Molecular geochemistry and geological significance of the Lower Carboniferous Zaduo Group in Eastern Qiangtang Basin,Xizang

Based on gas chromatography mass spectrometry (GC-MS) techniques, the authors in this paper give a detailed study of molecular fossils characteristics of the source rocks from the Carboniferous Zaduo Group in eastern Qiangtang Basin. The analytical results indicate that the n-alkanes in the Zaduo Group source rocks are composed of nC₁₃-nC₃₅, with nC₂₃ as the main peaks, and characteristic of single peak distribution. The nC₂₁-/nC_{22 +} values range from 0.22 to 0.74. The regular steranes have the predominance of C₂₉, and few have the predominance of C 27. In the C₂₇-C₂₈-C₂₉ diagram, most of them are mainly located in the mixed source area. The 1-methylphenanthrene/9-methylphenanthrene ratios range from 0.77 to 9.95, and 1, 7-dimethylphenanthrene is also detected, which are associated with higher plants, indicating a predominant source of organic matter derived from terrigenous higher plants. However lower aquatic organisms have also contributed to hydrocarbon parent material. The Pr/Ph ratios range from 0.83 to 1.43. The distribution of fluorenes (F), dibenzofurans (OF) and dibenzothiophenes (SF) is summarized as follows:OF > SF > F, SF > OF > F or OF > F > SF. In the ΣSF/(ΣF + ΣSF) -ΣOF/(ΣF + ΣOF) diagram, they mainly are projected near the oxidation-reduction environments which may be a weak oxidation-weak reduction environment, whereas some samples also fall into an oxidation lacustrine or in a coal environment. The OEP ratios of the source rocks ranging from 1.01~1.16, C₂₉ααα20S/ααα (20S + 20R) ratios ranging from 0.17 to 0.45, C₂₉αββ/(ααα + αββ) ratios ranging from 0.27 to 0.49, and the calculations of aromatic maturity parameters MPI1 and 4, 6/1, 4-DMDBT from the equivalent vitrinite reflectances show that the Carboniferous source rocks in the Zaduo Group are mature in organic matter.