Where has all the pressure gone? Evidence from pressure reversals and hydrodynamic flow

Direct measurements of pore pressure in shales are usually not possible because of the low permeabilities, but shale pore pressures may be estimated using well-log or seismic data. Measured pressures in adjacent reservoir beds are used to calibrate the estimated shale pore pressures, a methodology that works well where reservoirs are restricted in volume and without significant leakage. Analysis of pressure data from many basins with substantial hydrocarbon reserves shows significant pressure drawdown due to lateral drainage along inclined reservoirs and through faults to shallower levels. When the direct measurements of pore pressure used for calibration have been made in reservoirs that have experienced lateral drainage, the shale pore pressures are underestimated. Consequently, mud weights selected for drilling may be too low, leading to the risk of fluid influx and blow out when encountering other reservoir beds that are sealed by encasing shales. Furthermore,understanding the geological context for overpressure development is useful in exploration because of the implications for hydrocarbon migration, hydrodynamic trapping, and hydrocarbon column height assessment.