Separate phase hydrocarbon migration

Berg (1975) amongst others, has described separate phase hydrocarbon migration from a rock pore through an adjacent pore throat in a water-wet rock under hydrostatic conditions. The following outline of the migration process is largely based on his work. 

Initially, the hydrocarbons entering at the base of a horizontal carrier rock are very finely dispersed and the buoyancy forces are still too small to initiate hydrocarbon migration. 

Vertical upward hydrocarbon migration through carrier rock 

Continued supply of hydrocarbons from the source rock increases the vertical height of the hydrocarbon column (Zo). As soon as Zq is large enough, i e as soon as the buoyancy force of the hydrocarbon column is greater than the resistant force of the carrier rock, vertical upward migration through the carrier rock will start. 

Accumulation of hydrocarbons along horizontal barrier rock - carrrier rock interface 

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Leythaeuser and Poelchau, 1990 Mackenzie et al., 1987, 1988). The pressure-driven separate phase hydrocarbon migration is treated in Section 3.2.2. 

Figure 4.4 Secondary separate phase hydrocarbon migration under hydrostatic conditions (after Hobson and Tiratsoo, 1975. Reprinted by permission of Scientific Press Ltd.).

Assuming that the subsurface conditions are isothermal and isochemical and the migration system is in steady state, the flux of separate phase hydrocarbon migration at a certain location may be estimated from Equation 4.18. For hydrostatic conditions, the generalized Darcy equation 4.18,... 

Under hydrodynamic conditions, the actual flow of separate phase hydrocarbons through carrier rocks can also be approximated by applying Darcy Equation 4.18. In order to give an idea of the orders of magnitude involved, an estimation of the specific discharge of separate phase hydrocarbon migration under steady state hydrodynamic conditions is given below. 

The lateral flux of separate phase hydrocarbon migration through carrier rocks at depths of approximately 3 km, probably is in the order of millimetres to centimetres per year. The flux of hydrocarbons in aqueous solution is in the same order of magnitude as the flux for groundwater. 

As outlined in Chapter 4, the net driving force for separate phase hydrocarbon migration is (Equation 4.11). 

At a certain time during the evolution of a sedimentary basin the secondary separate phase hydrocarbon migration is driven by hydrocarbon potential gradients (Equation 4.11 Sections 4.1.1 and 5.2), which in turn are controlled by... 

In order to quantify the separate phase hydrocarbon migration under true or assumed hydrostatic conditions, the location of hydrocarbon expelling source rocks, the amount and characteristics of expelled hydrocarbons and the basin s hydrogeological framework should be known. 

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