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1909 Econ. Geol. 4 625 Engler thus enumerates the various stages which in his opinion occur in the formation of petroleum from organic matter:..4. Formation of liquid hydrocarbons and violent reaction with ‘cracking’ into light or gaseous products = formation of protopetroleum.

1938 B. T. Brooks in A. E. Dunstan et al. Sci. of Petroleum I. 52/1 Accordingly it might be expected that protopetroleums in transition stages will be found in geologically recent strata in the form of solid or semi-solid material.

1973 R. E. Chapman Petroleum Geol. ii. 32 There is general agreement that the main source of petroleum is the organic matter buried with a fine-grained sediment (usually a clay); and that diagenesis of this organic matter leads to a ‘protopetroleum’ which, before or during migration, becomes modified by the physical and chemical environment—particularly by increasing temperature during burial—until it eventually becomes petroleum.