Sedimentary Characteristics Of Gypsum-Salt-Bearing Layer Of The Lower Cambrian In The Middle And Upper Yangtze Region

The marine sediment of Cambrian in southern China is well developed and widely distributed,especially in the Cambrian strata,which is well exposed,has both abundant horizons and rock types.There are not only dark fine grained detrital rock(which can be used as hydrocarbon source rocks),but also carbonate rock(which can be used as reservoir).The Middle and Upper Yangtze Region is is regarded as one of the most important areas in the southern Marine oil and gas exploration,and the prospect is optimistic.As a high quality and important cap rocks,the gypsum rock is widely distributed in the Lower Cambrian in the Middle and Upper Yangtze Region,mainly distributed in the Lower Cambrian Longwangmiao Formation(Qingxudong Formation,Shilongdong Formation).The formation of the gypsum rock is closely related to the sedimentary environment.and the study of this has a great significance for regional sedimentary environment and paleogeography research.Some scholars have done a lot of work on the lithology,distribution and sedimentation patterns of the Cambrian gypsum rock,and several different sedimentary models have been proposed,but most of the studies were conducted in the Cambrian system as a whole,and there were few specific studies on the gypsum rock in different sedimentary backgrounds of Cambrian,so these patterns do not fully explain and apply to the sedimentation of the Middle and Upper Yangtze Region.Therefore this article through the field profile and drilling cores observation,and combined with regional geological data,logging data and laboratory test data to make sure about the rock types of the gypsun-salt rocks in the Middle and Upper Yangtze Region,spatial distribution and sedimentary environment,and to explore the Lower Cambrian in the Middle and Upper Yangtze Region anointed gypsum formation conditions and the sedimentary pattern,in order to offer a favorable theoretical basis for the development of oil and gas exploration.The study shows the rock types of the gypsum-salt rocks in the Lower Cambrian in the Middle and Upper Yangtze Region mainly include gypsum rock,carbonate rock and clastic rock.Through the analysis of the petrology characteristics of the gypsum-salt rocks,the vertical and horizontal distribution of the gypsum rock,the symbiotic relationship between the gypsum rock and the carbonate rock,according to rock composition,structure and other characteristics,based on gypsum,dolomite,terrestrial debris,the gypsum-salt rocks in the Lower Cambrian in the Middle and Upper Yangtze Region can be divided into gypsum?gypseous dolostone and gypseous sandstone?halilith,further divided into twelve small types,including pure gypsum rock ?dolomitegypsum?mud gypsum?gypsum breccia?gypsum dolomite?gypsum sandstone?gypsum mud rock?halilith,etc.According to mineral composition,carbonate rocks are divided into limestone and dolomite.By analyzing the structure,tectonic and special causes,the limestone can be further divided into granular limestone?argillaceous limestone?reef limestone et.And according to grain size,the dolomite can be divided into micrite-powder crystal dolomite?fine-medium dolomites and graniphyric micritic dolomite.In addition,the clastic rock is divided into conglomerate,sandstone,siltite,mudstone and shale according to structure and composition.Through the analysis of the longitudinal and transverse sections,gypsum rock in the Lower Cambrian in the Middle and Upper Yangtze Region has the features of wide distribution,large variation of thickness,small thickness of single layer,more interlayer,relatively more gypsun and relatively less salt rock.The gypsum rock in the Lower Cambrian mainly developed in the middle and upper Longwangmiao Formation,according to the characteristics of the basic composition and structure of the sections that containing gypsum-salt rock in different places,it could be divided into four categories: Fandian pattern ? Dingshan pattern ? Gongshen pattern and PaiShen pattern.On the whole,from the Southeast Sichuan-Northwest Guizhou to the Northeast sichuan-Northwest Hubei area,there is a tendency to decrease the thickness of the gypsum rock in the Lower Cambrian.In addition,the main development of the gypsum rock in the Lower Cambrian in the Middle and Upper Yangtze Region is in the South Sichuan-East Sichuan area and East Chongqing-West Hubei area and Jianghan plain area.The South Sichuan-East Sichuan area of gypsum rock accumulation area mainly include gypseous dolostone combination?gypseous sandstone combination and halilith combination;East Chongqing-West Hubei area and Jianghan plain area.mainly gypseous dolostone combination.The main developing sedimentary environment of the gypsun-salt rocks in the Lower Cambrian in the Middle and Upper Yangtze Region are evaporative platform?restricted-platform.The evaporative platform include evaporative tidal flat and evaporative lagoon,the restricted-platform include restricted tidal flat?lagoon and intra-platform.By using the method of qualitative and quantitative research,this paper expounds the lithofacies palaeogeography of the Lower Cambrian in the Middle and Upper Yangtze Region.The study selects seven single factors which can reflect the sedimentary environment,and draws the paleogeographic map of the lithofacies of early Cambrian Longwangmiao Period in the Middle and Upper Yangtze Region,including evaporative platform?restricted-platform?open-platform?slope?carbonate rock basin and shale basins.The comprehensive analysis suggests that the gypsum rock in the Lower Cambrian in the Middle and Upper Yangtze Region is mainly formed by evaporation,the main source are the salt in seawater and the more soluble salts in land sources.The hot and dry climate is an indispensable condition for the formation of gypsum rock.The lithofacies paleogeographic environment can control the deposition and accumulation of the gypsum rock.In the southwestern part of the research area,there is a relatively gentle intermittent exposure of the evaporating platform,the water is unstable,The gypsum rock can be formed by the deposition of regression?exposed to the surface of the earth or depression.The features of wide distribution,large variation of thickness,small thickness of single layer,more interlayer,relatively more paste and relatively less salt rock.In the South Sichuan-East Sichuan area and East Chongqing-West Hubei area,a large number of banded beach combinations make it long in a dry and hot closed environment,the sea water in the lagoon forms gypsum rock by evaporation and concentration.