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.