Study On Lithofacies And Palaeogeography Of Cambrian Xuchi Formation In Sichuan Basin And Its Surrounding Areas

The paper is based on the lithofacies paleogeography research of the Cambrian Xixiangchi Group of Sichuan Basin and its adjacent areas. Based on previous dates and relative research achievements, in order to123geological sections and41borehole dates as the main basis, using the correlation theory of lithostratigraphy, biostratigraphy, carbonate petrology and sedimentology etc, single factor analysis and multifactor comprehensive mapping method are adopted to compile Xixiangchi Group lithofacies-paleogeographic map, and the evolution and distribution characteristics of sedimentary period of lithofacies-paleogeographic will be uncovered for the study area of oil and gas exploration to provide reference. The main results achieved and understanding are as follows:Our analysis of the region geological background and the character of the residual thickness extension of Xixiangchi group suggests that the strata of Xixiangchi group generally are with the characteristics which are thin in the northwest while thick in the southeast and which have interphase reliefs and depressions. This area is neighboring Kangtian ancient land to the west,Motianling and Hannan ancient land to the north. The south and northest parts are respectively surrounded by Qianzhong Paleouplift underwater of Renhuai-Zunyi-Meitan and belt highlands underwater of Zhenba-Chengkou-Wuxi-Shennongjia. However,the inner basin has Leshan-Longniisi Paleouplift, two secondery depressions of Bazhong-Tongjiang and Chishui-Chuanjiang-Shizhu in southeast basin. Northeast and southeast of the study area are respectively neighboring Qinling Ocean and Jiangnan Basin.Carbonate rocks are the dominant rock type in the study area, subsequently the terrigenous clastic rocks and the metamorphic rocks. The main rock type of the carbonate rocks is dolomites,according to the features between composition,texture and structure, it can be divided into grain dolostone,fine-to mid-crystallized dolostone and mud-powder crystal dolostone; Limestones is distributed mainly in the eastern part of the study area, contain grian limestones, plaster limestone and transitional type; terrigenous clastic rocks is distributed mainly in the northwest part of the study area, include sandstones,siltstones and clay rock; The metamorphic rocks mainly include slates, phyllites and schists.Based on comprehensive research on field outcrops and wells from the rock type, sedimentary structure, sedimentary and logging response characteristics,and on previous research results and geological data, many kinds of sedimentary environment develop in the Cambrain Xixiangchi Group,such as ancient land, carbonate platform, platform margin, slope, basin, and so on, of which the carbonate platform is widely distributed and can be divided into diamictic tidal flat, platform interior flat, restricted lagoon, demi-restricted lagoon, evaporating-demi-restricted lagoon and platform interior shoal.Combined with the analysis above, taking the "single factor analysis and multifactor comprehe-nsive mapping method" as the methodology, sedimentary facies pattern were modeled after research on the lithofacies palaeogeography in the Cambrian Xixiangchi Group.Main viewpoints are as follows:ancient land exists to the northwest Sichuan Basin in Cambrian Xixiangchi period;restricted lagoon dominates the main sedimentary environment in Sichuan Basin, with low energy and limited environment, in which the grey-deep grey argillaceous dolomite and micrite dolomite are the main sediments; diamictic tidal flat with detrital material of terrigenous origin and carbonate develops in E’mei and Leshan southwest Sichuan Basin; evaporating-demi-restricted lagoon develops in Jiansha-Meitan area to the south of Sichuan Basin; limestone deposits as equal to dolomite in Zhenxiong-Gongxian area to the southwest of Sichuan basin and Yanhe-Xianfeng-Wufeng area to the east of Sichuan Basin, which belong to the demi-restricted lagoon with relatively open water; shoal is widely distributed in Leshan-Longnvsi paleo-uplift area, Jinsha-Meitan-Zunyi area of paleo-uplift of central Guizhou, Chengkou-Shenlongjia area to the northeast of Sichuan Basin and the highland underwater-Jiangkou-Yongshun-Sangzhi area to the northeast of Sichuan Basin; the northeast and southeast of Sichuan Basin are respectively transition to semi-deep slope and deep basin.