Study On The Quality Of Source Rock And Factor Of Sedimentary Environment In Lower Assemblage In Chuandongnan-Qianzhong And Nearby Area

After over five decades of study on southern China, geologists find that Sinian of Proterozoic and lower Palaeozoic are closely related in terms of formation and structure. Therefore, they are defined as Lower Assemblage in Southern China. The paper is mainly about source rocks in the south-east of Sichuan province, central areas of Guizhou province and surrounding areas. Through organic petrology and geochemistry, the paper depicts various formations and hiogenesis features of different hydrocarbon source rocks, identifies organic matter type and analyzes provenance, reserve and hydrocarbon potential. In addition, the paper explores facies significance of element geochemistry according to features of source rocks, and distinguishes and selects effective sedimentary and geochemical signs based on lithologic characters, sedimentary, biological and geochemical facies to demonstrate source rocks features, location and controling factors of residual basin of marine facies in areas studied to select best areas for exploration.During the development peroid of Doushantuo Formation source rocks, the areas studied belong to arenaceous pelitic facies, open platform facies. Until Dengying Stage, the area mainly belongs to restricted platform facies. During early Lower Cambrian, the areas belong to mesa-basin sedimentary facies and gradually developed to open platform-restricted platform facies. During Lower Cambiran, the areas belong to open sea shelf facies-open marine platform facies. Between late Ordovician and early Silurian, the areas experienced non-compensation stranded reduction marine basin-deep water shelf facies-dwelling sea-continental deposit.There are mainly three series of source rocks in the ares studied, i.e. Doushantuo formation, Niutitang formation( Zhalagou formation), Longmaxi formation( Wufeng formation ). Source rocks of Meitan and Duliujiang formation is inferior. In addition,the bitumen layer, such as Dingying and Mingxinsi formation should be given priority as potential source rocks. The organic type is Sapropel type ( Type I ).According to the trace element and the analysis in Sr/Ba, V/Ni, Sr/Ca, Ca/Mg, Th/U, it's proved that the sedimentary environment is marine. meantime,the salinity of seawater is becoming lower, so it's regression phase. LREE/HREE,δCe ( or Ce/Ce* ) andδEu ( or Eu/Eu* ) proved that the sedimentary environment is deep water shelf and far away from terrigenous.According to the characters of organic maceral, organic type, biomarker,it can be divided to five types of sedimentary facies: unrest restricted sea graptolite facies, alluvial restricted sea, carbonate mesa algal deposit facies, mud mesa algal deposit facies and deep water shelf algal deposit facies.