Quantitative constraints on hydrocarbon vertical leakage: Insights from underfilled fault-bound traps in the Bohai Bay Basin, China

The mechanism of hydrocarbon leakage in active fault-bound traps is not yet well understood. The underfilled fault-bound traps with different burial depths in the Qinan sag (NDG-3 trap and YN-1 trap) of the Bohai Bay Basin, China, provide a favourable setting to investigate the role of faulting in hydrocarbon leakage. Interpretation of seismic and borehole data, results of rock mechanics testing, and data from quantitative fluorescence analysis are used to evaluate hydrocarbon leakage. The results reveal that the trap-bounding faults display continuous growth faulting throughout the Cenozoic. The shale caprocks for the YN-1 trap and the NDG-3 trap are brittle–ductile transition deformation according to an analysis of brittle–ductile index (BDI). The presence of thick, effective top seals and favourable fault juxtaposition seal and fault membrane seal relationships suggests that vertical leakage caused by fault reactivation is the most viable mechanism for explaining the lost hydrocarbons. A model for the shrinkage of oil column height caused by fault reactivation can be established. According to this model, about 30% of the trapped column height will be lost from structural traps during reactivation in the case of brittle–ductile transition deformation of the shale caprock. These new results have implications for petroleum exploration in fault-dominated petroliferous basins in other areas of China.