THE LINK BETWEEN ANOMALOUS PRESSURES AND GEOLOGIC FORCING

Fluid pressures are close to hydrostatic in much of the subsurface, and some exceptions are attributable to topographic relief or fluid density differences. Many departures from hydrostatic conditions, however, are true anomalies. Anomalous pressures owe their existence to geologic processes that supply energy to generate and maintain them, such as rapid sedimentary burial and heating, tectonic loading, and diagenesis. The rate of the process, or the geologic forcing, Γ, has dimensions of volume change per volume per time, or time^-1. A dimensionless number, Γ_d, can be defined as

Γ_d = Γl/K

where l the distance from a domain's center to the nearest boundary and K is the domain hydraulic conductivity. Γ_d characterizes the ability of geologic forcing to generate and maintain anomalous pressures. Specifically, significant pressure anomalies will exist when Γ_d ≥ ~1. Thus, where anomalies are present, the value of Γ, l, or K can be estimated if the other two quantities in Γ_d are known. Recent findings, however, complicate this picture by suggesting another mode of pressure generation, osmosis, is significant in the subsurface. New in situ and lab experiments imply that shales and claystones can generate excess pressures up to ~ 30 MegaPascals where large differences in solute concentration exist. Osmotic pressuring does not result from dynamic geologic forcing; instead the pressure energy is derived from preexisting differences in chemical potential.