Geochemical Characteristics Of Lower Cambrian Source Rocks In Middle And Upper Yangtze Region

The source rocks of the Lower Cambrian Qiongzhusi Formation and its comparative formations in the Middle and Upper Yangtze region are one of the important Paleozoic marine source rocks in southern China.The lithology of source rocks is mainly shale,and the source rocks have high organic matter content,but they are deeply buried and have experienced high maturation process,so some conventional geochemical parameters are no longer applicable.Therefore,it is necessary to select new methods and applicable parameters to analyze and evaluate the high maturity source rocks and determine their distribution and development characteristics,which is conducive to guiding oil and gas exploration and development.In this paper,artificial neural network prediction method was selected to predict TOC for such highly matured source rocks,kerogen carbon isotope compositions were used to classify organic matter types,and asphaltene reflectance and marine vitrinite-like reflectance were selected to calculate vitrinite reflectance.Based on previous studies,the distribution characteristics of organic matter abundance,types and maturity of source rocks in the Middle and Upper Yangtze region are analyzed.It is concluded that the organic matter abundance of the Lower Cambrian Qiongzhusi Formation and its comparative formations in the study area is higher than0.5%,and the organic matter type is mainly type I sapropelic,and the R_o generally more than 2%,which is in the stage of high maturity,mainly gas generation.Taking the central Sichuan area of Sichuan Basin as the representative to analyze the molecular organic geochemical characteristics and elemental geochemical characteristics of the whole Middle and Upper Yangtze region,by analyzing commonly used biomarkers,such as n-alkanes,pristanes and phytane in isoprene,tricyclic terpenes,hopanes,C₂₇,C₂₈ and C₂₉regular steranes and related parameters of fluorene in aromatic compounds.It is considered that the maturity of source rocks is high and the main biological sources are derived from plankton,bacteria and algae,and the sedimentary environment is a reductive environment.Different element environmental indicator parameters also reflect the reductive sedimentary water environment.Through the analysis of the characteristics of main and trace elements and rare earth elements,the main elements are mainly Si O₂content,some trace elements such as Mo,Sb,U,Cd,V,Tl,Ni,Cr,Cu,Zn are relatively enriched,and rare earth elements are characterized by relatively enriched light rare earth elements.The source of hydrocarbon source rocks in the study area is diverse,and the geological structure background is mainly passive continental margin,which may be influenced by relatively weak deep hydrothermal activity,and has the characteristics of continental island arc.