Study On Lithofacies Palaeogeography Of The Lower Cambrian In The Sichuan Basin

Cambrian marine strata in South China were developed,Cambrian are widely distributed and well outcropped,the horizon is completed and rich in rock types,both dark fine clastic rocks which can be hydrocarbon source rock and carbonate rocks can be good a reservoir in certain conditions,in addition,the salt rock as a quality cover were developed too.Therefore,the Sichuan basin is one of the most important marine exploration areas in south,which has a very optimistic prospects in oil an gas.The study of lithofacies palaeogeography is the basis of the petroleum exploration.The distribution and combination of effective hydrocarbon source rock,reservoir and cap rock are controlled by lithofacies palaeogeography.According to the detailed and systematic studies,it is hopeful to ascertain the distribution and evolution of the lithofacies palaeogeography,clarify the relation between lithofacies palaeogeography and the development of the resource rocks,reservoirs and cap rocks,and these detailed basic geological data can be provided for the oil and gas exploration.Previous studies on the lithofacies and palaeogeography of the Cambrian in the study area are also more,but with more and more drilling,seismic and logging data,the relevant research on the core,geochemistry,etc are updated.Therefore,under the five-year national major project "Study on accumulation conditions and favorable exploration area of marine carbonate rocks evaluation in South China",and on the basis of quantitatives single factor analysis,the Lower Cambrian stratas can be studied detailed and multifactor comprehensive mapped with previous research results and the latest qualitative and quantitative data which guided by Sedimentology.And then from the angle of lithofacies palaeogeography unit,the favorable area of the reservoir cap is optimized,and the favorable exploration zone is divided.The type of rocks in Lower Cambrian of Sichuan basin is various,which can be divided into three types: clastic rock,carbonate rock and gypsum salt rock while the dominant type is dolomite of carbonate rocks.Clastic rock mainly develope in Qiongzhusian Stage and Canglangpuan Stage,inside,the coarse conglomerate and sandstone are mainly distributed in the northwestern margin of the basin,the fine siltstone is in the whole basin and the shale is mainly in the southeast margin.The carbonate rocks mainly develope and spread in Longwangmiaoan and Canglangpuan stage,in which the limestone can be divided into grain limestone,plaster limestone and reef limestone.Oolitic limestone is the main type of grain limestone;Limestone plaster is generally formed in quiet water;The reef limestone is rare and develope in theplatform edge.Dolomite including particle dolomite and crystalline grain dolomite which is most widely distributed in the whole basin.Among them,the sizing and rounding of particle dolomite in middle and southwest Sichuan is better than in other regions.The crystalline grain dolomite includes micritic dolomite,saccharoidal(fine,microtek)dolomite and granophyric dolomite,and sometimes coarse grained dolomite is filled in fractures of the granular dolomite.Gypsum salt rocks are mainly developed in Longwangmiaoan stage,located alone the line of Zhuazhuayan in Leibo and Wangjiawu in Jinyang,as well as the southeastern of Sichuan uplift.It can be divided into salt rock and gypsum rock,and the gypsum rock can be divided into ten types: pure gypsum rocks,dolomite and dolomitic gypsum rocks and gypsum breccia and so on.Based on the analysis and study of measured and observed profile,drilling section and related geological data,the sedimentary environment of main profile in study area is analyzed,which shows that the sedimentary environment of the Sichuan basin is various:The Qiongzhusian-Canglangpu stage develop including alluvial fan,delta littoral,shallow shelf and deep water shelf,that water deepening to southeast and northeast and developing slope-basin.The Longwangmiaoan stage develop clastic tidal flat and carbonate platform.The platform is divided into restricted platform which developing mixed tidal flat,lagoon and gypsum rock intraplatform and open platform including point beach and beach sea in the southeast of the basin,and the water deepening to southeast and northeast and developing slope-basin.Qualitative and quantitative research methods were used to study the lithofacies palaeogeography of the Lower Cambrian in Sichuan basin.The single factors that can reflect environmental characteristics have been selected including thickness,content of shallow water carbonate rocks,content of deep water sedimentary rocks,content of deep water carbonate rocks,content of black shale,content of shallow water grain carbonate rocks,content of gypsum,content of penecontemporaneous dolomite.And then,according to different periods,different single factors can reflect the sedimentary environment,to compilate Lower Cambrian Qiongzhusian stage,Canglangpuan stage and Longwangmiaoan stage lithofacies palaeogeography map.In Qiongzhusian stage,from the northwest to the southeast in the study area,palaeogeographic units include coarse clastic shore,fine clastic shore and shelf in which shallow shelf is mainly distribute in the interior of the basin while deep water shelf is mainly distributed in the northeast of Fengjie-Kaixian-Tongjiang area and the southeast side of the horse-Weiyuan-Gulin area.In Canglangpu stage,due to small scale regression,terrigenous clastic migrate to the East and palaeogeographic units from the northwest to thesoutheast include coarse clastic shore,fine clastic shore and shallow shelf.There are lots of changes of lithofacies palaeogeography in Longwangmiaoan stage,which can be summarized as follows: clastic platform in the west,carbonate platform is the main part,saline lake on the platform where both sides of the lake is granule beach,on the beaches is dolomitic flat and between beaches is beach interact sea,southeast and northeast margin is basin.Based on the analysis and test data,the favorable distribution area of oil and gas reservoirs is discussed from the perspective of lithofacies palaeogeography.Research shows that for hydrocarbon source rocks,fine clastic shore and shelf are favorable hydrocarbon environment,but the most favorable source rocks is the large thickness and wide range of dark mudstone and shale sendimented inside the craton rift in the shelf.For the reservoir,the powder crystal of dolomite formed by strong recrystallization of the granular dolomite in the dolomitic flat is the best reservoir property,which followed by the granular dolomite developed in granular beach,and then is the grain limestone,the worst is the micritic dolomite in beach interact sea.On the whole,upper beach.is more suitable to be used as reservoir than under beach.For the cap,gypsum salt rocks in saline lake of sealing performance is better than that of silty mudstone in fine clastic shore and shale in shelf environment.Finally,a comprehensive analysis of source-reservoir-cap favorable area,divided into three categories with favorable areas for exploration: class I is the most favorable zone,one is Weiyuan-Rongxian area,one is the Moxi area;II is a favorable zone,main in the northe Nanjiang-Chengkou area and southeastern of the basin Jinsha-Meitan.;the third is the general exploration area,in the southeast basin margin of Xishui-Zhongxian area,there is no indication of oil and gas,is not recommended as the goal in the near future.