Analysis of a Compartmentalized Bunter Structure in the Broad Fourteens Basin, Dutch North Sea

Understanding the impact of faults on reservoir compartmentalization, hydrocarbon migration and fluid contacts is a key for the exploration and development in structurally complex areas of the North Sea and requires an integrated analytic approach. The structure discussed here is located in the Broad Fourteens Basin and underwent a complex multi-phase structural evolution of extensional episodes followed by contractional phases. Two different gas-water contacts were encountered in fault bounded compartments of Bunter reservoirs. Gas pressure points lie on the same gradient in both compartments whereas water pressure points indicate differently pressured aquifers. Another key observation is that the compartments are under-filled. The integrated trap analysis applied, allows proposing a two-phase scenario in order to explain the present day trap configuration and the observed gas-water contacts. Initially gas charging of the structure took place and resulted in a single gas-water contact. During the subsequent contractional event one compartment was uplifted along the main fault of the structure preserving the trap integrity. The gas pressure was able to equilibrate during geologic time across an upper low Vshale window but no pressure equilibration was possible within the water leg, leading to the water overpressures observed in the uplifted compartment.