Abstract

Shales in which overpressure is generated are not always in equilibrium with their associated reservoirs. Shale pressures are determined by assessment of their "porosity" – using seismic interval velocities, resistivity, sonic travel time or density data. Comparison of actual values and expected values when normally pressured and compacted yields the magnitude of overpressure, but calibration is required, usually with direct measurement in interbedded or adjacent reservoirs. Lateral transfer, the rapid redistribution of overpressure within a tilted but continuous reservoir, leads to higher reservoir overpressure than in the surrounding shales, and the reverse occurs below an equilibrium point, known as the centroid. Lateral transfer leads to drilling surprises where the magnitude has not been assessed prior to entry into the reservoir. Lateral transfer may also raise pore pressures to the fracture pressure, leading to seal breaching and a failed trap.

Lateral drainage, whereby excess fluid within the reservoir escape via upwards linkage, perhaps due to regional uplift of one flank of the reservoir, results in lower overpressure in the reservoir relative to the shales above and below. In the case of both lateral drainage and lateral transfer beneath the centroid, the hydrocarbon sealing potential of the reservoirs is enhanced. Further, with lateral drainage, overpressure differences lead to tilted hydrocarbon-water contacts, with potential for non-structural traps.