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Fluid Flow and Reservoir Evolution |
Department of Geology, Box 1047, University of Oslo, 0316 Oslo 3, Norway
Flow of fluids in sedimentary basins causes transport of heat and dissolved mass and is therefore potentially important in relation to diagenetic reactions. Rather large fluid fluxes are required, however, for this type of transport to be significant in terms of dissolution and precipitation of minerals. Before diagenetic processes are attributed to pore water flow, semi-quantitative calculation of flow rates and duration of flow should be attempted.
Heat flow in sedimentary basins is normally dominated by conduction, except on a local scale. The upwards pore water flux due to compaction is on average smaller than the subsidence rate, and the thermal anomalies caused by compaction are moderate in modern basins. Dissolution and precipitation of minerals which are in equilibrium with the pore water occur when the flow is oblique relative to the isotherms. Due to the low solubility/temperature gradient of the common silicate and carbonate minerals, very large fluxes of pore water are required to transport significant volumes of mass in solution. The greatest potential for transporting solids and creating secondary porosity is during meteoric water flow, because the flow rate then may be several orders of magnitude higher than what is typical for compaction-driven flow. Quartz overgrowth corresponding to 3% of the rock volume requires a total flow of 108cm3 cm–2 if the silica is introduced by vertical compaction-driven flow from a source outside the sandstone. Pore water flow precipitating quartz due to cooling will dissolve carbonate cement at a much higher rate. An external import of silica through the flow of water would be expected to cement up the most permeable pathways, such as fractures and well-sorted permeable sand beds. In the case of carbonate cement the solubility gradient is negative and upwards flowing and cooling pore water would cause dissolution rather than precipitation. Also in the case of carbonate rocks, large-scale mass transfer such as dolomitization is easier to explain as occurring at relatively shallow depths rather than during deeper burial.
Ore minerals like galena have a steep solubility/temperature gradient, but concentrated precipitation requires rapid cooling of hot water. Such conditions are most likely to be met when hot fluids are cooled near the surface.
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