川东北地区普光2井飞仙关组储层沥青地球化学特征及成因分析

Geochemical Characteristics and Genesis of Reservoir Bitumen of Feixianguan Formation in Well Puguan 2,Northeast of Sichuan

根据储层中沥青的产状及类型、族组成、饱和烃色谱、生物标志化合物以及储层沥青反射率等,对普光2井飞仙关组储层固体沥青的地球化学特征及成因进行了详尽的剖析。研究结果表明,普光2井飞仙关组储层沥青为古油藏原油经裂解成气之后的残留物,属热演化的焦沥青。储层沥青的正构烷烃分布较为完整,碳数在C14～C30范围,没有受到明显的生物降解作用。Pr/Ph、Pr/nC17和Ph/nC18比值都小于1.0;OEP值1.25～1.43,CPI值0.98～1.19,无奇偶优势或偶奇优势;规则甾烷系列中C27化合物含量占优势,C27—C29规则甾烷呈不对称的"V"字型分布,甾烷异构化程度较低,明显低于异构化平衡值,认为是由于高、过成熟阶段S构型比R构型裂解速率更快的结果。飞一段、飞二段孕甾烷、升孕甾烷含量明显较飞三段高,可能是有机质生源或沉积环境性质上的不同所致。萜类化合物以五环萜烷为主,三环萜烷较为发育,其以C23三环萜烷为基峰,伽马蜡烷的含量较高。这些特征表明普光2井储层沥青主要来源于低等水生生物,有机质的热演化程度较高,主要处于高—过成熟阶段,有机母质形成于沉积水体咸度较高的海相沉积环境中。储层沥青中高含量25-降藿烷并非生物降解作用所致,而主要与源岩的热演化程度较高有关。

Based on the information about reservoir bitumen,such as occurrences,types,group composition,the characteristic of saturated hydrocarbon＇ gas chromatogram and biomarkers,as well as the reservoir bitumen reflectivity,this paper thoroughly analyze the geochemical characteristic of reservoir bitumen from Feixianguan Formation in Well Puguang 2 and its possible cause of formation.It is concluded that the reservoir bitumen which is classified to thermal pyrobitumen is the residue formed after ancient reservoir cracked.n-alkane in reservoir bitumen distribute integrally with the carbon number range from C14 to C30,without obvious trace of biodegradation.Pr/Ph、Pr/nC17 and Ph/nC18 are all smaller than 1.0,OEP varies from 1.25 to 1.43 and CPI varies from 0.98 to 1.19,without odd even predominance of normal alkanes.Among regular silane series,C27 plays a dominant role with the appearance of ＂V＂ shape of the distribution of C27-C29.Isomerization of sterane in initial stage,the ratio is obviously below average value.It is believed that ＂S＂ configuration crack faster than ＂R＂ configuration is the reason.The content of pregnane and litres of progesterone silane steroid in Fei1 Formation and Fei2 Formation is apparently higher than the counterpart of Fei3 Formation as the result of the difference of environment of sedimentation or source rock.Among terpene series,pentacyclic triterpenoids dominate,tricyclic ones develope well,especially,C23 is the base peak,the content of gammacerane is relatively high.All of these characteristics mentioned above demonstrate that reservoir bitumen in Puguang2 Well mainly derived from lower level hydrobiont,thermal evolution stayed at middle-higher stage.Parent materials formed in marine sedimentary environment with high salinity.The high content of C25-norhopane is associated with high thermal evolutional stage rather than biodegradation.