Geochemical characteristics and genetic types of crude oils from Qinjiatun and Qikeshu oilfields in the Lishu Fault Depression,Songliao Basin,northeastern China

The Qinjiatun and Qikeshu oilfields are new Mesozoic petroleum exploration targets in Lishu Fault Depression of Songliao Basin, northeastern China. Currently, researches on geochemistry of crude oils from Qinjiatun and Qikeshu oilfields have not been performed and the genesis of oils is still uncertain. Based on bulk analyses, the crude oils in the Qinjiatun and Qikeshu oilfields of Lishu Fault Depression from the Lower Cretaceous can be classified as three types. Type oils I, from Quantou and Denglouku formations of Qikeshu oilfield, are characterized by high C₂₄tetracyclic terpane/C₂₆tricyclic terpanes ratios, low gammacerance/C₃₀hopane ratios, tricyclic terpanes/ hopanes ratios, C₂₉Ts/C₂₉norhopane ratios and 17α(H)-diahopane/17α(H)-hopane ratios, indicating a brackish lacustrine facies. Type oils II, from Shahezi Formation of Qikeshu oilfield show low C₂₄tetracyclic terpane/C₂₆tricyclic terpanes, high gammacerance/C₃₀hopane ratios, tricyclic terpanes/hopanes ratios, C₂₉Ts/C₂₉ norhopane and C₃₀diahopane/C₃₀hopane ratios, thus suggesting that they originated from source rocks deposited in a weak reducing brackish lacustrine environment, or clay-rich sediments. Type oils III, from some wells of Qikeshu oilfield have geochemical characteristics intermediate between those two types and may be mixture of type I and II oils.