Bjørlykke, Knut1, Unn G. Gundersen1, Jens Jahren1
(1) University of Oslo, Oslo, Norway
ABSTRACT: Compaction Driven Fluid Flow in Overpressured Compartments
Over-pressured compartments in sedimentary basins provide some constraints on the pattern of fluid flow and it is possible to make semi-quantitative calculation of the fluid fluxes during progressive burial. In siliciclastic sediments the compaction processes are mostly mechanical at temperatures below 80-100„aC and a build up of overpressure will reduce the effective stress slowing down the rate of compaction and thereby also the compaction driven fluid flux. This provides a negative feedback on the build up of overpressure due to compaction. At higher temperatures the compaction is mostly chemical and a function of the temperature and can proceed even if the effective stress is reduced until fracture pressure is reached. The fluid flux is then essentially a function of temperature driven compaction as a function of increased burial and temperature. The water fluxes can be calculated from the rate of porosity loss in the underlying sedimentary sequence. At 2-4 km depth the water flux is typically 1-10 .10-6m3/m2yr. Modelling show that the focussing of flow at the top of anticlinal domes and rotated fault blocks may be up to 10-20 times and hydrofracturing will only cause a small transient increase in the fluid flux. Thin (<1mm) fractures at several meters spacing will be sufficient to maintain a flux in equilibrium with the rate of compaction serving as valves to maintain pressure near fracture pressure. Calculations show that the advective transport of solids and heat due to water flow from the top of a hydro-fractured reservoir will be small.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.