姥鲛烷、植烷立体异构体的色谱分离及地球化学意义

Chromatographic separation of pristane and phytane stereoisomers and its geochemical significance

4个源自不同类型沉积环境烃源岩的原油样品分别采自吐哈、大庆、塔里木和江汉油田,另有一样品采自南海SO177航次(TVG3)冷泉碳酸盐岩作对比用,它富含crocetane(2,6,11,15-Tetramethylhexadecane)和2,6,10,15,19-Pentamethylicosane(PMI)。用本分析条件120m的HP-5色谱柱可将植烷与植烷的立体异构体或与crocetane分离,分离度最高达0.9,同时将姥鲛烷与meso-姥鲛烷分离。饱和烃样品中正构烷烃被络合后测定其单体烃稳定碳同位素值,广33井样品在植烷混合峰处的δ13C值为?28.4‰(PDB),塔里木TZ-10井原油为?33.9‰(PDB)。与采自南海冷泉碳酸盐岩中crocetane的色谱保留位置、质谱特征和稳定碳同位素值进行对比,推测crocetane可能有两种成因:一种与甲烷菌有关,另一种与植烷的成因相同,为植烷的立体异构体。同分异构体分离前后,4个原油的姥植比(Pr/Ph)变化均不大,因此,传统姥植比(Pr/Ph)参数的指示意义仍然是有效的。

Four crude oil samples, representing different depositional environments, were collected from Tuha, Daqing, Tarim and Jianghan oil fields, respectively. A cold seep carbonate sample from the South China Sea, was used as a reference due to its richness in crocetane （2,6,11,15-Tetramethylhexadecane） and 2,6,10,15,19- Pentamethylicosane （PMI）. By using a 120 m long HP-5 column, phytane and its stereoisomer or crocetane could be well separated, and the resolution could be up to 0.9; pristane and meso-pristane were also separated at the same time. Stable carbon isotopic compositions of individual compounds of the saturated hydrocarbons were analyzed after the n-alkanes were removed by the molecular sieve method. The δ^13C value of phytane at the mixed peak for well Guang33 is 28.4 %0（PDB） and well TZ-10 is 33.9 %0（PDB） while that for the cold seep carbonates sample is 47.8%0（PDB）. Compared with the South China Sea carbonate sample, the presence of crocetane in the studied oils was deduced based on chromatographic retention time, mass spectrometry characteristics and stable carbon isotopic values. Two origins could be responsible for the crocetane. One is related with methanogens, the other is the stereoisomer of phytane with a similar origin with phytane. In addition, the Pr/Ph ratios of the four crude oils vary slightly before and after chromatographic separation of stereoisomers. Therefore, the conventional Pr/Ph ratio is still significant.