Questions concerning the origin and constitution of so-called heavy crude oils have multiplied immensely with escalating interest in recovery of these oils. Although defining the term heavy crude oils presents a major and as-yet-unsolved problem, substances such as naturally occurring tars, asphalts, and bitumens are all considered to be expressions of heavy crude oils and are so treated in this presentation. Much evidence indicates that some heavy oils are residues of biodegraded conventional oils or are conventional oils from which the light ends have been stripped. On the other hand, such observations do not readily explain the origin of all heavy oils, among which are those found at great depth where neither biodegradation nor evaporative stripping is a likely mechanism.
The association of vanadium and sulfur with heavy crude oils also suggests that many such oils may have been formed by mechanisms other than those thought to be basic to the origin of conventional oils. In particular, evidence has recently been accumulated to show that carbonate rocks may be source beds for many heavy crude oils and that mechanisms leading to the direct formation of heavy oils are probably unlike those by which conventional oils are formed. Moreover, oil formed in carbonate sequences may have accumulated in source-bed reservoirs if conditions of porosity and permeability were appropriate.