Several hundred heavy and extra-heavy oil and natural bitumen occurrences from 26 European countries (including European Turkey and the western borderlands of the USSR) were compiled. The definitions used for heavy crude oils and natural bitumens, as proposed by or prepared with the UNITAR/UNDP information centre, were applied.

Information on stratigraphy, lithology, and depth as well as on gravity, viscosity, and gas and water content, is given.

Deposits are characteristically distributed along the flanks of the basins or within the separating uplifts. Nevertheless, they are found from the surface down to depths of 3,000 m (9,800 ft). Up to now, big accumulations have been exploited in Albania and Sicily, but they have been discovered also in the British North Sea, France, Spain, and West Germany. In carbonates, they were mostly encountered in fractures of synsedimentary or tectonic origin. The accumulations are the result of either intrusion of immature heavy oil from a source rock or of the immigration of mature oil, which was biodegraded afterward. In many cases, there have been at least two separate migration/accumulation events. In some cases paleoseepages did supply a source rock with asphaltic material or became an eff ctive seal or a later hydrocarbon accumulation.

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