Remains of microorganisms (mostly; some other plant materials) and geochemical study of the range of organic compounds preserved in association with petroleum deposits have proven that the source is preserved organic materials transformed by heat and pressure of burial under the weight of sediments.

Geochemistry has proven to be of great value in determining the relationship of hydrocarbon deposits to source beds, due to the pioneering work of W. G. Meinschein, Huang, Staplin and others and, of practical value to petroleum explorationists, the identification and mapping of source-reservoir relationships which has contributed to important discoveries of reserves.