未熟有机质发酵后饱和烃中主要生物标志化合物的变化

Changes of main biomarkers in saturated hydrocarbon fractions after fermentation of immature organic matter

采用对未熟有机质富集培养本源菌再进行类发酵(Eh>-300mV)的模拟实验方法,重点研究了发酵样品中某些生物标志化合物的变化,探讨了在低演化阶段,由于细菌对有机质的参与和改造,液态烃类形成的地球化学特征。样品发酵后,烃类,尤其是低相对分子质量烃类含量增加,CPI和OEP值接近或略高于1。族组成中,沥青质含量减少,烃类和非烃增加。饱和烃中,C-13位上带有正烷基侧链的三环萜烷出现;高含量的伽马蜡烷被检出;生物构型藿类化合物向地质构型藿烷转化,C-22位上立体异构化平衡参数明显增大,接近或达到异构化终点值;单芳甾类烃与三芳甾类烃共存;甾体化合物含量的总体趋势是相对贫乏,平衡参数较低。

Immature organic matter was used to conduct the enrichment culture of original bacteria and then to carry out the simulated fermentation experiment(Eh >-300 mV), the emphasis was placed on exploring the changes of some biomarkers in the fermentative samples and the geochemical characteristics of liquid hydrocarbon formed during initial stage due to bacterial involvement in the transformation of organic matter. After the samples were fermented, the abundance ofhydrocarbon, in particular the low molecular weight hydrocarbons increased,with CPI and OEP values reaching 1 or somewhat more. The immature mudstone was collected from the Paleogene of Qianjiang depression in Jianghan Basin, with lowvitrinite reflectance of 0.3%. It is a typical example from saline lake stratum in fault subsidence basin, whose pristane/phytane ratio is 2.40, but thatvalue evidently reduced to 0.33 in the same sample through fermentation. Phytane was not found in the peat sample before fermentation, and high amount of phytane was observed in the same sample after fermentation, that ratio is 0.36. In the group composition, the asphaltene content decreased, while the hydrocarbon andnon hydrocarbon contents increased. In the saturated fractions of fermented samples, the 13α tricyclic terpane with alkyl side chain on the C 13 place appeared, it was presumpted that precursors of those tricyclic terpanes could bederived from hexaisoprene alcohol in cellular membrane of anaerobes; γ cerotane of high abundance was detected, it may be of halobacteria origin; biological configurational hopanoid compound was converted into geological configurational hopane, the distribution of homohopanes is tending to C35 >C34 >C33; stereoisomerization equilibrium parameter of C 22 rose significantly to approximate or reach the isomerization endpoint value; monoaryl sterane hydrocarbon and triarylsterane hydrocarbon were co existent. As a whole, the steroid content was relatively low and the equilibrium parameter tended to be lowered. After being transformed by bacteria, the distribution of steroids showed an obvious biogenic substanes indication. C21-C25 pregnanes, C22-C26 methyl pregnanes, C27-C29 regular steranes as well as C28-C30 4 methyl steranes were detected in the fermentative samples. Regular steranes had a higher abundance than pregnanes, anddistributions occurred C27 >C29 >C28, ααα20R > ααα20S characteristics. Only a trace amount of αββis present. 4 methyl steranes had a higher content and was dominated by 4α methyl configuration, with ααα20R higher peak occurred on C28-C29 and αββ20R higher peak occurred on C30. All this suggests that changes of steroid compounds be related to input of algae and terrestrial plants transformed by bacteria.