Qiangtang Basin is an important exploration target in Tibetan area that was conducted little exploration and research before this study. Xiaochaka Formation is major source rocks in Upper Triassic, Mesozoic in the basin. According to the evaluation standard of source rocks established specially in this basin, the occurrence features of this formation in different structural units was analyzed comprehensively and then the favorable source kitchens were predicted on the aspects of its sedimentary environment, thickness and organic geochemical characteristics. The work was conducted on the samples from many geological profiles in field survey routes and the analytical data on organic carbon, maceral, rock pyrolysis, vitrinite and bitumen reflectance, and carbon isotopes, etc. The conclusions are as follows: firstly, the organic matter abundance of limestone and mudstone in Xiaochaka Formation is high so that they belong to medium-good source rocks; however, the actual areal division of their distribution shows by the fact that the values are variable in different areas; this formation mainly distributes in the middle part of Southern Qiangtang Basin with mainly II B organic matter type and high maturity; limestone is slightly better than mudstone in organic matter type, but smaller in the favorable source kitchens. All the above-mentioned research has an important reference meaning for the further determination of major source rocks, resource evaluation and exploration prospect on oil and gas.

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To understand the biogas formation in geological basins, the present work investigated the reactive organic matter in sediments of the Sanhu depression of Qaidam Basin, a prolific region of biogenic gases with a proved reserve of 300 bil steres. The ROC (reactive organic carbon) was obtained by ultrasonic extraction from sediment samples in the solution of 6 mol/L HCl and 5% K 2 SO 4 . To investigate the effect of early diagenesis, parts of the samples were heated at 80°C before extraction. The results showed that the ROC content at a constant temperature decreased with increasing burial depths, which should be attributed to the microbial consumption. For the same sample, the ROC content heated at 80°C was dramatically higher than the unheated. The increment of the ROC content for some samples was as high as 200% in the experiment. The dramatically increasing ROC by thermal action should be the major nutrient substrate for the deep biospheres in most geological basins. There is a positive correlation between the reactive organic carbon (ROC) and the traditional insoluble organic carbon (TOC), not only for its absolute content of the ROC, but also for the 'ROC' produced in thermal action, all of these are clearly related with TOC. These data showed that higher abundance of organic matter can contribute more to the reactive organic matter, and is more favorable to the formation of biogenic gases. In the Sanhu depression of Qaidam Basin, more than 85% of the biogenic gas reserves occur in the lower layers (K5-K13) with a relatively high abundance of organic matter. The exploration has provided further evidence that deposits with higher abundance of organic matter are effective biogas source rocks. biogenic gases, gas source rock, Qaidam Basin, reactive organic matter, TOC Citation:Shuai Y H, Zhang S C, Chen J P, et al. Source of nutrient substrates for microbes in deep biosphere and characteristics of biogenic gas source rock.