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Characteristics and Origin of Crude Oils in Dawanqi Oilfield, Kuqa Depression of NW China


Kuqa depression is a petroliferous district of natural gas and crude oils. Previous studies focused on the prolific natural gas resources in central and southern areas, while the specific genetic affinities of the oils in Dawanqi Oilfield, an indispensable crude oil producing area in northwest of Kuqa depression, are still not clear, which impede the oils exploration in this area. In this regard, a suit of 33 oil samples were analyzed to discuss the geochemical characteristics and origin of crude oils in Dawanqi oilfield based on molecular geochemical features. The results of the oils showed a typical terrestrial origin of higher plant input with two families. Family A scattered in fault block DW1 distribute in shallow layers of the Neogene Kuche formation (Nk1) with slight biodegradation. Light hydrocarbons show some lost for compositions and complete benzene and toluene distribution, which is consistent with the biodegradation. The biomarkers of these oil present with abundant gammacerane and 4-methyl steranes but relatively poor in diahopanes and diasteranes. This oil family share the same compositional features to the oil found in Dabei area, indicating their genetic affinities which probably originate from lacustrine source rocks in Upper Triassic Huangshanjie formation (T3h) in the northern depression. Family B is widely distributed in Neogene Kuche formation (Nk2-Nk6) of fault block DW101, DW105 and DW109. This oil family is characterized by wide carbon range of alkane hydrocarbons without any biodegradation, and is much more mature than Family A. It is observed that the gammerance and 4-methyl steranes are less abundant while diahoptane and diasterane are relatively concentrated. The compositional characteristics of Family B are well in accordance with Qule and Yangta oils in southern depression, suggesting this oil family most likely stem from mudstones of Middle Jurassic Qiakelama formation (J1q) or lower Jurassic of Yangxia formation (J1y). This results are also supported by migration parameters of diamondoids and aromatic analysis. The widespread mudstone-sourced oil in southern blocks indicates the significance potential in deep layers. The new understanding of this study toward the origin of the oils in Dawanqi area would be prospective for the genetic mechanisms of crude oils and further oil exploration in this area.