羌塘盆地东部石炭系杂多群分子地球化学特征及地质意义

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

关键词:  石炭系  烃源岩  生物标志物  芳烃  羌塘盆地

Key words:  Carboniferous  source rock  biomarker  aromatic hydrocarbon  Qiangtang Basin

摘要:

对羌塘盆地东部刻莫地区石炭系杂多群16个烃源岩样品的饱和烃和芳烃组分的GC-MS分析表明,其烃源岩正构烷烃主要是以C₂₃为主峰碳的单峰型,nC₂₁-/nC₂₂₊值介于0.22~0.74之间,规则甾烷主要以C₂₉占优势,少数以C2 7占优势,在C₂₇-C₂₈-C₂₉三角图中主要位于混合源区域,芳烃组分中检测出了与高等植物有关的海松烯和1-甲基菲。这些结果表明,杂多群烃源岩有机质生源以高等植物输入为主,低等水生生物有机质对成烃母质也有一定贡献。杂多群烃源岩Pr/Ph比值为0.83~1.43,一般大于1;1-甲基菲/9-甲基菲比值为0.77~9.95,1-甲基菲含量较高,同时还检测出源于高等植物的海松烯(1,7-二甲基菲);"三芴"系列化合物总体分布特征表现为OF > SF > F、SF> OF > F或者OF > F > SF,在ΣSF/(ΣF+ΣSF)-ΣOF/(ΣF+ΣOF)关系图上主要落于氧化-还原环境附近,可能为弱氧化-弱还原环境,也有部分样品位于沼泽相或煤的氧化环境中。杂多群烃源岩OEP值为1.01~1.16,C₂₉ ααα20S/ααα(20S+20R)和C₂₉αββ/(ααα+αββ)比值分别为0.17~0.45和0.27~0.49,根据甲基菲和烷烃二苯并噻吩换算出的等效镜质体反射率平均值分别为0.70%和0.85%,反映出杂多群烃源岩具有成熟有机质的特征。

Abstract:

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.