Pore pressure within dipping reservoirs in overpressured basins

A systematic study of how mudstone permeability impacts reservoir pore pressure is important to understand the regional fluid field within sedimentary basins and the control of sediment properties on subsurface pressure. I develop a 2D static model to predict reservoir overpressure from information estimated from the bounding mudstones and structural relief. This model shows that close to a dipping reservoir, the mudstone permeability is high in the up-dip location and low in the down-dip location. This characteristic mudstone permeability variation causes the depth where reservoir pressure equals mudstone pressure (equal pressure depth) to be shallower than the mid-point of the reservoir structure. Based on the 2D static model, I constructed a nomogram to determine the equal pressure depth by considering both farfield mudstone vertical effective stress and reservoir structural relief. I find the equal pressure depth becomes shallower with decreasing vertical effective stress, increasing reservoir structural relief, and increasing mudstone compressibility. Pressure predicted by the static model agrees with pressure predicted by a more complete model that simulates the evolution of the basin and is supported by field observations in the Bullwinkle Basin (Green Canyon 65, Gulf of Mexico). This study can be applied to reduce drilling risk, analyze trap integrity, and facilitate safe and efficient exploration.