Although some hydrocarbon has been discovered in the C2k transitional reservoirs in the east of northern Qaidam basin, there remain fundamental unanswered questions about the C2k source rocks with mixed terrestrial and marine organic matter, ie, component and type of organic matter and discrepant hydrocarbon generation. Successive C2k source rock samples were collected from the Baishugou outcrop located in the southern slope of Gahainan Mountain for organic geochemistry, XRD, thin section, SEM analyses. The results indicate that the abundance of organic matter in source rocks is high, suggesting an excellent original productivity. Organic matter was already in the high- maturity thermal evolution stage and has primarily remained with humic kerogens, but the dissolved organic matter from the samples are hydrocarbon generation products of lower marine aquatic organisms, which represent mixed characteristics of marine and terrestrial organic matter. The contribution of marine OMs to hydrocarbon generation was higher than that of terrestrial organisms during the latest generation. Biogenic silica consists of remains of siliceous radiolarian organisms that were the main hydrocarbon generation material of marine kerogens. The archicontinent to the south, and the ocean trough to the north of the Ounan depression respectively provided terrestrial and marine OMs or silicon- rich upwelling, and the enrichment of marine OMs is related to the siliceous biological activity in the late Carboniferous. This study is of guiding significance for exploring the mechanism of organic matter enrichment in the Carboniferous and regional oil and gas exploration.