柴达木盆地西部典型油田原油地球化学特征对比

利用色谱-质谱(GC-MS)分析技术,系统剖析了柴达木盆地西部南区尕斯库勒、油砂山油田(A区)与北区南翼山、油泉子油田(B区)原油的生物标志化合物组成和分布特征,探讨了A区和B区原油的成因差异及其地质影响因素。结果表明:A区部分浅层原油正构烷烃碳数分布不完整,具有异常高的姥鲛烷(Pr)与n-C₁₇峰面积比值和植烷(Ph)与n-C₁₈峰面积比值,说明A区浅层油藏保存条件较差,部分原油遭受了轻微生物降解; A区和B区原油中甾藿比均大于2.4,且甾烷C₂₇R与甾烷C₂₉R峰面积比值多大于1,指示这2个区域原油母质来源都以浮游藻类为主,但B区原油中甾烷C₂₇R与甾烷C₂₉R峰面积比值和补身烷系列相对含量普遍高于A区,表明B区原油浮游藻类和细菌生源贡献可能相对较高; 所有原油都具有低姥植比、高伽马蜡烷含量的特点,且都检测出一定丰度的β-胡萝卜烷,说明生成原油的烃源岩形成于强还原咸水沉积环境,但A区原油中姥植比、伽马蜡烷指数以及β-胡萝卜烷与C₃₀藿烷峰面积比值明显高于B区,且升藿烷系列呈“翘尾巴”分布模式,而B区原油则没有这种现象,揭示生成A区原油的烃源岩沉积水体盐度更高、还原性更强; 甾烷异构体比值(C₂₉20S/(20S+20R)值和C₂₉ββ/(ββ+αα)值)表明A区均为低熟原油,而B区多为成熟原油。结合柴达木盆地西部第三纪湖盆沉积和构造演化史,认为南区与北区烃源岩沉积相带的不同空间展布情况是造成A区与B区原油地球化学特征差异的根本原因。

Comparison of the Geochemical Characteristics of Crude Oils from Typical Oilfields in Western Qaidam Basin

The composition and distribution characteristics of biomarker compounds in crude oils from Gasikule Oilfield and Youshashan Oilfield in the south part of western Qaidam Basin(Zone A)and from Nanyishan Oilfield and Youquanzi Oilfield in the north part(Zone B)were analyzed with GC-MS technology, and the genetic difference of crude oils from Zone A and Zone B and its geological influencing factors were discussed. The results showed that the distribution of carbon number of n-alkane in crude oils from part shallow reservoirs of Zone A with high ratios of peak areas of pristine(Pr)to n-C₁₇ and that of phytane(Ph)to n-C₁₈ was incomplete, so that preservation condition of shallow reservoirs from Zone A was poor, and part crude oils were slightly degraded by microorganism; ratios of sterane to hopane in crude oils from Zone A and Zone B were more than 2.4, and ratios of peak areas of sterane C₂₇R to sterane C₂₉R were mostly more than 1, so that biological sources of crude oils were mainly floating algae in the two zones; the relative content of drimane series and ratios of peak areas of sterane C₂₇R to sterane C₂₉R in crude oils from Zone B were generally higher than that from Zone A, so that the contribution of biological sources including floating algae and bacteria might be relatively higher; the characteristics of all crude oils included low ratio of Pr to Ph and high content of gammacerane, and a certain of β-carotane was found, so that the source rocks generating crude oils were deposited in strong reducing sedimentary environment with saline water; ratio of Pr to Ph, gammacerane index and ratio of peak areas of β-carotane to C₃₀ hopane in crude oils from Zone A were significantly more than that from Zone B, and the distribution of homohopane series displayed that the peak area of C₃₅ was more than that of C₃₄ in Zone A, so that the salinity of sedimentary water in source rocks of crude oils from Zone A was higher, and the reducing was stronger; ratios of isomer of sterane including C₂₉20S/(20S+20R)and C₂₉ββ/(ββ+αα)indicated that crude oils were low mature in Zone A, but most were mature in Zone B. Compared with lake basin sedimentation and tectonic evolution of Tertiary in western Qaidam Basin, the different spatial distributions of sedimentary facies belt of source rock in the south and north parts were the basic reason for the difference of geochemical characteristics of crude oils between Zone A and Zone B.