Mass Transfer of Petroleum During Secondary Migration: The Main Fractionation Processes
MICHELSEN, JOHAN K. and KHAVARI KHORASANI, GANJAVAR
The distribution of different petroleum types in the subsurface depends on the compositional changes of petroleum expelled from the source rocks with increasing maturity, and the relative molecular velocities during secondary migration. The molecular velocities, approach phase velocity only during one-phase convective mass transfer at low petroleum saturation. During two-phase flow, relative phase mobility imposes the largest slow down of molecules which preferentially reside in the slowest moving or immobile phase. In petroleum systems charged by siliciclastic marine source rocks, liquid-vapor fractionation dominates phase fractionation while in petroleum systems with marine carbonate or terrestrial source rocks, liquid-liquid fractionation can play an important role. However, probably the most important fractionation process which affects the efficiency of secondary migration is linked to the irreversible thermodynamics of petroleum columns. In reservoirs, a combination of convection and molecular diffusion will set up compositional gradients to eliminate emerging mechanical and compositional instabilities in the column. Therefore, during fill and spill involving an oil column, the spill composition will represent an under-saturated oil, even at a time where a saturated fluid is entering the column. The reservoir stabilizes fluids which are transmitted further, and these fluids can move to lower pressures without phase transitions. This process explains the high velocity of heavy molecules and the low migration losses associated with giant heavy oil deposits in foreland basins.