塔里木盆地寒武系—奥陶系烃源岩油源特征与超深层油气来源

The biogeochemical features of the Cambrian-Ordovician source rocks and origin of ultra-deep hydrocarbons in the Tarim basin

塔里木盆地寒武系—奥陶系烃源岩的生源特征及其对超深层油气贡献是塔里木盆地超深层海相油气勘探面临的关键地质问题。根据分子与同位素地球化学分析和生烃热模拟实验研究,系统梳理油源对比的地球化学参数,认为原油中C_(26)~C_(30)甾烷的分布、伽马蜡烷指数、三环萜烷与五环萜烷比值和正构烷烃同位素等油源示踪指标,易受热演化程度、生物降解和水岩氧化还原作用影响,掩盖母源输入和沉积环境的差异。正构烷烃奇偶优势,异常的三环萜烷分布,全油和正构烷烃δ^(13)C同位素明显偏重以及高含量的多环芳烃的出现主要归因于高过成熟热演化作用、低温热液或侵入岩浆等异常热事件的影响,而并非反映■—O_(1)烃源岩中生烃母质的地球化学特征。局部的硫酸盐热化学还原反应导致无机硫并入形成次生的噻吩类化合物,造成有机硫同位素值明显升高,无法反映母源特征,目前被选做寒武系典型烃源岩和端元油的生烃母源特征很可能是经历多种次生地球化学作用改造的结果。通过分析寒武系—奥陶系两套中等热成熟度烃源岩的形成环境和生物母质类型,表明塔里木盆地台盆区寒武系—奥陶系烃源岩的油源特征差异主要受控于沉积相,塔北中西部海相原油主要与透光厌氧环境烃源岩有关,塔中地区原油主要来源于深水盆地相烃源岩。重新建立不同物化性质原油与高过成熟烃源岩之间可靠的油-源对比参数,需要同时考虑成熟度和次生改造等复杂地球化学作用的多重影响才能确定其成因和来源。

The origin characteristics of Cambrian-Ordovician source rocks in the Tarim basin and their contribution to ultra-deep hydrocarbons were the critical fundamental issues for the marine exploration of the Tarim basin.According to molecular and isotopic geochemical analysis and hydrocarbon generation thermal simulation experimental research,the geochemical parameters of oil source correlation are systematically analysed.It is considered that the distribution of C_(26)~C_(30) sterane in crude oil,gamma wax index,the ratio of tricyclic terpane to pentacyclic terpane and n-alkane isotope and other oil source tracing indicators are susceptible to the influence of thermal evolution degree,biodegradation and water rock redox,covering up the differences of parent source input and sedimentary environment.Odd even dominance of n-alkanes,abnormal distribution of tricyclic terpanes,heavierin δ~(13) C of whole oil and nalkanes,and the appearance of high content of polycyclic aromatic hydrocarbons are mainly due to the influence of abnormal thermal action caused by high-post maturity and magmatic intrusion or hydrothermal activity,rather than reflecting the geochemical characteristics of hydrocarbon generating biological materials in €-O1 source rocks.Local sulfate thermochemical reduction reaction leads to the incorporation of inorganic sulfur into thiophene compounds,resulting in the obvious increase of organic sulfur isotope value,which cannot reflect the characteristics of original source.The characteristics of typical source rocks and end member oils currently selected are likely to be the result of multi-superposition of secondary alterations.By analyzing the formation environment and biological parent material types of middle mature Cambrian-Ordovician source rocks with significant differences in thermal evolution,we show that the oil source differences of Cambrian Ordovician source rocks in the platform basin area of Tarim basin are mainly controlled by sedimentary facies;the marine crude oil in the middle and west of northern Tarim is mainly related to the source rocks in anaerobic environment,and the crude oil in Central Tarim is mainly from the source rocks of deep-water basin facies.To re-establish reliable oil source correlation parameters between crude oils with different physicochemical properties and high over mature source rocks,we need to consider the multiple effects of complex geochemical processes such as maturity and secondary transformation at the same time to determine their genesis and source.