The Tight Sandstone Reservoir Characteristics Of Xujiahe Formation's 5^th Member In Xinchang Area

Triassic Xujiahe Formation is main gas reservoir in west Sichuan depression,with quite plenty of natural gas resources.The 2^th and 4^th member of Xujiahe Formation have already invested heavily in research work about oil-gas exploration,but the 5^th member hasn't been paid much attention.This article aims at studying sedimentary facies,reservoir physical property,division of diagenesis stage,diagenetic facies,reservoirs controlling factor and distribution of favorable reservoir of T₃X⁵'s each member by observation of rock cores,SEM,casting and conventional thin section inspection and analysis of logging data,XRD diffraction data and mercury injection data,which is based on sedimentology,reservoir geology.According to research,three categories of T₃X⁵ included in sandstone are lithic quartzite sandstone,lithic sandstone and Feldspar lithic sandstone,mainly is fine or extremely fine particle size,fairly bad sorting and fairly bad roundness.The content of feldspar is low in clastic compositions.The cement is the main component of the interstitial material,which is combined first by carbonate mineral and siliceous cementation,and next by clay mineral.The reservoir pore type of target strata in study area was combined by intergranular dissolved pore,intragranular dissolved pore,mold porosity,fracture and less primary porosity.Porosity is mainly distributed in 0.4⁸%,with an average of 2.85%.The permeability is mainly distributed in the 0.002⁰.902×10^-3?m²,with an average of 0.064×10^-3?m².It is totally a very low porosity and permeability sandstone reservoir.The reservoir diagenesis of target strata in study area include compaction,cementation,metasomatism and corrosion,compaction and cementation are the main causes of tight sandstone reservoir,while corrosion is beneficial to improve reservoir physical property.The sandstone reservoir of target strata is in middle diagenesis stage A,and some are in middle diagenesis stage B.Based on the analysis of core observation and logging data,it is concluded that the diagenetic facies of this area can be divided into 5 categories,which are unstable dissolution facies,siliceous cementation facies,filling of clay mineral facies,compaction facies,carbonate mineral cementation facies.Combined with the analysis of sedimentary facies,unstable denudation facies,siliceous cementation facies developed in distributary channel of fan delta front,and the filling of clay mineral facies,compaction facies,carbonate mineral cementation facies developed in the underwater distributary channel and bay or littoral-shallow lacustrine.Combined with reservoir evaluation standard,the reservoir of target strata are divided into 4 types.The type I and II which the porosity is generally greater than 3%and the permeability distributed in the0.01¹×10^-3?m²,mainly developed in the west and south area,some developed in the northeast of the area.