塔里木轮南14井三叠系油砂中长链单甲基支链烷烃及其单体碳同位素组成

Molecular and Stable Carbon Isotopic Compositions of Long-Chain Monomethylalkanes from an Oil sand Sample in the Lunnan Oilfield,Tarim Basin,NW China

塔里木轮南14井三叠系油砂中烷烃类呈双峰型分布,前峰型正构烷烃(nC_(12)～nC_(22))单体碳同位素值较轻(-33.72‰～-32.84‰),碳同位素组成呈明显的轻奇碳数优势;后峰型正构烷烃(nC_(23)～nC_(32))单体碳同位素值略重(-32.02‰～-30.90‰),碳同位素组成呈明显的轻偶碳数优势。上述低碳数和高碳数正构烷烃在分子组成和单体碳同位素组成上的分布差异明显反映两种不同生源的生烃贡献。同时,在高碳数正构烷烃之间检测到了丰富的长链(C_(25)～C_(35))单甲基支链烷烃系列化合物,该类化合物与高碳数正构烷烃具有相近的单体碳同位素组成(-30.52‰～-31.64‰),指示二者具有共同的母质成因。但是,文献及实测单体碳同位素组成表明长链单甲基支链烷烃属异养细菌等微生物来源,因此,对传统的双峰型分布中高碳数正构烷烃属高等植物来源的认识提出质疑,高碳数正构烷烃也可能来自异养细菌等微生物生源。

The bimodal distribution of n-alkanes has been explored in an oilsand sample from well Lunnan-14, Lunnan oilfield, Tarim Basin, NW China. GC-C-IRMS Analyses indicate that the stable carbon isotopic compositions of n-alkanes nC_(23)～nC_(32)(— 32. 02‰ to —30. 90‰) are isotopically heavier than that of n-alkanes nC_(12) ～ nC_(22)( — 33. 72‰ to —32. 84‰), which reflects the contributions of two different organic matter inputs. In the mean time, high contents of mono-methyl branched alkane homologues have been detected in the n-alkanes nC_(23) ～nC_(32) which have the same or similar stable carbon isotopic compositions ( — 30. 52‰ to —31. 64‰) with the n-alkanes nC_(23)～nC_(32).This seems to mean that they share the same genesis. According to voluminous literature and measured stable carbon isotopic compositions, mono-methyl branched alkanes should be derived from microorganism. Thus, a suspicion is raised for the general biological precursor that the high molecular weight n-alkanes (nC_(23)～nC_(32)) are derived from higher plants.