Changing conditions along the migration path

The viscosities of water and oil are strongly influenced by temperature. Increasing temperatures reduce the oil and water viscosities (Chapter 1 Dake, 1978). The viscosity of gas is directly proportional to the pressure (Dake, 1978). The viscosity of subsurface gases increases, whereas the viscosity of subsurface oil decreases with increasing depth (England et al., 1987 Table 4.2). 

The capillary pressures are determined by the hydrocarbon-water interfacial tension, y, and the diameters of the interconnected pore throats of the carrier rock. The interfacial tension between oil and water increases a little with depth it varies between 25 x 10 Nm and 35 x 10 Nm (Berg, 1975). The gas-water interfacial tension decreases with depth from 75 x 10 Nm at ground surface conditions to 35 x lO Nm at depths of 2 km (Berg, 1975). At depths of more than 2 km, the gas-water interfacial tension is similar to the oil-water interfacial tension (Berg, 1975 England et al., 1987). The size of the pore throats is determined by the physical properties of the carrier rock. 

The effective permeability of a carrier rock to hydrocarbons depends on the physical properties of the rock and on its hydrocarbon saturation. Both factors may change along the migration path. The hydrocarbon saturation of the carrier rock is influenced by the supply of hydrocarbons from the source rock, the characteristics of the carrier rock and the losses of hydrocarbons during the migration. 

---