The Characteristics And Evaluation Of The Source Rock In Permian And Triassic, Changliang-liyu Mt. Region, Qiangtang Basin

Qiangtang Basin contains rich oil and gas resources. But previous studies on hydrocarbon focused on the Triassic and Jurassic strata of Qiangtang Basin while Paleozoic source rocks lacked of studying. With petrology, sedimentology, petroleum organic geochemical methods, field investigation, and comprising of the relevant information and academic research, the measured stratigraphic sections of Changliang Mt.-Liyu Mt. Upper Permian Raggyorcaka formation and Upper Triassic Tuzhonghu formation in the North Qiangtang Basin are studied.Through studying the section in the field and microscope identification, the results show that the study area Upper Permian Raggyorcaka formation is a set of terrigenous clastic flysch containing a bit of carbonate rocks, which the main lithology is dark shale and shale with lithic sandstone, siltstone interbedded while Upper Triassic Tuzhonghu formation is a set of medium-layer calcareous sandstone with the thick-layer of volcanic breccia, with epimetamorphic phenomenon.The analysis results of sedimentary facies depositional environment of Upper Permian Raggyorcaka formation and Upper Triassic Tuzhonghu formation show that in the study area Raggyorcaka formation is deoxidized saltwater environment, with shallow shelf facies, and Tuzhonghu formation is bathyal environment, with bathyal basin facies.Through Biomarker analysis showed that in the study area the types of organic source of Upper Permian Raggyorcaka formation are zooplankton(bacteria) and low phytoplankton(algae, red algae, etc.), then accept a certain percentage of terrestrial plant sources after recession in Late Permian.Through the source rocks organic geochemical analysis, the results show that organic carbon content of 13 samples are more than 1.0%, which are good source rocks, and accounting for 68.4% in 19 hydrocarbon source rock samples of Raggyorcaka formation. There are 6 sample are between 0.6%-1.0%, which are moderate source rocks. Organic carbon content of 7 sample of Tuzhonghu formation are less than 0.4%, which is not a valid source rocks. The type of the most hydrocarbon source rock samples is sapropel group, indicating the good hydrocarbon generating capacity, and its content is between 57%-78%, with an average of 64%. The hydrocarbon potential of vitrinite group is lower than sapropel group, and the percentage of vitrinite group is substantially zero. Hydrocarbon generation capacity of inertinite group is weak, and its percentage is in 22%-43%, with an average of 35.5%. The type index, Ti, shows that the main type of Raggyorcaka formation is humic-sapropelic kerogen, accounting for 84.2%, while the other is sapropelic-humic kerogen, accounting for 15.8%. According pyrolysis parameter analysis, the type of Tuzhonghu formation is humic kerogen. All samples of vitrinite reflectance of Raggyorcaka formation are greater than 1.3%, which is in 1.316-1.457%, with mean of 1.368%, while Tmax values are in 323-522 ℃, with an average of 392 ℃, and kerogen colors are brown. All of results show that maturity of organic matter is in a stage of high maturity. The vitrinite reflectance of Tuzhonghu formation samples is an average of 0.884%, as the maximum of 0.924%, the minimum of 0.845%, which indicates that the maturity is only in the mature stage.Through a comprehensive evaluation of source rocks, all the results indicate that source rocks of Raggyorcaka formation have good hydrocarbon generating potential, and speculate that Paleozoic source rocks have better Hydrocarbon generating prospects in the North Qiangtang Basin.