Light-Oils Transformation to Heavy Oils and Asphalts—Assessment of the Amounts of Hydrocarbons Removed and the Hydrological-Geological Control of the Process
Published:January 01, 1987
E. Tannenbaum, A. Starinsky, Z. Aizenshtat, 1987. "Light-Oils Transformation to Heavy Oils and Asphalts—Assessment of the Amounts of Hydrocarbons Removed and the Hydrological-Geological Control of the Process", Exploration for Heavy Crude Oil and Natural Bitumen, Richard F. Meyer
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Heavy oils frequently represent a residue left after removal of saturated and aromatic hydrocarbons by various alteration processes. They are characterized by a high content of asphaltenes and polar compounds and by higher sulfur content than the light oils from which they were derived.
In the Dead Sea area (Israel), light oils contain 20% asphaltenes plus polar compounds and approximately 2.5% sulfur. Asphalts, which were shown to be genetically related to the above oils, but have been altered, contain 80% asphaltenes plus polar compounds and approximately 10% sulfur.
The present study attempts to explain: (1) the quantities and types of oil constituents that were removed via the alteration processes, and (2) the geological-hydrological control of these processes.
We have applied material balance calculations in which we assume that saturated and aromatic hydrocarbons are preferentially removed from the oils at different rates, while the polar compounds and the asphaltenes remain behind as inert components. The different removal rates are explained by contrasting intensities of water washing and biodegradation. These processes seem to be affected by the extent of mixing between brines that are in contact with the oils and meteoric water.
According to these calculations, more than 75% of the light oil constituents have been removed by the alteration processes, and the asphalts represent a residue of 10-20% of the original oils. The calculations also imply that there is no reason to assume secondary enrichment of sulfur (by addition), and concentration of sulfur-rich compounds can account for the high sulfur content of these heavy oils.
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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.