沉积有机质微生物降解与生物气源岩识别——以柴达木盆地三湖坳陷第四系为例

生物气的主要组分甲烷是不同微生物菌群协同降解复杂有机质的终端产物,研究生物气生成过程中微生物对沉积有机质降解的生物标志化合物特征,对柴达木盆地东部生物气气源层识别具有重要意义。柴达木盆地三湖坳陷涩北一号气区第四系沉积物饱和烃生物标志化合物的研究发现,一些泥岩层内饱和烃组分遭受微生物降解,使部分正构烷烃和无环类异戊二烯烷烃消失,色谱不可分辨的复杂有机混合物显著升高。微生物降解作用明显的泥岩,降解参数升高,可溶有机质含量与烃转化率也显著升高,反映第四系部分泥岩层中高的可溶有机质含量与烃含量,是微生物活动和降解原始沉积有机质的产物。在微生物降解作用明显的泥岩中,检测到了丰富的产甲烷菌特殊生物标志化合物2,6,10,15,19-五甲基二十烷等,表明降解程度高的泥岩中,不仅厌氧降解细菌活动强烈,产甲烷菌活动也很强烈,是生物气的优质烃源岩。因此可以得出:可溶有机质含量,特别是其中烃含量以及微生物降解参数,是评价生物气气源岩的重要依据。

Biodegradation of depositional organic matter and identification of biogenic gas source rocks: an example from the Sanhu depression of Q aidam Basin

The major component of biogenic gas, methane, is an important end product of biogeochemical processes associated with degradation of complex sedimentary organic matter by communities of microbes. Efforts to better understand the characteristics of biomarkers as a result of biodegradation of sedimentary organic matter during gas generation are therefore valuable in assessing and constraining the potential of mudstone as source rocks for biogenic gas in the eastern Qaidam Basin. Based on the study of biomarkers in saturated hydrocarbons of the Quaternary sediments from the Sebei 1 gas field in the Sanhu depression of the Qaidam Basin, it is found that saturated hydrocarbons in some mudstone samples were biodegraded and resulted in a partial removal of n-alkanes and acyclic isoprenoids and a significant increase in the "unresolved complex mixture". Mudstone samples undergone obvious biodegradation processes show increased biodegradation index and remarkable increase in the contents of soluble organic matter and hydrocarbon conversion ratios, suggesting that the high contents of soluble organic matter and hydrocarbons in some of the Quaternary mudstone layers are the results of microbial activities and biodegradation of original sedimentary organic matter. Abundant particular biomarkers for methanogens, such as 2, 6, 10, 15, 19-pentamethylicosane, were detected in mudstone samples undergone obvious biodegradation processes, indicating that both methanogenic archaea and anaerobic bacteria intensely developed in these sediments and they are therefore proposed to be excellent biogenic gas source rocks. It is the contents of soluble organic matter and hydrocarbons and biodegradation index that significant for assessing and constraining the potential of mudstone as source rocks for biogenic gas.