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

摘要

Based on GC-MS data of saturate and aromatic hydrocarbon fractions in crude oils from Well TD 2 and the Yingmaili and Tazhong area in the Tarim Basin, the end-member oil derived from Cambrian-Lower Ordovician 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 rocks. The geochemical characteristics of crude oils from Well TZ11 and TZ30 are completely 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 Cambrian-Lower Ordovician source rocks. In addition, these crude oils don’t contain 25-norhopanne series, indicating that they haven’t suffered biodegradation and satisfy with geochemical conditions of end-member oil. It is noteworthy that the two kinds of marine oils have similar triaromatic steroids and methyl triaromatic steroids, but very different C27-29 steranes and C28-30 methyl steranes, showing that they are not a good indicator of organic matter input. 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 relative abundances between crude oil from Well TD 2 and two kinds of end-member marine oils suggest that they are easily affected by thermal maturity and should be cautiously used in marine oil-source correlation.