Groundwater burial-induced

Intermediate subsystem of burial-induced groundwater flow Lateral flow of groundwater is away from the depocentre and focussed along unconformities and through relatively permeable, sandy, silty and limestone units... 

Figure 2.9 Cross-section showing hypothetical distribution of the three subsystems of burial-induced groundwater flow (geological cross-section of the Viking Graben, North Sea, adapted from Doligez et al., 1987. Reprinted by permission of Graham and Trotman Ltd.).

Figure 2.10 Characteristic pressure-depth relations in the three subsystems of burial-induced groundwater flow.

The shallow and intermediate subsystems of burial-induced groundwater flow may develop in shaly basins with moderate subsidence rates (0.1 mm - 1 mm/year). In rapidly subsiding shaly basins (burial rates > 1 mm/year), all three subsystems may occur. 

Figure 2.13 Calculated directions and velocities of burial-induced groundwater flow in a subsiding inhomogeneous basin, after 50 and 100 million years of subsidence (after Bethke, 1985, Journal of Geophysical Research, Vol. 9, no. B8, Fig. 5, p. 6822. Copyright by the American Geophysical Union).

In addition to the grovmdwater pressure distribution, the burial-induced flow of groundwater is associated with several physical and chemical characteristics of the sedimentary basin. These include the distribution of temperature, salinity and chemical composition of the groundwater, and the distribution of diagenetic minerals in the basin. 

The flow of water through a semi-permeable membrane (clay, shale) from water with a small concentration of dissolved solids to water with a greater concentration is called osmosis (e.g. Bredehoeft et al., 1982 Neuzil, 1986). The osmotically-induced flow of water occurs because of a difference in vapour pressure across the membrane (Hinch, 1980). The aqueous activity will be relatively small in water with a relatively large concentration of dissolved solids, because more water molecules are bonded on the dissolved ions (Hinch, 1980). In a sandstone-shale sequence with water of equal chemical concentration, the aqueous activity of the shale water will be less than that of the sandstone-water, because water molecules are adsorped on the large mineral surfaces of the shale (Hinch, 1980). As a consequence, the water salinity differences that may exist in sandstone-shale sequences in the intermediate and deep subsystems of burial-induced groundwater flow may actually be in osmotic equilibrium. 

Figure 2.30 Calculated distribution of gravity- and burial-induced groundwater flow systems in the Gulf of Mexico Basin for present-day and Miocene times (after Harrison and Summa, 1991, American Journal of Science, Vol. 291, Fig. 25. Reprinted by permission of American Journal of Science).

The hydrocarbons expelled from the mature source rocks in separate phase, may initially occur in a very finely dispersed state. At depths corresponding to the peak phase of hydrocarbon expulsion in actively filling and subsiding basins, the hydrodynamic condition is characterized by the intermediate or the deep subsystem of burial-induced groundwater flow. Initially, the very finely dispersed hydrocarbons will move along with the burial-induced groundwater... 

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