The Forming Conditions Of Unconventional Gas Of The Fifth Member Of Xujiahe Formation In The Western Sichuan Depression

The exploration and production of in-source continental shale gas and near-source tight sand gas reservoirs are still in the early stage in China,therefore it is necessary to analyze the formation mechanisms,the accumulation status,the main control factors of gas content,and the evaluation standards of shale gas and tight sand gas of the fifth member of Xujiahe formation—a composite shale gas reservoir.To research on the sedimentation,reservoir and accumulation of the fifth member of Xujiahe formation in the Western Sichuan Depression,then to explore the research methods of"in-source"continental shale gas,and to summarize the conditions for the forming conditions of the continental shale gas.So it will effectively promote the breakthrough of unconventional gas exploration,realize the resources replacement and reserve growth.The following conclusions and understanding are obtained:(1)The fifth member of Xujiahe shale gas formation can be divided into three submembers and nine layers.There are three layers in the upper submember,mainly interbedded sandstone and mudstone;There are four layers in the middle submember,mudstone(shale)with lenticular sandstone;There are two layers in the lower submember,mainly interbedded sandstone and mudstone.(2)The distribution of underwater distributary channel and mouth bar,which is at the bottom part of the upper and middle submembers and lower submember,in the fifth member of Xujiahe formation is limited in the Western Sichuan Depression.with strong reworking of sand body,thin thickness(mostly less than 10 meters),discontinuous distribution,fast lateral change and strong heterogeneity.Mudstone(shale)is rare,thin thickness of single layer is thin(mostly less than 10meters),but its cumulative thickness is thick.(3)There are three sets of relatively stable mudstone(shale)in the fifth member of Xujiahe formation.The top of middle submember(TX₅⁴)association and the top of lower submember(TX₅⁷/₅⁸),whose source of the unconventional gas accumulation is good in the fifth member of Xujiahe formation.(4)The fifth member of Xujiahe formation has the characteristics of fine grain,thin layer,overlapping and compacted,so the reservoir properties are poor.The shale of the fifth member of Xujiahe formation is organic matter-rich,nanoscale pores and micro-fractures.Its porosity and permeability are higher than sandstone,however the shale has high clay content and strong plasticity;the pore type of sandstone is complicated,and the pores are mainly secondary pores.Micro-fractures are mainly diagenetic fractures and structural fractures.The shale has strong diagenetic evolution,high brittle mineral content,more than 50%quartz composition and medium-high brittleness index.The high-energetic distributary channels and mouth bar have high sandstone thickness,abundant micro-fractures and high gas content.The favorable reservoir model in the fifth member of Xujiahe formation is a combination of high quality shale,high energy depostion of sandstone,sandstone-rich lithologic association and micro-fractures developed zones.(5)The natural gas in the fifth member of Xujiahe shale gas formation is mainly hydrocarbon gas,whose methane content and dry coefficient change a lot and the average value is low.It is typical coal type gas and has characteristics of mature to medium maturity thermogenic gas.The natural gas is a mixture of free gas and adsorbed gas.The adsorbed gas is more than 70%and free gas is less than 30%.(6)The vertical distribution of the fifth member of Xujiahe formation is dominated by high quality shale,which has near source reservoir system and near source accumulation.Deep lacustrine face,underwater distributary channel and mouth bar in the delta front are rich in natural gas.The difference of fault distribution,hydrodynamic field and fracture distributions determine the location of sweet points.(7)The two target layers of the fifth member of Xujiahe shale gas formation:one is the interbedded sandstone and mudstone deposition in the TX₅⁷/₅⁸,the other is the TX₅³ sandstone association,whose source rock is the coastal marsh in TX₅⁴.The two target layers are evaluated to be good in the areas of Majing-Shifang,Chongzhou-Pixian and Xinchang-Fenggu.