Geochemical Characteristics of Crude Oils from Wunan Oilfield,Qaidam Basin

By measuring the stable carbon isotopes of satisfied hydrocarbon,aromatics and single hydrocarbons in crude oil collected from the Wunan oilfield in southwest margin of Qaidam basin,we discuss the geochemical features of the crude oil.The results indicate that the n-alkanes present a bimodal distribution pattern(i.e.,odd-to-even and even-to-odd carbon predominance),associated with low ratios of Pr/Ph(0.5) and C35-Hopane/C34-Hopane(1.0),high abundance of tricyclic-terpanes and gammacerane,and occurrence of hopane(C35).The aromatic hydrocarbons compounds were mainly composed of naphthalene and phenanthrene,and a relatively high amount of triaromatic steroid compounds occurs.These results confirm that the crude oils in the Wunan oilfield were formed in a mild-salty to salty redution depositional environment,and the parent input matter mainly derived from the lower aquatic organisms.The biomarker parameters of thermal maturity(i.e.hopanes and steranes) suggest a low thermal maturity of the crude oil.Carbon isotope of crude oils(δ13C bulk) was relatively heavy,in contrast that the carbon isotope values of single molecule hydrocarbons(δ13C n-alkanes) remain unchanged with exception of carbon isotope increase in high carbon number,which also support that the source matter of crude oils were derived from aquatic organisms,as well as the sapropelic organic matter.