Sedimentary Characteristics Of Gypsum-salt-bearing Layer Of Middle Cambrian In Middle-upper Yangtze Region

Gypsum-salt rock plays a key role in the formation of many oil and gas bearing basins at home and abroad.With the continuous breakthrough of deep-ultra deep carbonate exploration,the research on symbiotic composition of gypsum-salt rock-carbonate is mentioned.Previous studies have shown that sub-salt field of the Middle Cambrian of the Upper and Middle Yangtze Region has large potential for oil and gas exploration,although some scholars have done some research on the petrological characteristics,distribution,and deposition patterns of the Middle Cambrian gypsum-salt rocks.There are many different depositional models,such as the carbonate slope model,the carbonate platform mesa evaporation model,the carbonate gentle slope-grain shoal and the lagoon sedimentary diagenetic model,but these models do not completely explain the sedimentary phenomena revealed in the study area.Therefore,through the analysis of the petrology characteristics,the spatial distribution of gypsum-salt rocks and carbonate symbiotic relationship of gypsum-salt bearing layers,It is to explore the depositional environment and depositional model of gypsum-salt rocks of the Middle Cambrian in Middle-Upper Yangtze Region.It provides theoretical basis for the exploration and development in the study area.There are various rock types of the Middle Cambrian rocks in the Upper-Middle Yangtze Region.It is mainly clasolite,carbonate and gypsum-salt rock.Clasolite can be further divided into conglomerate and breccia,sandstone,siltstone and mudstone and shale.Carbonate can be divided into limestone and dolomite.And limestone can be further divided into granular limestone,marl limestone and stromatolite limestone;dolomite can be further divided into silt-crystal dolomite and fine-medium crystalline dolomite.The gypsum-salt rock can be divided into four categories: gypsum rock,gypsum-bearing dolomite,gypsum-bearing mudstone(sandstone)and salt rock.It can be further divided into pure gypsum rock,gypsum breccia,dolomite-bearing gypsum-salt rock,dolomitic gypsum-salt,gypsum dolomite,gypsum-bearing dolomite,gypsum-bearing mudstone(sandstone),and salt rock.According to the basic composition and structural features of the section of the gypsum-salt-bearing layer in the study area,the section of the Middle Cambrian of the Upper-Middle Yangtze area can be divided into four categories: Firstly,the characteristic of Fandian typy is gray mudstone,mudstone and breccia interbebbing.There is no gypsum-salt rock and it represent the gypsum-salt rock profile is under the surface environment.Secondly,The Jianshen type is mainly characterized by the development of a thick gypsum-salt rock,which develops a clastic dolomite and gypsum-salt rock from the bottom up,which represents the gypsum-salt rock profile is developed in the lagoon environment;Thirdly,the Dingshan type whose main feature is the development of ooid dolomite,microcrystalline dolomite,argillaceous dolomite and gypsum-salt rock from bottom to top.The cycle represents that the gypsum-salt rock profile is developed in the lagoon environment;and the forth type is mainly characterized by the development of ooid dolomite,gypsum siltstone,and gypsum dolomite from bottom to top,which represents the evaporative mesa environment.Based on the analysis of the drilling and profile data of the Middle Cambrian in the study area,it shows that the gypsum-salt rock of the Middle Cambrian in the study area has three gypsum-salt rock development areas.the gypsum-salt rock in the eastern Chongqing and western Hubei.It overall is distributed from northeast to southwest.The gypsum-salt rock in the southwestern Sichuan and northern Guizhou.It overall is distributed from northwest to southeast.The gypsum-salt rock in the southwestern Hubei and central southern Hubei.It overall is distributed from west to east.Among them,there are two development zones of gypsum-salt rock in the Douposi Formation,which is the eastern Chongqing,western Hubei and eastern Hunan and southwestern Hubei and central southern Hubei.The former gypsum-salt rock is spread northwest-southwest and the latter gypsum-salt rock spread in the east-west direction.Compared with Douposi Formation,the gypsum-salt rock distribution range of Xiwangmiao Formation has been expanded.In addition to the above-mentioned two gypsum-salt rock development areas,there are also concentrated distributions in the southwestern Sichuan and northern Guizhou,and the gypsum-salt rock stretches from southwest to the northeast.Based on the petrological characteristics,profile characteristics,and spatial distribution characteristics of the gypsum-salt-bearing layer of the Middle Cambrian in the study area,In combination with measured and observed profiles and the collection of predecessor profile data,it is believed that the gypsum-bearing layer of the Middle Cambrian in the study area are a platform for carbonate sedimentation and can be divided into two types of depositional environments: platform evaporite facies and restricted platform.At the same time,platform evaporite facies can be further divided into evaporation tidal flats and evaporation lagoon;restricted platform is further divided into confined tidal flats,confined lagoon and intra-tank.On the basis of petrological studies,a “multi-factorial comprehensive mapping method for single factor analysis” was used,In combination with regional geological data and other quantitative and qualitative data,the paleo-geographic maps of the Middle Cambrian lithofacies in the study area were compiled,and the features of the lithofacies and paleogeography in the study area were visually displayed.The paleogeographic units in the study area develops lands,mixed mesas,carbonate platforms,slopes,and basins from west to east,among which main is carbonate platform.Based on the petrological characteristics,spatial distribution characteristics,sedimentary environment,and lithofacies paleogeographic of the gypsum-salt-bearing layer in the Upper-Middle Yangtze areas,a depositional model of the symbiotic system of gypsum-salt rock-carbonate was established.The comprehensive analysis of gypsum-bearing layer is mainly distributed in the lagoon environment.