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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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