Recent advances in organic petrology and organic geochemistry: an overview with some reference to ‘oil from coal’
Authors: Murchison D. G. 
Published: October 2007

Although organic petrology and organic geochemistry differ greatly in age, it is during the last 25 years that there has been an almost exponential expansion of research and literature in both fields. Much of the support for the developments that have taken place has come from industrial sources. The steel industry underpinned applied coal petrology in the 1960s to improve carbonization practice related to the production of metallurgical cokes. The petroleum industry has generally supported both organic petrology and organic geochemistry, with a principal aim of improving methods to assess rank (level of maturity) and type (source input) of both coals and kerogens. As precise a determination as possible of both these parameters is as essential in hydrocarbon-prospect appraisal as it is in the raising of coke quality.

In organic petrology the construction of fully automated microscope fluorometric systems has been a major advance that has helped to define more precisely the low maturity levels that exist prior to and through most of the ‘oil window’. The same optical systems have not been so successful in producing an acceptable single petrographic typing method for kerogens. Used in conjunction with chemical methods, however, microscope fluorometry has certainly contributed substantially towards more precise kerogen typing, allowing more satisfactory discrimination of the liptinite macerals, several of which are known ‘oil formers’. Another group of potentially economic rocks, the oil shales, has also been successfully classified, principally through fluorescence methods.

Within organic geochemistry, the growth and application of analytical pyrolysis methods have been of relevance and importance. While not capable of high chemical resolution, bulkflow pyrolysis systems (
cf.
‘Rock-Eval’) are now widely used to gain a preliminary estimation of maturity level, source input and generating potential of source rocks by the screening of large numbers of samples. The most important single advance in the pyrolysis field, however, has probably been the evolution of a typing method, which, although it utilizes information from both petrography and bulk-flow pyrolysis has, as its essential core, quantitative high-resolution pyrolysis data. Kerogens can now be subdivided into eight different end-members and referred to specific depositional environments.

When these fluorometric and pyrolysis data are combined with information from biological-marker studies, which have expanded rapidly as a result of the advances in computerized gas chromatography-mass spectrometry over the past decade, it is not surprising that hydrocarbon exploration efforts are better rewarded. There is now less doubt that terrestrial sources have generated economic accumulations of oil. The principal areas where oil has been generated from land-plant material are the Australian and the Canadian Frontier sedimentary basins, and the Chinese intermonbane basins, but other similar environments have been identified and no doubt other basins with terrigenous, oil-forming source rocks will be discovered in the future.

For further rapid advances in microscopic fluorometry in organic petrology a thorough understanding of the underlying chemistry will be important. Although pyrolysis-gc systems will continue to contribute to kerogen typing, surely the principal competitor (and further stimulus) to petrographic typing methods will be pyrolysis-ms. Close collaboration between investigators in the two fields is essential. A serious reexamination of the prospects for laser-micropyrolysis systems in kerogen typing, first mooted more than 20 years ago, but then unsatisfactory because the ‘state of the art’ in both microscope optics and in laser and mass spectrometer technology was insufficiently refined, should be considered. Continuing sophistication and success in the detection, identification and application of biomarkers will result from the use of a variety of techniques, for example, tandem mass spectrometry, high performance liquid chromatography-mass spectrometry and gas chromatography-isotope mass spectrometry. Selected metastable ion monitoring and high-resolution gc-ms seems an inevitable core procedure for the specific identification of biomarkers in the future, probably replacing current conventional gc-ms systems.