| The Sichuan Basin,which is located on the ancient plate of the Yangtze,is a polycycle congruence sedimentary basin.It has led to the evolution of the Sichuan Basin,which has the sedimentary structure of three sags circumfluent one wale,after the Middle Triassic and Late Triassic Indosinian movement.Based on the plane section of field actual measurement,descriptingthe coresand analysis thin slice,counting the data of the cores,the electron microprobe and the scanning electron microscope are expounded,the stratigraphic and sedimentary-petrological characteristics of the Xujiahe Formation in the central Sichuan area are discussed.Also from the perspective of sedimentary diagenesis,the study area is further discussed,It can help to predict the favorable reservoir location in the area.Finally,according to the paleogeothermal calculation method of authigenic chlorite in clay minerals,It can obtainthe paleogeothermal temperature and its gradient evolution in this area,which providessome theoretical basis for recoveringthe thermal-alteration history of sedimentary basins.The main conclusions are as follows:(1)The study shows that,the reservoir rocks of Xujiahe Formation of the Upper Triassic in the central Sichuan area,which are dominated by lithic feldspar sandstone,feldspar lithic sandstone,feldspar quartz sandstone and lithic quartz sandstone.By the analyzing of limestone components in the sandstone,shows that the single crystal quartz has the most,followed by flint,and other crystalline quartz appears little;the total content of feldspar is low,with the potassium feldspar and plagioclasebased,and the content of plagioclase is less than that of K-feldspar in general.The content of bit cuttings is high,and the three major cuttings are found in the study area.The sedimentary rock has the highest amount of cuttings,and the relative content of themagmatic rock cuttings is little.(2)On basis of geochemistry analytic method,It shows thatis relative enrichment of light rare earth element and the relative loss of the heavy rare earth about the Xujiahe Formation in the central Sichuan area;The REE elemental partition of Xu 2 member was familiar with Xu 4 member,The distribution pattern of Xu 6 member was obviously different from the first two,it indicated the provenance of Xu 6 member was different fromXu 2 member and Xu 4 member;the drawing picture of the main element shows that the sedimentary environment of Xujiahe Formation,integrally is manifested as the tectonic background environment of passive continental and active continental margin.(3)After the process analysis a lot of the thin slice,the results indicated that the diagenetic processes of the Upper Triassic Xujiahe Formation have experienced similar diagenetic processes,and discussed the characteristics of diagenesis,such as compaction pressure dissolution,cementation(metasomatism);build the pore evolutionary,and divided five major diagenetic facies.It is believed that chlorite-siliceous cementation and dissolution of siliceous cementation are two favorable diagenetic facies,which can help to predict favorable reservoirs in thelater.(4)According to the case of enrichment of authigenic chlorite in the Xujiahe Formation,the formation temperature byauthigenic chlorite calculated was 157 ~ 243 ?,and the average temperature was 165 ~ 213 ?.The results of the paleo – geothermal gradient is 34 ? / Km,and it is compared with the commonly used paleogeothermal method,the data is in agreement with those of previous studies which is using different methods,which can provide a favorable theoretical basis for studying the thermal-alteration history of sedimentary basins in China. |
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