Chemistry of Iranian bitumen
Extra-heavy oil and natural bitumen rep-resent crude oils which have been severely degraded by microbial action. As evidenced by their paucity of low-molecular-weight saturated hydrocarbons. Fig provides a comparison of salient attributes of crude oil and natural bitumen.
The numbers of reservoirs involved show that many fewer analyses are available for extra-heavy oil and natural bitumen, which causes the averages to be weighted by a few large deposits with numerous analyses. Never the less, the chemical and physical differences among the oil types are clear. From conventional oil to natural bitumen there are increases in density: residuum yield: pour point: dynamic viscosity; and in vanadium and non-metals nitrogen and Sulphur. Conversely, the API gravity, gasoline and gas oil yields, and volatile organic compounds (benzene, Toluene, Ethylbenzene and Xylenes-BTEX and volatile organic compounds-VOC) all degrease. there is also a decrease in average reservoir depth. Very little of the extra-heavy oil and natural bitumen originated with these chemical attributes, which are rather the result of the degradation of originally conventional crude oils, with the consequent loss of most of their low-molecular weight volatiles.
The degradation has resulted in crude oils which are very dense, highly viscous, and black. The degradation, principally bacterial, requires an active water supply to carry the bacteria, inorganic nutrients and oxygen, and to remove toxic by – products, such as hydrogen Sulphur; contact with the reservoir containing the low-molecular-weight hydrocarbon food: and temperatures generally below about 200 F (Barker 1979). Other low-molecular-weight components are lost thermal fractionation, and evaporation when the reservoir. Thermal fractionation, and evaporation hen the reservoir is breached at the earth surface.