Research On Accumulation Characteristic And Model Of Deep Buried Tight Sandstone Gas In The Kuqa Depression

In this dissertation,aiming at the geochemical problems of accumulation in the deep tight sandstone gas exploration in Kuqa Depression,the organic geochemical characteristics and gas-prone property of source rocks in Kuqa Depression were studied by means of petrology of source rocks,pyrolysis of source rocks and thermal simulation on the basis of sample collection.Based on the analysis of components and carbon isotope characteristics,the origin and source of deep-buried tight sandstone gases were further studied.Through the analyses of reservoir physical properties and diagenetic evolution sequence,the pore evolution process is quantitatively revealed.Based on inclusion lithofacies,microthermometry,quantitative grain fluorescence,the hydrocarbon charging periods in deep-buried gas reservoirs were systematically expounded.According to the sequence of reservoir densification and natural gas charging and the anatomy of typical gas reservoirs,the types of deep tight sandstone gas reservoirs were classified,the genetic mechanisms and main controlling factors of gas reservoirs were clarified,and then the reservoir accumulation models of different types of gas reservoirs were revealed.The main achievements and understandings of this dissertation are as follows:(1)Jurassic-Triassic source rocks are well-developed in Kuqa Depression.The thickness in central Baicheng Sag and Yangxia Sag is generally over 400 m,and the gas generation intensity is over 70×108 m3/km2.The maturity of source rocks in the center of Baicheng Sag is over 3%,showing feature of high maturity.The kerogen in source rocks is mainly type III and has obvious gas-prone property.Source rocks in Kuqa Depression have better gas generation characteristic.When the temperature increases to 550?600? at 2?/h rate,the methane yield of Jurassic coal-bearing source rocks exceeds 158 mL/g·TOC,and the maximum of lacustrine source rocks is 84 mL/g·TOC.Generally speaking,the gas supply conditions of source rocks are superior,which providing gas sources for deep tight sandstone gas reservoirs.(2)In the Kuqa Depression,methane is the main component of deep tight sandstone gas,which is 82.11%?98.50%;the heavy hydrocarbon gas(C2+)is mainly between 0%and 8.9%,showing the the coexistence of wet gas and dry gas.The carbon isotopes of alkane gas components are generally heavier,?13C1 is-36.9‰?-27.6‰,?13C2 is-27.6‰?-16.3‰,?13C3 is-25.87‰?-15.7‰ which are mainly in positive carbon isotope sequence but with partial reversal.Natural gas maturity ranges from 0.66%to 3.03%,the larger numerical differences imply that natural gases were derived from source rocks with different maturity.Comprehensive analysis shows that the deep-buried tight sandstone gases in Kuqa Depression belong to mature,high-over mature coal-derived gases,which were mainly sourced from Jurassic coal-bearing source rocks,but the contribution of Triassic source rocks can not be ignored.(3)The main gas-bearing intervals in Kuqa Depression are Jurassic,Cretaceous and Paleogene,which are mostly low porosity and low permeability reservoirs.The main frequency distribution of porosity and permeability are 2%?8%and 0.01?0.1×10-3um2,respectively,which are lower than shallow area in Kuqa Depression.Deep reservoirs have undergone syngenetic-supergene and early diagenesis,and now in the early stage of middle diagenesis.According to the diagenetic evolution sequence and pore evolution model,it is considered that the compaction times of Jurassic and Cretaceous reservoirs are earlier,which occurrd in the middle and late stages of Kangcun Formation depositional stage.The Paleogene reservoir densification is relatively late,which occurrd in the late of Kuqa Formation depositional stage.Compaction is the majority to reservoir densification and run through the densification process.(4)Fluid inclusions,including aqueous inclusions,oil inclusions and gas inclusions were observed in deep tight sandstone reservoirs in Kuqa Depression.The homogeneous temperature of fuild inclusions ranges from 81.5 to 176.4?,and salinity is 1.39%-26.3%.Quantitative grain fluorescence shows that the early chargd oil in deep reservoirs was destroyed or even disappeared under later gas invasion.Based on the analysis of inclusion petrography,microthermometry,quantitative grain fluorescence,hydrocarbon generation kinetics and authigenic illite K-Ar isotope dating,it is considered that there are two stages of oil and gas charging in deep-buried tight sandstone reservoirs in Kuqa Depression.Oil charging mainly occurred from late Jidike Formation to the early and middle of Kangcun Formation,and gas charing mainly occurred in the Kuqa Formation.(5)Two types of deep tight sandstone gas reservoirs were classfied in Kuqa Depression.The "first densification and then accumulation" gas reservoir was dense when gas charging and capillary and gas expansion force are migration forces,which distributed in the basin margin,deep depression and anticline.The accumulation process is:reservoir compaction?hydrocarbon generation and expulsion of source rock?natural gas charging and accumulation?structural adjustment.The "first accumulation and then densification" type gas reservoir was not dense when the gas charging and the migration forces are buoyancy and hydrodynamic force.Gas distribution is controlled by structure,and the lower water layer of the gas reservoir has a unified interface.The accumulation process is:hydrocarbon generation and expulsion of source rock?natural gas charging and accumulation?reservoir compaction?structural adjustment.(6)The main controlling factors of "first densification and then accumulation"type gas reservoir are better source rock conditions,widespread "stable zone",better"fault cap" combination,and the accumulation model is "oil charging and reservoir densification occurred in the middle and late stages of Kangcun Formation sedimentary period,gas invasion in the Kuqa Formation and gas reservoir formed after structural modification".The main controlling factors of "first accumulation and then densification" type gas reservoir are better source rock conditions,favorable relatively high porosity and permeability reservoirs,relatively complete regional cap rocks,existence of structural high point and dominant migration channels,and the accumulation model is "oil charging in mid-sedimentary period of Kangcun Formation,mature gas charging in sedimentary period of Kuqa Formation and reservoir densification in late Kuqa Formation,and then gas reservoir formed by structural transformation".