Geochemistry Characterics Of Late-Triassic Source Rocks In Qiangtang Basin, Qinghai-Tibet Plateau

The Qiangtang Basin is located in the Qinghai-Tibet Plateau,and covers an area about180000km2.This basin is located in the central section of the Tethyan tectonic domain in which there is the most oil and gas production in the global. The structure of the Qiangtang basin is complex, the basin was divided into three sections, two sag and one uplift zone.The previous practice and research show that the sedimentary strata of this area belongs to the Mesozoic strata. In all strata, the Upper Triassic strata are widely distributed, large thickness. The black mudstone which belongs to the Upper Triassic strata contains abundance of organic matter and is an important source rock. So the Upper Triassic strata is key layer of achieveing breakthroughs in oil and gas exploration field.In the paper, the work of measuring formation profile, collecting and testing the samples of the Upper Triassic source rock was made on four formation profiles, Zangxiahe formation profile of Duoseliangzi area, Tumengela formation profile of Woruo-Mountain, Zhana formation profile of Xiaocha lake area, Tumengela formation profile of Zhanalongba area. Base on analysing the abundance, type, maturity, source of organic matter, and the formation environment of source rock,we study the character of the source rock and made some important progress.First, the organic geochemistry characters of source rock show that the source rock of the Tumengela formation in which the organic matter abundance is high, the type of organic matter is batter, and the overall high degree of thermal evolution to achieving high mature stage, has the. But the source rock of the Zhana and Zangxiahe formation in which the organic matter abundance is low, the type of organic matter is batter, and the overall high degree of thermal evolution to achieving dry gas stage, is provided with weakly production of oil and gas.Second, the biomaker of source rock show that all source rock samples were degradated by the animalcule. But the Biodegradation is weak, so the parameter of biomarker is effective. The character with single and double peak in saturated hydrocarbons gas chromatography, and the distribution characteristics of C27, C28, C29in sterane indicate that the organic matter is form aquatic organism and terrestrial. The ratio of Pr/Ph between0.06to0.93, and low ratio between gammacerane and hopane show that the environment in which the source rock form is the reducing environment, and the salinity of the water is higher than the fresh water. The carbon preference index(CPI),20S/(20S+20R) of sterane indicate and22S/(22S+22R)of C32hopane indicate that the organic matter is mature. And the Ts/(Tm+Ts)>0.5show that the thermal evolution of source rock has achieved high mature stage.Third, base on the paleomagnetic,the major and trace element date of the Tumengela formation, we analyse the characters of paleoclimate, palaeosalinity and redox which are master factors of formation and evolution about organic matter. The results which agree well with the fact of coal measure strata in Tumengela, show that the environment is littoral acies, and esemi-humid to moist environment,low paleosalinity and reducing environment is well for breeding of organism and forming high quality source rocks.