Erosional Unloading and Fluid Pressures in Hydraulically "Tight" Rocks

Abstract

When rocks deform in response to changes in stress, the small variations in pore volume that occur affect the pore fluid pressure. The changes in fluid pressure can be significant if the rate of change of stress is large relative to the rate at which pressure perturbations are dissipated by flow. It has been proposed previously that the gradually increasing loads on sediments undergoing burial can cause excess fluid pressures. We hypothesize that the opposite effect, depression of pore pressure resulting from rebound during erosional unloading, also may occur in certain geologic settings. Simple theoretical treatment of the problem indicates that rocks with small hydraulic diffusivity, such as some shales, could experience significant pore pressure decreases when unloaded by moderate rates of erosion. The analysis further indicates that decreases sufficient to produce negative pressures can occur. No theoretical bar to this condition exists, and it has been achieved in the laboratory. However, in subsurface environments, it is perhaps more likely that degassing of the pore fluid and resulting increase of gas volume, or desaturation, would occur. Thus it appears that a mechanism exists for generating significant underpressuring and unsaturated conditions below the water table in thick sequences of poorly permeable rock. The analysis also shows that in formations with depressed fluid pressures the groundwater flow is inward from the permeable boundaries for long periods of time. These results may be of importance if toxic materials are to be isolated in "tight" rocks.