| The Upper Triassic Xujiahe Formation in the Sichuan Basin has abundant natural gas resources and superior accumulation conditions.It is difficult to explore because of great burial depths,super compact and strong heterogeneity.The second member of Xujiahe Formation(Xu2 member)was found to have the most reserves,but the scale of development was small.In the early stage,the study of Xu2 member reservoir mainly focused on the recognition of fractured reservoir;In recent years,porous reservoir(including pore type and fracture?pore type)was found in Xu2 drilling of West and Middle Sichuan Basin,which has the potential to accumulate large-scale reserves and stable production of gas.Therefore,it is of great significance for the exploration and development of Xujiahe formation which make clear the basic geological characteristics and formation mechanism of porous reservoirs in the tight sandstone of Xu2 member.This paper investigates a large number of domestic and foreign related literature and previous research results.According to the drilling,logging,core and experimental analysis data in the study area,the sedimentary microfacies and reservoir properties of the tight sandstones in the Xu 2 Membe are analyzed,and the porous reservoir is defined.Based on the previous analysis,the geological characteristics,microscopic geochemical characteristics of porous reservoirs are studied,the evolution and formation mechanism of porous reservoirs are discussed,and the main controlling factors,development models,and favorable exploration direction of porous reservoir are summarized.The main research results are as follows:(1)The sedimentary microfacies in Xinchang area of foreland depression belt and Hechuan,Tongnan areas of foreland uplift belt is dominated by underwater distributary channel and estuary dam in Xu 2 Member sandstones;The sedimentary microfacies in Jinhua,Qiulin and Zhongtai area of foreland slope belt is dominated by estuary dam and distal bar,and no distributary channel is developed.According to the characteristics of tight sandstone reservoir in Xu2 member,the physical of the foreland depression belt are poor,and the reservoir space is dominated by intragranular dissolution pores;the physical of the foreland slope belt are medium,and the reservoir space is dominated by intragranular dissolution pores,but contains a small amount of residual primary pores;the physical of the foreland uplift belt are good,and the reservoir space is dominated by residual primary pores and intergranular dissolution expansion pores.The classification evaluation criteria of porous reservoir in different structural zones were determined,the class I,class II and class III pore type and fracture?pore type reservoirs are referred to as the generalized porous reservoir,in which the lower limit of effective porosity in the foreland depression zone,the foreland slope zone and the foreland uplift zone are 4%,5.5% and 6%,respectively.(2)The basic geological characteristics of the porous reservoirs in the Xu 2 Member are analyzed by means of thin section observation and SEM.The foreland depression belt is mainly composed of lithic sandstone,lithic quartz sandstone and feldspar lithic sandstone,and the diagenesis is mainly compaction and authigenic quartz cementation.The diagenesis stage has entered into late diagenesis stage;foreland slope belt and foreland uplift belt is mainly composed of feldspar lithic sandstone,but the diagenesis of slope belt is mainly compaction and dissolution,and the diagenesis stage has entered the middle diagenesis stage B;the diagenesis of foreland uplift belt is mainly chlorite cementation and dissolution,and the diagenesis stage has reached the middle diagenesis stage B,some of them are in the middle diagenesis stage A.According to its main geological characteristics,porous reservoirs are divided into three types: hydrocarbon filling + chlorite + dissolution + fracture type,hydrocarbon filling + chlorite + dissolution type,fracture + dissolution type.(3)The pore evolution characteristics of porous reservoirs in Xu2 member of each structural belt are inversed in time and quantity.In the foreland depression belt,compaction and cementation are the main ways to reduce the porosity,and the initial porosity decreases rapidly,the dissolution in the Late Triassic and early Jurassic improves the porosity greatly,and the reservoir is densified in the late Jurassic;The foreland slope belt also suffered relatively strong compaction and siliceous cementation which reduce porosity,but the middle Jurassic dissolution increased the largest amount of pores,until the early Cretaceous reservoir densification;The coating,liner chlorite,and early quartz overgrowth maintained the porosity in foreland uplift belt,and the initial porosity decreased the slowest,the late Jurassic organic acid dissolution increased part of the pores,and the reservoir densification in Late Cretaceous.The densification time of each structural belt is later than the period of oil and gas accumulation.(4)The formation mechanism of porous reservoirs and the diagenetic fluid evolution characteristics were analyzed by macro and micro geochemical methods such as burial history,SEM,energy spectrum,X-diffraction,quartz oxygen isotope,calcite carbon oxygen isotope and electron probe.The formation mechanism of porous reservoirs includes slow compaction under favorable burial mode and low geothermal gradient;early hydrocarbon filling,early coating,liner chlorite and early quartz overgrowth maintain the pore;multi-stage dissolution.The porous reservoirs of Xu2 member in the study area has experienced(shallow buried)acid diagenetic environment?(shallow buried)alkaline diagenetic environment?(organic acid enters)acid diagenetic environment?(deep buried)alkaline diagenetic environment.(5)The main controlling factors,development mode and exploration direction of the porous reservoirs in Xu2 member are summarized.The foreland depression belt contains the most sandstones and hydrocarbon source,and the main controlling factors of reservoir are clastic composition,dissolution and fracture.When looking for porous reservoirs,we should pay attention to the relationship between fault and source rock configuration in the Late Triassic-early Jurassic dissolution period.The hydrocarbon source of foreland slope belt is relatively developed,and it is close to the hydrocarbon center of the depression belt in Western Sichuan,and the sandstones is also relatively developed,the main controlling factors of reservoir are clastic composition,dissolution,paleo-uplift and fracture.When looking for porous reservoirs,we should pay attention to the paleo-uplifts in the middle-late Jurassic dissolution and hydrocarbon emplacement periods,and superimpose fault fractures.The hydrocarbon source and sandstones in the foreland uplift belt is inferior to that in the depression belt and slope belt,and the main controlling factors are clastic composition,sedimentary microfacies and fractures.When looking for porous reservoirs,we should pay attention to high-energy sedimentary microfacies and superimpose the fractures formed by diversity compaction in the ancient residual hills at the top of Leikoupo Formation. |
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