The pre-Pennsylvanian reservoirs of the Appalachian region are predominantly lenticular. Fluid pressures in many gas fields in these reservoirs are subnormal, but some are normal and a few are above normal. Each reservoir, across large areas, has a characteristic departure from normal of the mean values of the fluid pressures. Abnormally low fluid pressures tend to occur in lenticular reservoirs closely associated with shales in areas which have undergone erosion. A possible explanation is that erosion causes a reduction in the fluid pressure in the pore space of shales, and that this reduction is transmitted to the closely associated reservoir rocks. The pressure reduction in shales may be due to the increase in pore volume and absorption of water in clay minerals as the overburden pressure decreases, and to the absorption of water during mineral transformations that occur because of the decrease in temperature.