Combination of basin modeling and pyrrolic nitrogen compounds to investigate the secondary oil migration pathway in the Dongying Depression of Bohai Bay Basin, China

This study investigates the pathway of secondary oil migration which leads to the oil accumulation in Dongying Depression using basin modeling and pyrrolic nitrogen compounds. Modeling of oil migration pathways have been conducted for the third and fourth member (Es₃&Es₄) of Shahejie Formation including the reconstruction of burial history, thermal maturation and hydrocarbon generation processes, which are calibrated by parameters of pyrrolic nitrogen compounds and further reinforced by distributions of oil wells and fields. Carbazole parameters, such as 1-/4-methylcarbazole (1-/4-MC), 1,8-/2,7-dimethylcarbazole (1,8-/2,7-DMC) and benzo[a]carbazole/(benzo[a]carbazole + benzo[c]carbazole) ([a]/[a] + [c]) are presented to trace the oil migration pathways. The investigated 38 oil samples can be classified into three groups considering the effects of biodegradation, thermal maturation and the variation in source facies. The group I oils are derived from the lower interval of Es₃, the group III oils are originated from the upper interval of Es₄, and the group II oils are admixing of the source-rock intervals of Es₃ and of Es₄. Nine migration pathways can be determined using the abovementioned carbazole parameters. In specific, ratios of 1-/4-MC and 1,8-/2,7-DMC increase and [a]/[a] + [c] decreases as the increasing of oil migration distance. Meanwhile, the oil migration pathways resulted from basin modeling are in good agreement with those determined by pyrrolic nitrogen compounds. Most of the investigated oil wells and fields are located on the predicted migration pathways. In a nutshell, two favorable oil accumulation areas have been predicted along the migration pathways of oil in the Es₄ member of Dongying Depression.