Organic Geochemical Characteristics and Geological Significance of Source Rocks in Late Carboniferous Harjiawu Formation of Santanghu Basin

The Upper Carboniferous Harjiawu Formation in Santanghu Basin occurs as a set of intermediate basic volcanics,pyroclastic rock and sedimentary rocks which develop many layers of source rock with different thicknesses.The results of Rock-Eval and saturated hydrocarbon GC-MS reveal that the organic matter abundance of source rocks is higher in Harjiawu Formation.The type of organic matter include typeⅢ,type I and the mixture of the both.The degree of thermal evolution of most samples has achieved mature stage.The higher values of Pr/Ph ratios(1.1-3.01),low values of G/C30hopane(0.047-0.158),and the tricyclic terpanes distribution indicate that the water environments are weak suboxic-weak and low saline during source rock deposition.Low C31R/C30 hopane ratios(0.186-0.310),very low C22/C21ratios(0.164-0.260),higher C24/C23 tricyclic terpane ratios(0.472-0.684)are consistent with the source rocks being deposited in a lacustrine environment.∑C21-/∑C22+,regular sterane distribution(C29C27C28and C29C28C27)and sterane/hopane ratios(0.06-0.20)point out that the biological sources of organic matternot only have higher plant,but also have a mass of bacteria and algae.