Research On Sedimentary Characteristics And Paleoenvironment Of Cambrian—Ordovician Carbonate Rocks In Liujiang Basin

Carbonate rocks are also important oil and gas reservoirs in the world because of their wide distribution and large area.Especially in the past two decades,considerable carbonate oil and gas resources have been found in many basins in China.Therefore,in-depth exploration of the genetic environment and influencing factors of carbonate rocks can provide a theoretical basis for oil and gas exploration and development.There are relatively complete Cambrian-Ordovician carbonate rocks in the Liujiang Basin,with many types and complete outcrops,which provide favorable conditions for the study of carbonate rocks.Taking the carbonate rocks of Liujiang Basin as the research object,the Cambrian-Ordovician stratigraphic sequence is established through field exploration,measurement,description and sampling analysis.According to the components of rock fabric,the types of peliferous clastic limestone,bioclastic limestone,oolitic limestone,algal limestone,stromatolith,panther limestone,micritic limestone,chert nodule or strip limestone,dolomitic limestone and marl are subdivided.Based on the study of regional background and sedimentary characteristics,nine microfacies types are classified,including basin facies,open shelf facies,platform slope foot facies,platform front slope facies,platform edge bioreef facies,platform edge shoal facies,open platform facies,restricted platform facies and platform evaporative salt facies.A large number of geochemical experiments have been carried out to measure the contents of major and trace elements in different types of carbonate rocks.The distribution of each element under different environmental and climatic conditions was analyzed,and the element distribution patterns under different temperature and humidity conditions,different salinity,redox environment,and land and sea environment were established,which provided a basis for analyzing the paleoenvironmental conditions.Based on the sedimentary characteristics of rocks,the content of finger elements and background conditions,the evolution process of the climatic environment and geological environment from the Cambrian to Ordovician in the Liujiang Basin was deeplyanalyzed.The early Cambrian climate was arid and hot,and gradually transitioned from a land-erosion environment to a shallow-shore environment;the humidity in the Maozhuang period increased,and trilobites and brachiopods Angiosperma began to appear.The Middle Cambrian transgression expanded,forming a wide-sea sedimentary environment,and peaked during the Zhangxia period.During the late Cambrian,there was an obvious regressive phenomenon.During the multifunction period,the coastal oxidation environment exposed sediments above the water surface and suffered from storms and waves,which made the unconsolidated and semi-consolidated carbonate sediments break into gravel chips and then redeposit and cement in situ or nearby,forming a large amount of gravel debris.In fengshan period,the water became deeper again,and the platform was in deep water.The early Ordovician transgression expanded again,and the sedimentary environment gradually changed into the normal shallow and deep water environment.The climate was warm and humid,which was suitable for biological growth.In the late Middle ordovician and late Ordovician,due to caledonian movement,the whole north China region was uplifting,seawater receded,and the sedimentary base level dropped.The region was again elevated to land,with discontinuous deposition and in a state of denudation.