An improved method and indications for the compound specific isotopic analysis of hopanes in source rock extracts
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摘要: 以5Å分子筛/氧化铝混合填料的层析柱对烃源岩抽提物进行了分离,成功获取了以藿烷类化合物为主的亚组分,可直接用于单化合物碳同位素分析。通过对典型海、陆相烃源岩中的藿烷碳同位素特征进行对比研究,结果表明准噶尔盆地二叠系芦草沟组湖相页岩中藿烷系列单体化合物的碳同位素值(δ13C)分布在-40.7‰~-62.7‰,且呈现出随着碳数的增加同位素值变轻的特点;而川西北二叠系海相页岩中藿烷化合物δ13C则相对较重,分布在-20.5‰~-45.4‰,具有随碳数增加先变轻再变重的趋势。两类页岩中藿烷类化合物的碳同位素变化幅度可达±20‰以上,尤其是川西北二叠系海相页岩中藿烷类化合物单体碳同位素的差异可达到±24.9‰,表明藿烷类化合物具有明显的多源特征。藿烷类化合物的分子碳同位素受到原始沉积环境、母质来源与演化过程的影响而可能具有较大的差异,但这并不影响其有望成为新的油源对比指标。Abstract: With a mixed stationary phase of 5Å molecular sieve/alumina, extracts of various kinds of source rocks are separated by column and sub-fractions dominated by hopanoid compounds were successfully obtained. The sub-fractions are suitable for the direct analysis on GC-IRMS to obtain compound specific isotopic values of hopanes. Comparison is carried out on isotopic signatures of specific hopanes in a lacustrine oil shale (Permian Lucaogou shale, Junggar Basin, NW China) and another Permian marine shale (NW of Sichuan Basin). Results show that the Permian Lucaogou shale had the δ13C values of hopanes distributed between -40.7‰ and -62.7‰, and decrease with the increasing of carbon number. The δ13C values of hopanes in marine shale of Sichuan Basin are apparently heavier, distribute between -20.5‰ and -45.4‰, and they appeared to be firstly decreased and then increased as carbon number increases. In both of the shales analyzed, the variation range of carbon isotopes of hopanes can be more than ±20‰ (±24.9‰ for the Sichuan Basin shale), which indicates multiple contribution to these compounds. The isotopic signatures of hopanes may be constrained by paleoenvironment, sources as well as maturation, however, compound specific isotope of hopanes is still a useful indicator for oil-source correlation.
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Key words:
- bacteria /
- hopanoid compounds /
- carbon isotope /
- marine source rock /
- continental source rocks /
- Sichuan Basin /
- Junggar Basin
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图 3 准噶尔盆地二叠系芦草沟组湖相烃源岩藿烷化合物GC-IRMS测定谱图
图中数字表示的化合物见图 2。
Figure 3. GC-IRMS chromatogram of hopanes in lacustrine source rocks from Permian Lucaogou Formation, Junggar Basin
表 1 实验样品地球化学信息
Table 1. Basic geochemical parameters of experimental samples
序号 样品编号 地区 层位 ω(TOC)/% S1/(mg·g-1) S2/(mg·g-1) Tmax/℃ Ro/% 1 KSL 川西北 P2d 6.82 1.15 46.49 448 0.65 2 J36-6 吉木萨尔凹陷 P2l 2.21 9.25 12.05 442 0.85 3 J36-11 吉木萨尔凹陷 P2l 1.16 6.36 5.59 433 0.90 -
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