source rock?+Dengying Formation?reservoir?+Jiuluodong Formation?caprock?;?2?Jiuluodong Formation?hydrocarbon source rock?+Permian Qixia-Maokou-Emeishan Basalt Formation?reservoir?+Middle and Lower Triassic?caprock?;and?3?Jiuluodong Formation?hydrocarbon source rock?+Leikoupo Formation?reservoir?+Upper Triassic?caprock?.in first two sets of source-reservoir-cap assemblages,sourcerocks are in direct contact with reservoirs,while the third type is cross-layer assemblage.?4?The deep hydrocarbon accumulation system in the study area is obviously controlled by structural evolution,which has the characteristics of reservoir modification,migration enhancement,thick capping and favorable source-reservoir-cap assemblage.Reservoir modification refers to:?1?initial karst transformation of Dengying Formation reservoir by Tongwan movement;?2?secondary karst transformation of Dengying Formation reservoir by Caledonian movement;?3?primary karst transformation of Maokou Formation reservoir by Dongwu movement;hydrothermal transformation of Permian Qixia Formation by Emei taphrogenesis and basalt weathering crust.Migration enhancement refers to:?1?Tongwan movement,Caledonian movement and Dongwu movement formed three sets of unconformities,which are important channels for hydrocarbon migration,and the superposition of Tongwan unconformity and Caledonian unconformity improves the efficiency of oil and gas migration;?2?Himalayan tectonic activities formed three types of faults,one of which played a constructive role,enabling deep Sinian and Permian paleogas can migrate along the faults to shallow middle Triassic Leikoupo Formation to form secondary reservoirs.Thick capping refers to:the middle and lower Triassic gypsum-salt rocks with thick layers developed in the study area,due to the influence of tectonic movement,hug thick continental clastic rocks?Upper Triassic-Quaternary?are deposited in this area,so the caprock conditions are better.Favorable source-reservoir-cap assemblage refers to:structural activities in study area have constructive effects on the source-reservoir-cap assemblage.First,the denudation of Caledonian movement resulted in the assemblage of Jiulaodong Formation?hydrocarbon source rock?+Permian Qixia-Maokou-Emeishan Basalt Formation?reservoir?+Middle and Lower Triassic?mudstone and gypsum-salt caprock?.Second,in Late Himalayan period,deep and large buried faults formed in Area?Mingshan syncline concealed structural belt,which enabled deep Sinian and Permian paleogas reservoirs to migrate along the fault to shallow layers,and formed cross-layer assemblages of Jiulaodong Formation?source rock?+Leikoupo Formation?reservoir?+upper Triassic?caprock?.?5?The hydrocarbon accumulation activities in the study area is obviously controlled by structural movement,which is mainly divided into paleo-reservoir stage?Late Triassic-Middle Cretaceous??paleo-gas reservoir stage?Late Cretaceous-Late Paleogene??present gas reservoir adjustment and destruction stage?since Neogene?.Early structural movements control the basic prtroleum geological conditions,but late deformations decide the final hydrocarbon accumulation.Middle and Lower Triassic gypsum-salt rocks play a very important role in structural deformations.In beds above gypsum-salt rocks,structural deformation is intense,faults developed and preservation conditions are seriously damaged.However,structural deformation is weak under gypsum-salt rocks,paleo-structures are well preserved and hydrocarbon accumulation conditions are good,which is conducive to the preservation of paleo-oil and gas reservoirs.Therefore,in the marine strata beneath gypsum salt rocks in south segment of West Sichuan Depreesion,hydrocarbon source rocks?reservoirs and source-reservoir-cap assemblage well developed,exploration prospects are good.Based on the current studys,it is concluded that:The main favorable reservoir-forming areas in study area are Area III Xiongpo-Longquan fault-fold belt?paleogas reservoir?and Area II Mingshan syncline concealed structural belt?paleogas reservoir+secondary gas reservoir?.While in Area I extended frontal deformation zone of Longmenshan and Area IV Emei-Washan fault-fold belt have poor condition of hydrocarbon preservation and poor prospect of hydrocarbon accumulation because structural destruction since Neogenesince.