Distribution Of Sandstones With Chlorite-Cemented Facies Of The Upper Triassic Xujiahe Formation,Yuanba And Tongnanba Area

The tight sandstone reservoirs in the Upper Triassic Xujiahe Formation has become a key target for gas exploration in the Sichuan Basin.The Yuanba and Tongnanba area is located in the northeastern part of the Sichuan Basin,and the sandstone reservoirs has poor qualities,and only those with chlorite rim show a higher porosity.Therefore,the distribution of this kind of reservoirs plays a significant role in the gas exploration in the Yuanba and Tongnanba area.Based on lithology,geochemistry and well-logging geology,petrological and diagenesis characteristics of the Xujiahe sandstones in Yuanba and Tongnanba area were systematically analysed,and development conditions and control factors of formation of chlorite rim are identified,and thus to predict the distribution of favorable pore zones.The main rock types of the Xujiahe sandstones are litharenite and feldspathic litharenite.Most Xujiahe sandstones are typified by medium texture maturity and lower composition maturity.Chlorite-cemented facies usually developed in feldspathic litharenite and litharenite.With chlorite's content of more than 1.5%,other authigenic cementations such as quartz and calcite are poorly developed,and physical properties are generally better(porosity usually more than 5%).This diagenetic facies is identified as chlorite-cemented facies.Chlorite-cemented facies usually developed in distributary channel deposit,with lower content of matrix(<5%)and a certain amount of volcanic rock fragments(2~6%).Chlorite-cemented facies are mainly distributed in the lower T3x~2 member in the northwest area and the upper T3x~4 member in the east area of the Yuanba area.