Subsurface fluid flow in response to a pressure differential follows Darcy＇s Law such that it increases with an increase in the pressure applied and with the permeability of the matrix, and decreases as a function of the viscosity of the fluid. Overpressure is considered to be caused by either mechanical means (disequilibrium compaction due to rapid sediment loading or the horizontal equivalent, tectonic stress) or by fluid volume expansion due to geothermal heating, kerogen conversion to bitumen or mineral transformation. An additional factor that has not been considered as a cause of overpressure is the reduction of effective permeability. Total or effective permeability (as opposed to theoretical or intrinsic permeability) of a rock (the sum of relative permeabilities of all of the separate fluid phases) decreases rapidly in response to the introduction of immiscible fluid phases. Familiar two-phase relative permeability diagrams indicate that effective or total permeabilities drop to one third of intrinsic permeability as the saturation of the non-wetting phase increases. Experimental results in three-phase systems (oil, gas, water) suggest that effective permeability will fall by a factor of more than four when all three phases are present in significant concentrations. A reduction of the effective or total permeability of a rock by a factor of four would be expected to be balanced by a decrease in the fluid flow rate and; simultaneous increase in the pressure gradient. The simple algebra of Darcy＇s Law requires that the pressure gradient/fluid flow rate would also have to change by a factor of four. For example, the pressure gradient might double while the fluid flow rate halves (2/.5 = 4). The significance of these simple observations is that the introduction of one or more non- aqueous phases into a water saturated system (hydrocarbon generation) could make a significant contribution to the development of overpressure merely by virtue of the introduction of the additional phases.