塔里木盆地寒武系-下奥陶统烃源岩的端元油

Typical end-member oil derived from Lower Ordovician-Cambrian source rocks in the Tarim Basin, NW China

依据塔东2井和英买力与塔中地区海相原油饱和烃和芳烃馏份的GC-MS资料,重新确立了塔里木盆地寒武系—下奥陶统烃源岩的端元油。分析结果表明,塔东2井原油富含3～6环无烷基取代的多环芳烃,显示其经历过异常高温的改造;而完整的正构烷烃系列与25-降藿烷系列并存表明其是正常油与生物降解油的混源油,因此不宜作为寒武系烃源岩的端元油。塔中11井和塔中30井原油特征明显不同于英买101井和英买201井来源于中—上奥陶统烃源岩的原油,判断它们应该来源于寒武系—下奥陶统烃源岩,且这类原油中缺乏25-降藿烷系列,表明没有遭受生物降解的改造,满足端元油的地球化学条件。但值得注意的是两类海相原油中三芳甾烷和甲基三芳甾烷系列分布与组成十分相似,这与其甾烷和4-甲基甾烷系列形成了鲜明对照。三芳甲藻甾烷是塔里木盆地海相油源研究最常使用的一类标志物,它在塔东2井原油和过成熟海相烃源岩与两类成熟端元油中分布特征上的显著差异,表明它们易受成熟度的影响,因此在海相原油油源研究中应谨慎使用。

Based on GC-MS data of saturated and aromatic fractions in crude oils from Well TD 2 and the Yingmaili and Tazhong area in the Tarim Basin,the end-member oil derived from Lower Ordovician-Cambrian source rocks was determined.The analytical results show that the crude oil from Well TD2 is rich in 3-6 ring polycyclic aromatic hydrocarbons without alkyl group,suggesting that this oil was altered by an abnormally thermal event.It contains a complete series of normal alkanes and 25-norhopanes,indicating that it is mixed oil of normal oil and biodegraded oil,so it can’t be taken as end-member oil derived from Cambrian source rock.The characteristics of crude oils from Well TZ11 and TZ30 are different from the crude oils from Well YM101 and YM201 generated by Middle–Upper Ordovician source rocks,so it is inferred that they are derived from Lower Ordovician-Cambrian source rock.In addition,these crude oils don’t contain 25-norhopanne series,indicating that they haven’t suffered biodegradation,satisfying with geochemical conditions of end-member oil.It is noteworthy that the two kinds of marine oils have similar triaromatic steroids and triaromatic methyl steroids,but different C27-29 steranes and C28-30 methyl steranes.Triaromatic dinosteroids are a kind of biomarkers most usually used in marine oil-source correlation in the Tarim Basin,but the big differences in their distributions between crude oil from Well TD 2 and post-mature marine source rocks and two kinds of end-member oils suggest that they are easily affected by thermal maturity and should be cautiously used in marine oil-source correlation.