Fluid flow in sedimentary basins

Abstract

The pore waters in sedimentary basins are ultimately derived from sea water, meteoric water, mineral bound water, or from the underlying basement. Geochemically these waters initially have different signatures; however, the pore water compositions are easily altered by reactions with minerals or by mixing with other fluids. Meteoric water is driven downwards into sedimentary basins by the gravitational potential, which approximately corresponds to the elevation of the ground water table above the sea or lake level. It is difficult to model such flow, however, because the permeability on a large scale is almost impossible to represent realistically. The continuity of confined sandstone and limestone aquifers plays an important role in determining the flux of meteoric water received by rocks in different parts of the basin, but this is again difficult to predict. Pore water flow driven by compaction typically has velocities several orders of magnitude lower than what is commonly found in meteoric water flow regimes. The average rate of upwards flow is lower than the rate of subsidence and the difference is the rate of incorporation of sea water in the topmost layer at the sea floor. The salinity of formation water provides important constraints on fluid flow in sedimentary basins. In the North Sea Basin and the Gulf Coast Basin, these types of data indicate that vertical mixing by compaction-driven flow and convection is limited. Compaction-driven flow obeys Darcy's Law but there are complex interdependencies between pressure, permeability and compaction. Given the low compressibility of water, the flow which can result from pressure release alone is very limited, except on a local scale along permeable faults and fractures. The main flow of pore water during burial is driven by compaction, which is a slow process, even when overpressure is released abruptly with a resulting increase in the net effective stress. In the case of high permeability faults, the flow into the fault from low permeability sedimentary rocks at depth may be rate-limiting. The displacement of pore water in adjacent sediments near the top of the faults may also slow down fluid flow on fractures that do not reach the sea floor. Modelling of fluid flow in sedimentary basins should be based on permeability distributions which are mostly determined by primary facies, diagenesis and the tectonic history of the basin. Extensional tectonics produce a predominantly vertical fracture network which may serve as conduits for pore water flowing deep into the crust. Upward flow of hot fluids in basement fractures must have a high velocity in order to produce large thermal anomalies. Concentrations of dissolved components precipitate at the surface and form deposits of quartz and ore minerals. If basement fractures are overlain by a thick sequence of soft sediments, the rate of flow on the fractures will be reduced because of the low permeability in the sedimentary cover, and conductive heat transport will dominate. Flow of water at high enough velocities to bring hot water to the surface and produce hot springs is most likely to occur in fractured basements rocks or well-cemented sedimentary rocks. Concentrated precipitation of dissolved components will occur near the surface, where the rate of cooling is highest. Also, sediment-hosted ores precipitated from flow of hot water on fractures are therefore likely to form before accumulation of thick overlying sedimentary sequences.