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Secondary migration of heavy oil in low dip basins
1Red Sea Exploration Department, Aramco, Dhahran, Saudi Arabia; [email protected]
Buoyancy is the primary driver of secondary oil migration. It can be significantly affected by a reduced oil column and a small density differential between the migrating oil and the water in the carrier bed. One of the main causes of oil column height reduction is diminishing structural dip, typically found in homoclinal basins. Freshly generated hydrocarbons migrating to shallower portions of the basin will begin losing their lighter fractions, which, once free, migrate faster, updip in the carrier beds, leaving behind an increasingly denser and more viscous liquid phase. Long-range secondary migration may also increase the exposure of the oil to biodegradation, which deteriorates the quality of the oil, further increasing its viscosity. The combined effect of weak buoyancy and high oil viscosity continually slows down the migrating front to the point at which it can stall it. Stalled migrating oil fronts are not in hydrostatic equilibrium and can produce large accumulations. The accumulation is preserved in the carrier bed itself, without any structural or stratigraphic trap, its location determined by buoyancy and oil viscosity alone. These accumulations usually are restricted to the upper portion of the carrier bed and present inclined oil–water contacts. The key to identifying these potentially large accumulations is understanding the effect of buoyancy and viscosity in the migration velocity. The proposed name for this unique trapping mechanism is stalled front trap. The Rubiales field of Colombia is an example of this trapping mechanism.
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