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.