BIOMARKERS OF THE LOWER JURASSIC BLACK SHALE IN THE SHUANGHU AREA OF THE QIANGTANG BASIN, NORTHERN TIBET, AND THEIR GEOLOGICAL SIGNIFICANCE

The organic-rich black shale in the Qiangtang Basin, northern Tibet, has long been the focus of petroleum geologists. This paper discusses the characteristics and maturity parameters of such biomarkers of the shale as n-alkanes, acyclic isoprenoids, terpenoids and steranes, using gas chromatographic (GS) and mass spectrometric (MS) techniques. n-Alkanes are characterized by a typical unimodal distribution, with dominant low carbon numbers of nC(16)-nC(19), which suggests that the organic matter sources are mainly algae and planktonic organisms along with terrestrial higher plants. Furthermore, acyclic isoprenoid alkanes pristane (Pr) and phytane (Ph) are the most abundant, the Pr/Ph ratio being 0.06-0.70. The phytane dominance indicates that organic matter deposited in a reducing environment. In addition, the C(31)22S/(22S + 22R) hopanes ratio is 0.58-0.59, that of C(29)20S/(20S + 20R) regular steranes 0.43-0.59, and alpha beta beta/(alpha beta beta+ alpha alpha alpha) 0.39-0.65. These maturity parameters show that organic matter is in the maturity stage, while it is slightly more mature in the Biluocuo (BLC) section of the Qiangtang Basin than in the Ganbeixiama (GB) section. As indicated by the biomarkers values, the Jurassic source rocks in the two researched sections have a certain hydrocarbon potential.