辽河原油好氧生物降解模拟过程中化学组成及其碳同位素值的变化

Variations in chemical and stable carbon isotopic compositions of Liaohe crude oil during aerobic biodegradation simulation

对辽河盆地中央凸起南部潜山原油进行了好氧生物降解模拟实验,分析了辽河原油在轻度至中度(PM 1~4级)的好氧生物降解过程中化学组分的变化,以及在此过程中原油族组分和正构烷烃单体的稳定碳同位素值的变化。结果显示,饱和烃组分的消耗速率最快,正构烷烃的消耗速率高于异构烷烃,低碳数正构烷烃的消耗速率高于高碳数正构烷烃。正构烷烃的疏水性及其在原油中的相对浓度可能是影响微生物选择性地消耗正构烷烃的主导因素,即低碳数的正构烷烃由于在油水界面上的表面张力较小而更容易被微生物降解。正构烷烃单体(C_(14)~C_(30))稳定碳同位素值在生物降解过程中较稳定,未产生明显的碳同位素分馏效应。正构烷烃的好氧降解产物主要是脂肪酸、醇和CO_2。辽河原油中正构烷烃组分的δ^(13)C值较饱和烃、芳烃、胶质和沥青质组分都要偏重,降解产物中水溶性较低的高碳数脂肪酸和醇大部分进入胶质中,少部分通过氢键缔合和形成酯键等方式进入到沥青质组分中,造成沥青质组分的δ^(13)C值发生相应的变化,即对沥青质的δ^(13)C值产生"拉动效应";而降解产物中高水溶性的低碳数烷基酸和醇会进入水相而脱离原油,导致残余油的整体δ^(13)C值相应变轻。因此,假如易受生物降解的烃类如正构烷烃等的δ^(13)C值与原油的δ^(13)C值差别比较大,那么生物降解过程中随着原油化学组成的改变,原油本身及其族组分的δ^(13)C值均可能发生明显的变化。

Aerobic biodegradation simulations were performed on the Liaohe crude oil samples. Variations in chemical compositions of the oils and stable carbon isotopic compositions of oil fractions and individual n-alkanes were investigated by determining the degraded products in slightly to moderately biodegraded oils. The results showed that saturated hydrocarbons were preferentially degraded, and the normal alkanes were biodegraded prior to iso-alkanes. In addition, n-alkanes with lower carbon numbers were consumed prior to higher carbon number n-alkanes. No significant carbon isotopic fractionation occurred for the n-alkanes （C14-C30） during aerobic biodegradation. Therefore, the relative abundance and the hydrophobicity of n-alkanes should be the dominant factors controlling their degradation. That is, the n-alkanes with lower carbon numbers facilitate access to petroleum degrading microbes as substrates due to lower interfacial tension between oil and water. The aerobic biodegradation products of n-alkanes were mainly fatty acids, aliphatic alcohols and carbon dioxide. The δ13C values of n-alkanes were heavier than those of other fractions in the studied Liaohe crude oil. Most of the oil-soluble fatty acids and aliphatic alcohols were combined into resin fraction while a small portion of them into asphaltene fraction through hydrogen bonds and/or ester bonds, resulting in a δ13C shift to asphaltene fraction. Namely, the biodegradation products of n-alkanes might have a ＂pull effect＂ on δ13C of asphaltenes. Whereas the water-soluble fatty acids and carbon dioxide that were dissolved into water phase would result in 13C depletion of the residual oil. Therefore, if δ13C of the hydrocarbons susceptible to biodegradation, such as n-alkanes, is significantly different from that of crude oil, the variation of chemical compositions of crude oil would result in significant changes in the δ13C of both bulk oil and oil fractions during biodegradation.