Groundwater subsystems

A shallow subsystem, characterized by cross-formational vertical upward flow of groundwater. The groundwater potential increases slightly with depth. The pressure-depth gradient is near hydrostatic to slightly superhydrostatic (Figure 2.10). 

Shallow subsystem of burial-incluced groundwater flow Groundwater flow is vertically upwards and cross-formational... 

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. 

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. 

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... 

Figure 4.9 Schematic cross-section of a Nigerian field showing focussed vertical upward escape of hydrocarbons from a deep geopressured subsystem of burial-induced groundwater flow (from Weber, 1987. Reprinted by permission of Elsevier Science Publishers BV).

Shallow subsystem of burial inducdd groundwater flow Separate phase hydrocartx>n migration is lateral in the deeper parts... 

The shallow subsystem of burial-induced groundwater flow is characterized by cross-formational vertical upward flow of groundwater and near-hydrostatic pressure-depth gradients. In comparison with hydrostatic conditions, the... 

The largest influence of hydrodynamic conditions in the intermediate subsystem on the entrapment conditions can be expected near parts of the basin with the largest vertical and lateral groundwater potential gradients, i.e. near the basin s depocentre. 

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