Geochemical characteristics and significance of Early Cretaceous bitumens in Esima area, southern Qiangtang Basin of Tibet
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摘要:
首次在南羌塘安多县鄂斯玛地区早白垩世地层中发现沥青。从沥青有机碳含量、族组分及生物标志化合物方面综合研究了其有机地球化学特征，并进行了油源对比。研究结果表明，样品的有机碳含量为3.42%~75.01%，显示其有较高的沥青含量；族组成中重烃组分（非烃+沥青质）质量分数最高，其次为芳香烃，饱和烃质量分数最低。生物标志物研究表明，沥青的成熟度较高，沥青母岩的沉积环境为还原环境，其有机母质主要为混合来源，特别是藻类做出了重要贡献。运用生物标志化合物的相对含量指标进行油源对比研究表明，沥青可能来源于索瓦组泥岩。该研究成果对南羌塘的油气勘探具有一定意义。
Abstract:Early Cretaceous bitumens were found for the first time in Esima area in southern Qiangtang Basin. The authors analyzed the organic geochemical characteristics from the bitumen content of organic carbon and group composition and biomarkers in this area and discussed the oil-source correlation. The result reveals that the total organic carbon content of the bituments samples varies in the range of 3.42%~75.01%, indicating abundant bitumens. The bitumen samples have the highest fraction of heavy hydrocarbons (nonhydrocarbon and asphaltene), followed by aromatics hydrocarbon, and the lowest fraction is saturated hydrocarbon. The maturity of bitumens are not in high maturity and are mainly in the mature stage. The source rock was deposited under reductive conditions, and the original organic matter was algae and high-grade plants, with especially important contribution made by algae. Oil-source correlations were studied by using the biomarkers, and the results show that the bitumens were derived from the Suowa Formation mudstones. The results obtained by the authors are important for oil and gas exploration in southern Qiangtang Basin.
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Key words:
- biomarkers /
- oil-source correlation /
- Early Cretaceous /
- Qiangtang Basin
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表 1 鄂斯玛地区早白垩世沥青及可能烃源岩基本有机地球化学参数
Table 1. Geochemical parameters of the bitumens samples and their potential source rocks in Esima area
样品
编号层位 岩性 TOC/% S/% 氯仿沥青
“A”/%氯仿沥青“A”族组成/% 饱和烃 芳烃 非烃 沥青质 B1 J3K1s 含沥青灰岩 3.54 0.24 0.0276 12.52 28.49 31.94 27.04 B2 J3K1s 含沥青灰岩 6.53 0.10 0.0195 16.67 31.54 31.03 20.77 B3 J3K1s 含沥青灰岩 7.30 0.49 0.0234 9.71 26.64 24.38 39.28 B4 J3K1s 纯沥青 75.01 1.59 0.4583 6.99 24.26 40.81 27.94 B5 J3K1s 含沥青灰岩 3.42 0.03 0.0244 13.43 23.88 37.31 25.37 B6 J3K1s 含沥青灰岩 5.38 0.03 0.0123 19.57 30.43 28.26 21.74 A1 J3K1s 泥岩 0.72 0.34 0.0332 24.28 29.86 20.21 25.64 A2 J3K1s 泥岩 0.75 0.44 0.0300 21.37 28.71 15.19 34.72 注：J3K1s为索瓦组 
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表 2 鄂斯玛地区沥青及可能烃源岩正构烷烃和类异戊二烯烃分析结果
Table 2. Data of saturated hydrocarbon and isoprenoid hydrocarbon for the bitumens samples and their potential source rocks in Esima area
样品
编号主峰碳 CPI OEP Pr/Ph Pr/
nC17Ph/
nC18nC21-/
nC22+(nC21+nC22)/
(nC28+nC29)B1 19 1.02 0.99 0.92 0.35 0.26 0.80 1.73 B2 19 1.05 1.03 0.84 0.44 0.34 0.73 1.79 B3 20 1.07 0.97 0.91 0.54 0.45 0.54 1.73 B4 19 1.16 1.07 0.69 0.72 0.37 0.94 3.44 B5 19 1.14 1.07 0.73 0.75 0.47 0.70 2.55 B6 21 1.17 1.02 0.57 0.81 0.42 0.69 2.68 A1 21 1.07 1.01 0.89 0.40 0.32 0.90 1.94 A2 20 0.98 1.02 0.98 0.56 0.28 0.78 1.66
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表 3 鄂斯玛地区沥青及可能烃源岩甾烷和萜烷标志物分析结果
Table 3. The parameters of biomarkers from the bitumens samples and their potential source rocks in Esima area
样品
编号规则甾烷/%
C27 C28 C29C29ααα20Sααα/
(20S+20R)C29αββ/
(ααα+αββ)Ts/（Tm+Ts） C29藿烷/
C30藿烷C(29+30)莫烷/
C(29+30)藿烷C31 αβ22S/
（22S+22R）γ蜡烷/C31
(22S+22R)/2B1 33 19 48 0.38 0.51 0.38 0.48 0.12 0.50 0.29 B2 28 25 47 0.5 0.48 0.39 0.54 0.11 0.43 0.18 B3 35 21 44 0.42 0.4 0.20 0.67 0.16 0.40 0.13 B4 34 22 44 0.47 0.44 0.22 0.59 0.15 0.43 0.18 B5 36 21 43 0.5 0.41 0.42 0.52 0.10 0.48 0.26 B6 39 20 41 0.4 0.44 0.37 0.58 0.17 0.40 0.16 A1 30 25 45 0.5 0.50 0.28 0.50 0.14 0.38 0.19 A2 31 23 46 0.42 0.50 0.38 0.57 0.17 0.36 0.09
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