Oil source and accumulation mechanism of the Damoguaihe Formation in the southern Wuerxun Sag, Hailaer Basin, China

Abstract

The Damoguaihe Formation, situated in the southern area of the Wuerxun Sag, has been proved to be petroliferous according to numerous hydrocarbon exploration results. However, whether the oil is originated from the Damoguaihe Formation or Nantun Formation source rock has long been debated. Oil-oil and oil-source correlations suggest that the crude oil of the two formations has many similarities including physical properties, n-alkane distribution, and kerogen type, which are indicative of a same hydrocarbon origin, the Nantun Formation source rock. The determined faulting period and hydrocarbon accumulation period, when combined with the current hydrocarbon distribution and the results of paleo-oil reservoir testing using oil-bearing inclusion, demonstrate that the hydrocarbon accumulation of the Damoguaihe Formation is induced by the fault-induced destroy of the Nantun Formation trap integrity that leads to the oil migration toward the Damoguaihe oil reservoir. In addition, faulting can strongly destroy the trap integrity, and the corresponding fault throws that destroy the trap integrity in the Nantun Formation are different in different formations. The throw threshold in the caprock is 120 m, while that in the post-hydrocarbon-accumulation Nantun Formation reaches 200 m. When the actual throw is greater than the throw threshold, the trap is destroyed, and thus oil and gas migrate and accumulate in shallow layers. In addition, we discuss the different fault growth history, the throw selection may be different when using the throw threshold method to determine trap integrity. It should be noted that the throw selection in this study may also be applied to similar geological conditions in other hydrocarbon systems.