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ABSTRACT: Hydrodynamic Analysis of Pressure and Fluid Migration Systems

Otto, Claus , CSIRO Land and Water, Perth, Australia

The interdisciplinary application of hydrodynamic techniques can yield valuable information on fluid migration systems, trap integrity, and pressure compartmentalisation.

The existence of pressure compartments that are isolated from the surrounding hydrodynamic regime by low-permeability seals (eg faults) is inferred from offsets in plots of fluid pressure/hydraulic heads versus depth or elevation. However, deviations from a hydrostatic pressure profile can also be expected in an open system as a result of topography-driven regional flow system. Downward flow is associated with subhydrostatic pressures (underpressures) at depth. Upward flow is characterised by superhydrostatic pressures (overpressures). Flow across a low-permeability confining unit (e.g. fault) is accompanied by a steep hydraulic gradient. Although abrupt offsets in a pressure profile generally do result from a large permeability contrast, they do not necessarily indicate limited hydraulic communication within a regional flow system. In a closed system (i.e. not controlled by a continuous source of fluid), abnormally high or low pressures are caused by a number of geological factors, including burial, uplift, tectonism, and hydrocarbon generation, all of which require that the pressure compartmentalised regime be maintained by sealing units. Unlike the open system: (1) the pressure in a closed system will only increase if the source is contained within the pressure compartment (e.g. hydrocarbon source) or if the compartment is subjected to stress changes; and (2) the overpressure and underpressure can be severe because of its isolation. The open and closed pressure systems are not mutually exclusive within the same basin. However, they vary in terms of the dynamic and temporal characteristics of the abnormally pressured rock, as shown in the reconstruction of fluid flow histories in sedimentary basins.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia