Section III. Geological Environments and Migration
Published:January 01, 1987
The migration of an oil into a trap is governed by its buoyancy, the capillary pressure, and the hydrodynamic forces. For heavy oils the buoyancy is low; therefore, they can only saturate high-permeability zones, which are also preferentially swept by steam in a steam-drive recovery operation.
However, the nature of the heavy oils as well as their geological history must be taken into account.
Several case histories are reviewed.
If biodegradation and water washing occur after the primary migration of a mature light oil into a trap possessing poor reservoir characteristics, recovery will be difficult. The Likouala oil field and its counterpart, the Emeraude oil field, both in the Congo Republic, exemplify this case.
If water washing increases and partially sweeps the reservoir, recovery will be impossible; this is the case in some parts of the Warner reservoir (Tri-State area, United States).
If a mature conventional oil migrates through an open hydrogeological system, it undergoes biodegradation before entering into the trap. Its low buoyancy allows saturation only of the high-permeability, high-porosity zones of the reservoir. The less permeable areas remain water saturated. Recovery by steam drive should be more efficient. Some areas of the North Poso Creek oil field exemplify this case.
The most attractive heavy oils from the point of view of steam drive recovery are immature to marginally mature oils that migrate, unaffected by biodegradation, into the trap. Heavy, mature oils generated through the combined effect of biodegradation and water washing prior to the accumulation in the trap also deserve attention.
Figures & Tables
Exploration for Heavy Crude Oil and Natural Bitumen
Gross volumes of oil, which must be kept in mind to address the volume/size framework, may be thought of in order from largest to probably smallest volumes as follows: (1) generated; (2) dissipated; (3) degraded/ partially preserved; and (4) trapped and conventionally producible. Basic knowledge of these volumes may be from greatest to least in essentially reverse order.
The 332 largest known accumulations (less than 1% of the total number) account for more than three-quarters of the known 7.6 trillion bbl of oil and heavy oil or tar in more than 40,000 accumulations in the world. About 2.4 trillion bbl of estimated undiscovered conventional oil added to the known volume of 7.6 trillion bbl yields a total of 10 trillion bbl known or reasonably estimated. World-wide cumulative production of about 500 billion bbl of oil accounts for only 5% of the gross.
Oil in place must be estimated for conventional oil fields before comparison with heavy oil and tar accumulations. The size range of accumulations considered in the size distribution of the 332 largest known accumulations is from 0.8 to 1850 billion bbl of oil. The smallest conventional fields in the distribution are about 1 billion bbl because the size cut-off is 0.5 billion bbl of oil recoverable. The size distribution of the 332 largest known accumulations approaches log normal and is overwhelmed by the largest three supergiant tar deposits that hold nearly half of the total 5495 billion bbl.
Globally, the largest three accumulations, all heavy oil or tar, are in South and North America; the two largest conventional oil fields are in the Middle East. Prudhoe Bay and East Texas fields rank 18 and 34, respectively, in descending size order.