Extra-heavy oil is recorded in 91 deposits some of these represent separate reservoirs in a single field, of which some are producing and others abandoned. the deposit are found in 21  countries, with 11 of the deposit being offshore or partially offshore.

Natural bitumen is found in 183 identified deposits in 21 countries, These are generally reported as tar sands or, in Canada, oil sands, Clearly, many more such deposits are identified but, as in the case of oil seepages, no resource deposits are known in eastern Siberia in the Russian Federation but insufficient date are available to make more than conservative size estimates.

Two types of basin contain respectively. Most of the world’s natural bitumen and extra-heavy oil and, indeed, contain about three-quarter of all the oil reserves in the world. These basins are architecturally similar, either lying within or accreted to continental cratons. In profile, the sediments are thick and strongly folded or rift-faulted in the seaward direction folded or rift-faulted in the seaward direction and become thinner and structurally higher as they encroach upon the craton. Oil is generated in the deeply buried, thick seaward sediments and migrates upward to be trapped adjacent to the craton. Biodegradation is promoted at the carbonic edge, where the sediments have been brought near to the earth s surface. This permits an influx of fresh water, providing oxidizing conditions, and both evaporation and washing out of light, high API gravity oil components, Complete degradation results highly viscous very low-API gravity bitumen exemplified by the Alberta deposits, If the edge is reburied before the oil is completely degraded, the result is likely to be extra-heavy oil like that in the Venezuelan Orinoco Oil Belt, which is somewhat less viscous and of higher API gravity than the natural bitumen.