ABNORMAL PRESSURES AS HYDRODYNAMIC PHENOMENA

So-called abnormal pressures, subsurface fluid pressures significantly higher or lower than hydrostatic, have excited speculation about their origin since subsurface exploration first encountered them. Two distinct conceptual models for abnormal pressures have gained currency among earth scientists. The static model sees abnormal pressures generally as relict features preserved by a virtual absence of fluid flow over geologic time. The hydrodynamic model instead envisions abnormal pressures as phenomena in which flow usually palys an important role. In this paper I develop the theoretical frBmework for aabnormal pressures as hydrodynamic phenomena, showX that it explains the manifold occurrences of abnormal pressures, and examine the implications of this approach. Abnormal pressures can be explained as occurring in flow regimes that are both equilibrated and disequilibrated hydrodynamically. The former are ''adjusted'' to their geologic and hydrologic surroundings while the latter are not. Equilibrium-type abnormal pressures generally result from topographically-driven flow (as in artesian basins) but may also occur as a result of osmosis or fluid density contrasts. The more common and varied disequilibrium-type abnormal pressures are caused by natural geologic processes such as compaction, diagenesis, and deformation. These processes have the effect of distributed fluid sources or sinks and are manifested either as actual fluid sources/sinks or as virtual ones involving changing porosity and/or temperature. Disequilibrium-type abnormal pressures can be characterized in terms of three quantities: the size and permeability of their domains and the vigor of the ''geologic forcing'' or magnitude of the source/sink term. Published estimates reveal aa surprising number of distinct geologic processes that can generate forcing sufficient to maintain abnormal pressures in dynamic systems. This observation is crucial to understanding why abnormal pressures need not indicate regimes that have been static over geologic time. Low-permeability regions play key roles in both equilibrium- and disequilibrium-type abnormal pressures. Extensive low-permeability strata are generally required to generate abnormal pressures in topographically-driven flow regimes, while osmotically-driven flow and hydrodynamic disequilibrium occur only within regions composed of or encompassed by low-permeability media. Analyzing abnormal pressures as hydrodynamic phenomena often allows these low permeabilities to be estimated, something that is otherwise difficult to do. In some instances hydrodynamic analyses also shed light on rates of geologic processes and a region's geologic history.