Carrier rock

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. 

Capillary pressures excerted by the barrier rock resist vertical upward hydrocarbon migration. Hydrocarbons accumulate and spread out along the horizontal barrier rock - carrier rock interface. 

Lateral updip hydrocarbon migration along inclined barrier rock - carrier rock interface... 

Hydrocarbons accumulated along an inclined barrier rock - carrier rock interface, will start to migrate laterally updip through the carrier rock when the critical height of the hydrocarbon column is exceeded again by addition of hydrocarbons. 

When the source rock is on top of the carrier reservoir rock, the downward expelled separate phase hydrocarbons will initially accumulate along the source rock - carrier rock boundary, as the fine-grained source rock acts as a top seal boundary. Once the critical vertical height of the hydrocarbons has been reached, the hydrocarbons will migrate vertically updip along the source rock - carrier rock boundary. 

England et al. (1987) use the following expression to estimate the effective hydrocarbon permeability of a carrier rock at a certain point in the subsurface... 

Table 4.1 gives the characteristics of the carrier rock (sandstone) and the fluids (oil and water) considered to be representative for conditions prevailing at about 3 km depth (England et al., 1987). These characteristics are used in the following example calculations. Introducing the values given in Table 4.1 into Equation 4.19 results in a specific discharge... 

For updip flow along a carrier rock - barrier rock interface with an inclination of a = 2°, Qo becomes... 

These estimated specific discharges for the buoyancy-driven migration of oil through a carrier rock are about 5 orders of magnitude greater than the estimated expulsion rates for oil from source rocks as given in Chapter 3. 

In a heterogeneous basin, in a carrier rock with groundwater flow parallel to the inclined upper boundary of the rock, the updip migration of a hydrocarbon stringer of length t accumulated along the carrier rock - barrier rock interface will start if... 

As outlined in Section 4.1, the migration of very finely dispersed oil droplets or gas bubbles with diameters smaller than those of the smallest pore throats of the carrier rock will not be influenced by capillary forces. In addition, according to Tissot and Welte (1984) the very finely dispersed oil droplets will not strictly follow the law of buoyancy. In the initial stages of secondary migration, the hydrocarbons in separate phase may occur as droplets or... 

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. 

Hydrocarbons, that is principally the light hydrocarbons, may leave a source rock in aqueous solution, and initially may stay in solution in the carrier rock (Chapter 3). Hydrocarbons in aqueous solution are transported through the subsurface by convection, molecular diffusion and dynamic dispersion (Section 1.3.2). 

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. 

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