柴达木盆地开特米里克油田凝析油成因研究——基于金刚烷烃类化合物

Origin of the condensates from Kaitemilike Oilfield in the western Qaidam Basin:Diamondoid hydrocarbons

以往的研究结果表明开特米里克油田的凝析油属于典型的未成熟油,依据是它们具有异常低的C29甾烷异构体比值[C2920S/(20S+20R)<0.25]。但是,生物标志化合物的定量分析结果则显示这类凝析油中甾烷、藿烷系列的浓度明显偏低,且与研究区的高成熟原油相当,而远低于其他低成熟原油中的甾烷、藿烷系列浓度,显然这与其未成熟特性不符。金刚烷类化合物具有高的热稳定性,它们在成熟后期和高成熟地质样品中才开始出现。但值得注意的是这些凝析油较其他原油明显富含烷基金刚烷类化合物,其浓度是所研究原油中最高的,显示高成熟特征。依据甲基单金刚烷指数MAI(>70)值和甲基双金刚烷指数MDI(>40)值与镜质体反射率RO值之间的对应关系,判断其生成时的RO值应大于1.3%,表明它们应该形成于高成熟的凝析油气阶段。此外,在C29规则甾烷与3+4-甲基双金刚烷浓度关系图版上,这类凝析油均分布在原油强烈热裂解区域,可见这是一类遭受了热裂解作用改造的高成熟原油。由此可见,该类凝析油的未成熟性是由生物标志化合物相对组成引起的一种假象,运移过程中上覆围岩中富含甾烷、萜烷的可溶沥青的浸染是导致这一假象的原因,这已得到了研究区新近系烃源岩地球化学特征的证实。因此,热稳定性高的金刚烷类化合物在确定原油成因时可提供更加可靠的信息,它可以弥补其他分子成熟度参数的不足。

Previous researches showed that the condensates from the Kaitemilike Oilfield in the western Qaidam Basin were immature oils based on the ratios of C29 sterane isomers（20S/（20S＋20R）and ββ /（αα＋ ββ）0.25.However,the new analytical results show that the concentrations of steranes and hopanes in these oils are about 0.5μg/mg and 3.5μg/mg,respectively,much less than 5.0μg/mg and 15μg/mg in lower-mature oils,but very similar to that in high-mature oil from the well Nan 10 in Nanyushan Oilfield.This feature is completely not consistent with their immaturity.Diamondoid hydrocarbons are a kind of thermally stable compounds and mainly identified in mature and high-mature geological samples.The results show that the condensates from Kaitemilike Oilfield are rich in diamondoid hydrocarbons including alky l admantanes and diamantanes,and their concentrations are highest in all studied oil samples from different oilfields,indicating of the high maturity.Based on the relationship between methy l adamantane index MAI（70）,methyl diamantane index MDI（40）and vitrinite reflectance RO,the vitrinite reflectance ROvalue could be more than 1.3%,corresponding to condensate generation stage,when those condensates were generated from their source rocks.In addition,the condensate samples from Kaitemilike oilfield are distributed in zone of intensive oil cracking in the plot between the concentrations of C29 regular steranes and 3＋4-methy l diamantane,indicating that they are a kind of crude oils to have been cracked to some extent.Therefore,the immaturity for those condensates from C2920S/（20S＋20R）ratio is not true,and mainly results from soluable bitumen rich in steranes and triterpanes in immature source rocks during migration.It is consistent with the geochemical characteristics of Neogene source rocks in the study area.In this case,diamondoid hydrocarbons with higher thermal stability have a specific significance in determining the maturity and origin of crude oils compared with steranes and triterpanes.In addition,the new recognition for the maturity of those condensates plays a positive role in further petroleum exploration in this study area.