Characteristics of biomarker compounds in the mudstone of Upper Jurassic Suowa Formation from QZ3 Well in Qiangtang Basin, northern Tibet

In this paper, the authors carried out detailed research on geochemical characteristics of the Suowa Formation mudstones from Well QZ3. The origin of organic matters and the sedimentary environment were investigated according to the characteristics of biomarkers. Abundant biomarkers, including n-alkanes, isoprenoid, terpanes and steranes from mudstones, were detected by GC and GC-MS. The n-alkanes have nC₁₇, nC₁₈, nC₂₃or nC₂₅ as the mean peaks. OEP values vary from 0.4 to 1.24, Pr/nC₁₇ from 0.68 to 0.99, Ph/nC₁₈ from 0.55 to 0.98, and Pr/Ph from 0.50 to 0.90. In the m/z191 GC-MS, C₃₀ hopane is most abundant in the terpanes, with the presence of tricyclic terpanes and gammacerane. In the m/z217 GC-MS, regular steranes are characterized by the highest abundances. The regular steranes C₂₇-C₂₈-C₂₉ααα 20R are distributed in the asymmetric "V" shape. C₂₉ααα 20S/(20S+20R) values vary from 0.30 to 0.49 and C₂₉αββ/(ααα+αββ) from 0.40 to 0.54. GC/MS characteristics of saturated hydrocarbon show that organic matter is characterized by mixed sources composed of phytoplankton, suggesting that organic matter is formed in a low-salinity anoxic environment. Different kinds of parameters indicate that the organic matter of the asphalts and mudstones is the peak of oil generation. At the same time, biomarker parameters of the asphalts and mudstones from QZ3 Well show similar characteristics in such aspects as organic matter source, sedimentary environment and maturity. They have very close genetic relationship.