Discussion on oil sources and petroleum migration in the Wunan Oilfield, Qaidam Basin

The geochemical features of the crude oil samples collected from the Wunan Oilfield in southwestern margin of Qaidam Basin were analyzed.The result shows that the n-alkanes presented a bimodal distribution pattern(odd-toeven and even-to-odd carbon predominance),with the low ratios for both Pr/Ph( 0.5) and C 35-Hopane/C 34-Hopane( 1.0),and high contents of tricyclic-terpanes and gammacerane,which indicates that the crude oil formed in a midsalty to salty water reduction depositional environment,and the parent material are mainly from the lower aquatic organisms.Moreover,maturity parameters of hopane and sterane reflect its lowmaturity of crude oil.According to the oilsource correlation,the crude oil derived from the Xiaganchaigou Formation(E 3 2) of Zhaha-Qiekelike depression.The type of crude oil is more consistent from the same source,thus the significant oil-migration effects of nitrogen compounds in the oil are observed.The composition and relative abundance of nitrogen compounds in the crude oil were used to discuss the petroleum migration in the Wunan oilfield.With the increasing of the migration distance,the concentration of nitrogen compounds is decreased,which indicates that crude oil generated on its west adjacent depression,and migrated from west to north and from northwest to southeast.It is concluded that the crude oil in the study area came from the western Zhaha-Qiekelike depression.