迈陈凹陷徐闻X3井油砂中C_(27~39)双杜松烷两个新同系列的发现及意义

Detection and significance of two novel C_(27-39)bicadinane homologues in oilsand samples from well Xuwen X3 in the Maichen Sag

对北部湾盆地迈陈凹陷徐闻X3井27个油砂样品的饱和烃和芳烃馏分进行色谱-质谱分析,以确定C_(27~39)双杜松烷同系列的色谱、质谱特征及其意义。结果表明,徐闻X3井油砂样品具有高姥植比(介于3~7之间)、指示达玛树脂贡献的双杜松烷类化合物(包括W、T和R化合物)异常丰富的特征,(W+T)/C_(30)藿烷(C_(30)H)值大多介于1~6之间,但奥利烷(O1)/C_(30)H值变化幅度大(介于0.1~1.0之间),显示出煤成油的典型特征。这些生物标志物的分布特征与研究区发育的渐新统涠洲组含煤烃源岩的背景相吻合。首次检测到C_(27~29)和C_(31~39)两个新系列同系物,其质谱特征与顺-顺-反-和反-反-反-双杜松烷相似,均存在M^(+)-43(丢失一个异丙基)特征离子,推测它们属于具有相似立体构型的双杜松烷同系列。它们的碳数与其保留时间之间呈现起伏的折线而非线性关系,表明其分子结构中不存在基团大于异丙基的烷基侧链,据此推测C_(31~39)升双杜松烷系列中增加的碳数可能以若干甲基的形式取代在环系上。因此,在这两个同系列中,C_(27~29)同系物为脱烷基(甲基或异丙基)双杜松烷系列,C_(31~39)同系物为由若干甲基取代的升双杜松烷(即甲基化双杜松烷)系列。C_(27)和C_(29)降双杜松烷含量与油砂成熟度呈正相关性,表明成熟度升高有利于双杜松烷发生脱烷基作用;而C_(31)和C_(33)升双杜松烷含量与成熟度呈负相关性或没有相关性,暗示油砂中甲基化双杜松烷的形成与成熟度间没有因果联系。因此,油砂中C_(27~29)降双杜松烷系列可能属于次生成因,而C_(31~39)升双杜松烷系列则具有原生性。

To detect C_(27–39)bicadinane homologues and determine their geochemical significance,the saturate and aromatic hydrocarbon fractions of 27 oilsand samples from well Xuwen X3 in the Maichen sag of the Beibuwan Basin were analyzed by using GC-MS method.The analytical results demonstrated that the crude oils in those oilsands are coal-derived oil as evidenced by the high Pr/Ph ratios(3–7),the unusually abundant bicadinanes indicative of dammar resin input,mostly between 1 and 6(W+T)/C30H values,and the highly varied Ol/C30H ratios(about 0.1–1.0).The coal-derived origin of the oil is also supported by the occurrence of coal-containing sequences in the Oligocene Weizhou Formation in the study area.The two novel homologous series of C_(27–39)compounds were first detected in the saturated hydrocarbon fractions of some oilsand samples.Their mass spectra are very similar to those of cis-cis-trans-and trans-trans-trans-bicadinanes characterized by a specific ion M^(+)-43 similar configurations.Based on their mass spectra and no linear relationship between their carbon number and the retention time,it is inferred that there is no larger than isopropyl group in the alkyl side chains in their molecules,and additional carbon numbers in C_(31-39)homobicadinanes are probably substituted on the ring system of bicadinanes as methyl groups.Certainly,the precise positions of those methyl groups remain unknown in the ring system.Therefore,C_(27-29)compounds are dealkylated(methyl and isopropyl)bicadinanes,and C_(31-39)compounds are homobicadinanes with extra methyl groups in the C_(27-39)two bicadinane homologues.It is noted that there is a positive correlation between the relative contents of C_(27)and C_(29)norbicadinanes and aromatic hydrocarbon maturity parameters in the oilsands from well Xuwen X_(3),suggesting that increasing maturity could promote dealkylation of the bicadinanes.However,weak negative or no correlation can be observed between the contents of C_(31)or C_(33)homobicadinanes and the aromatic hydrocarbon maturity parameters,demonstrating that the formation of methylated bicadinanes is independent of maturity of the oilsand samples.Therefore,the C_(27-29)norbicadinanes were probably secondary,but the C_(31-39)alkylated bicadinanes were mainly primary in the oilsand samples.