Organic petrology of source rocks, history and state of the art

The development of organic source rock petrology depended on the methods applied. A procedure derived from coal petrology is the study of polished sections of the untreated rock under incident light. This method, combined with the fluorescence techniques, is best suited to characterize and identify the discrete organic matter (macerals) and permits the most reliabble maturity measurements on the basis of vitrinite reflectance and of various fluorescence parameters. Another common method treats the rock with acids and studies strew slides of organic matter concentrates. This destroys the natural texture of the rock and often some of the individual macerals as well, but on the other hand it allows recognition of extremely fine particles which are not resolvable in polished sections. Maturity studies based on maceral colour changes as observed in strew slides are rapid and practical although they are rather subjective. The relative merits of the various methods are discussed. The individual organic constituents as seen in the whole rock and in organic matter concentrates are described and illustrated in two colour plates. Some classification systems are compared in tabular form. Finally, the history of microscopic observations and measurements with respect to the generation of petroleum-like substances in coal is briefly reviewed.