烃源岩中单甲基烷烃化合物分析与鉴定

对鄂尔多斯盆地中生界三叠系延长组烃源岩饱和烃进行了GC-MS分析。在吴56井长8烃源岩样品中检出碳数为C₁₂~C₃₁的单甲基烷烃(MMAs)系列化合物，由甲基位置为C₂至中间碳原子的不同异构体组成，其出峰位置在同碳数正构烷烃之前；甲基取代基为C₂和C₃的MMAs相对丰度最高，C₄以上甲基烷烃相对丰度较低。根据单甲基烷烃质谱图的离子碎片特征和化学键重组方式，并结合保留时间，以C₂₂单甲基烷烃系列化合物为例，对甲基位置不同的单甲基烷烃系列化合物进行了鉴定，结果表明:C₂₂单甲基烷烃的甲基位置为C₂~C₁₁；其质谱图与同碳数正构烷烃相似，质谱图基峰仍为m/z57或m/z71，并呈现出按碳数增长(C_nH_2n+1)规律相对丰度降低的特征，有明显的分子离子峰(M+)和M+-15质量碎片；与正构烷烃相比，甲基取代基在C₅以上位置时，C₂₂单甲基烷烃质谱图中存在m/z126、140、154…C_nH_2n等偶数离子特征碎片，被认为是鉴定单甲基烷烃的重要依据。据此，总结了单甲基烷烃系列化合物特征离子表，并利用特征离子的质量色谱图的反褶积来确定气相色谱中未被分离的单甲基烷烃的保留时间，进一步利用Kovats指数和同形性因子(Hp)对单甲基烷烃进行鉴定和预测。

Analysis and Identification of Monomethylalkanes in Source Rocks

Saturated hydrocarbons in source rock from member 8 of Yanchang Formation(Upper Triassic) in Ordos Basin were analyzed by GC-MS. Different isomers of monomethylalkanes (MMAs)( carbon number ranging from 12 to 31) have been measured on a rock sample from Well Wu 56 located in Wuqi area. MMAs, with different substituted methyl ranging from the second carbon to the middle carbon, have shorter retention time than the corresponding n-alkanes with identical carbon number.2-methylalkanes and 3-methylalkanes are the most abundant compounds among MMAs, while 4-methylalkanes and other MMAs are relatively low. The positions of the substituted methyl in the MMAs can be identified through typical fragment ions, recombination pattern for chemical bonds, and retention time by mass spectrometry. For example, it has been found that the substituted methyl positions of C₂₂-MMAs are C₂-C₁₁.The mass spectrometry is similar to the n-alkane with the same carbon number, m/z57 or m/z 71 is their base peak. They also show a predominant M+ peak and (M-15)+peak, suggesting a branched methyl. Moreover, the relative abundance decreases as the total carbon increases. When the methyl position is exceeding the fifth carbon, in comparison with n-alkanes, their major characteristic fragment ions are such m/z126,140,154,as even C_nH_2n,which are important to identify MMAs. Generally, a table about the characteristic fragment ions of MMAs are concluded. As well as gas chromatographic unseparated monomethylalkane isomers were resoluted by mass spectrometric deconvolution. Finally, by using of the Kovats index and the homomorphy factors for precise retention prediction and identification of the monomethylalkanes.