Influence of Biodegradation on the Chemical Composition of Heavy Oil and Bitumen
Published:January 01, 1987
Biodegradation of petroleum is a selective metabolism of special organic compounds by an ensemble of microorganisms. It apparently starts under aerobic conditions, which are provided in areas invaded by surface- derived/ oxygen-rich formation waters. Bacteria introduced into an oil pool with meteoric waters apparently utilize dissolved oxygen for consumption of certain types of hydrocarbons. The selective removal of hydrocarbons seems to occur in the following sequence: n-alkanes, isoprenoid alkanes, naphthenes, and aromatics. Under anaerobic conditions, bacterial growth is possible if supplies of aerobic synthesized metabolites are available or else by reduction of dissolved sulfate, which results at least in the production of reduced sulfur compounds.
The fate of the synthesized microbial material is at present unclear. Part of the more stable and oil-soluble compounds may be dissolved in the crude oil and increase the fraction of nonhydrocarbons, especially asphaltenes. The microbial alteration processes yielding heavy oils are an effect of the selective depletion of light components coupled with enrichment of heavy NSO compounds, especially sulfur compounds.
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.