Migration and occurrence of high wax oils in the Damintun Depression, Northeast, China: Implication for primary controls of petroleum migration pathways in heterogeneous carrier beds

This paper discusses migration styles and primary controls of petroleum migration pathways in heterogeneous carrier beds in the Damintun Depression, Bohai Bay Basin. There are two types of crude oils in the Damintun Depression: a high wax oil (wax hydrocarbon content between 8 and 60%) and a normal oil (wax hydrocarbon content lower than 8%). The high wax oil and normal oil are distinctly different both in biomarker compositions and in distribution patterns of carbon isotopic ratios for individual alkanes, and were confirmed to have been generated from different source rocks. Modeling of secondary migration pathways of both the high wax oil and the normal oil was conducted using a simple three-dimensional model, which assumes that the positions of petroleum migration pathways are controlled by the morphology of the sealing surfaces without taking into consideration the influence of permeability heterogeneity (the morphology-controlled migration model). The modeling results have reasonably well predicted the occurrences of both the high wax oil and the normal oil. All commercial petroleum accumulations are along the predicted preferential petroleum migration pathways (PPMP) formed by focusing of numerous “small petroleum streams” close to the kitchens. The focusing of oils originating from a large area of the generative kitchens is essential for the formation of large oilfields. The strong porosity and permeability heterogeneities of the carrier beds, and the relatively high prediction accuracy of the modeling, suggest that the preferential petroleum migration pathways, although influenced by permeability contrast at local scales, can be effectively predicted at a basin or depression scale under certain circumstances using a model that does not take into consideration the effect of heterogeneity. Most dry holes are not on the predicted petroleum migration pathways, suggesting that the three-dimensional migration pathway modeling may provide a useful tool to reduce exploration risk.