Geochemical Characteristics Of Paleozoic Hydrocarbon Source Rocks In Cuqen Basin, Tibet

Based on the previous studies, analysis of outcrop samples from Carboniferous-Permian in Cuqen Basin, combined with vast amounts of data, this paper comprehensively analyses the sedimentary environment, abundance, types and maturity of organic matter in Carboniferous-Permian hydrocarbon source rocks from Cuqen Basin to evaluate the potential of hydrocarbon generating of the source rocks. This study mainly used the organic geochemical method:evaluation of the abundance of organic matter based on the analytical data of TOC and chloroform extract; restoration of organic types conducted in terms of insoluble organic matter and dissoluble organic matter, the former include microscopic examination, elementary analysis and isotope composition analysis of kerogen, the latter include pyrolysis parameters, composition of dissoluble organic matter and biomarker character; evaluation of maturity of organic matter based on data of Ro, Tmax and Biological markers isomers ratio.Comprehensive evaluation reveals that possible source rock of Yongzhu Formation is dark mudstone and marlstone which deposit in continental shelf, the marlstone belong to medium to good source rock classes, and only less than half of the mudstone reach the standard of source rock, which belong to medium to good classes; the type of organic matter is mainly type Ⅰ-Ⅱ2, most of them are high-mature to over-mature. Possible source rock of Laga Formation is dark mudstone which is in continental shelf environment and belong to invalid to poor source rock classes, partly good source rock, organic type include type Ⅰ to type Ⅲ, and mostly high-mature to over-mature. Possible source rock of Xiala Formation is dark marlstone which form in carbonate platform environment and are poor to medium source rock, partly are good source rock, organic type is mainly type Ⅱ1-Ⅱ2,partly type Ⅰ, and they are of mature to high-mature stage.Biomarker character also demonstrated that type of organic matter are mainly type Ⅱ, which reflect organic origin from mixture of lower hydrobiont and terrestrial higher plants and belongs to reducing and highly salinity environment. Meanwhile, the data of biomarker also indicates that organic matter in source rocks of all formation have come to late stage of maturity. The high level of thermal evolution has a great effect on the molecular geochemical characteristics of Carboniferous-Permian source rocks in Cuqen Basin, which cause the reduction of the content of dissoluble organic matter, alteration of carbon isotope composition and serious damage of both steranes and terpanes.