Characteristics and Hydrocarbon-Accumulation Mechanism of Source Rocks with Low Abundance and High Efficiency in Quaternary Period of Eastern Qaidam Basin

In order to investigate the characteristics and hydrocarbon accumulation mechanism of source rocks with low abundance and high efficiency in the Quaternary in the eastern Qaidam basin,the rock samples selected from the outcrop of Quaternary were analyzed,including the organic carbon content,the parameters of pyrolysis and biomarker characters of saturated hydrocarbon by means of Rock-Eval,GC-MS and so on.The results show that the content of organic carbon ranges from 0.15% to 0.75%,and the value of S1+S2 ranges from 0.25mg/g to 2.5mg/g,which displays its limited hydrocarbon-generating potential.There were mainly type Ⅲ of kerogen,a few of type Ⅱ of kerogen,and few of type Ⅰ of kerogen.There was predominance of phytane to pristane,and the ratio of pristane to phytane was lowerer than 0.5.The values of the parameters of C29αββ/(αββ+ααα) and C29ααα20S/(20S+20R)were 0.34～0.38 and 0.27～0.33,and the organic matter was being in the stage of low maturity.The water temperature of the depositional lake was low and its salinity was high.The unique depositional environments of this area in the Quaternary are the important conditions for forming the large scale biogenic gas pools.The water environment with a high salinity slowed down the degradation rate of the organic matter.The methane bacteria activity was restrained in cold environment for a long time and the generation peak of the biogenic gas was delayed.Moreover,the massive Quaternary dark mudstone and frequent developed sandstone in Sanhu area provide abundant gas source and constitute a favorable combination of reservoir.