Recent advances in coalification studies and their application to geology

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Abstract
Recent advances in coalification studies since 1970 are reported, concerning microscopic methods, causes of coalification, the coalification process as such and application to geology. The degree of coalification (‘rank’) is mostly determined on the basis of vitrinite reflectance measured on finely dispersed material in rocks. For rocks bare of vitrinite other microscopic methods are used, such as fluorescence properties of liptinites and graptolite reflectance under incident light or coloration of microfossils like palynomorphs and conodonts in transmitted light. The validity of the various methods is discussed. Maximum rock temperature and its duration are the main causes of chemical coalification reactions. Heating rates during subsidence influence the attainment of rank equilibrium. Contact-metamorphic coals and natural cokes formed from bituminous coals are characterized by a relatively high reflectance and anisotropy. Strong frictional heat may cause local increase of rank adjacent to thrust planes. Graphitization is thermodynamically not possible in nature without pressure and shearing. Confining pressure retards the chemical coalification reactions. Tectonic stress, especially shearing, promotes the increase of maximum reflectance and of anisotropy. It commonly leads to bi-axial optical properties instead of the more typical uni-axial negative geometry of vitrinite in undisturbed coal seams. Bi-axial optical properties are common for vitrinite inclusions in clastic rocks. On the basis of combined fluorescence microscopical and geochemical studies, ‘bituminization’ has been recognized as the characteristic part of coalification in the range between sub-bituminous and bituminous coal. It is characterized by the generation of petroleum-like substances which enable the coal to soften and to agglomerate during carbonization. Hydrogenation properties are favoured by the bituminization process. The different liptinite macerals behave differently during coalification and pass coalification ‘jumps’ at different rank stages. A considerable part of total inertinites is formed during early coalification (brown coal to sub-bituminous coal stage) as ‘rank inertinite’. This new knowledge appears to become important in relation to hydrocarbon exploration (‘diagenetic methane’). Some examples for the application of coalification studies to geology refer to vitrinite reflectance as an indicator of rock diagenesis and metamorphism, palaeogeographical and structural history, palaeogeothermics and hydrocarbon exploration. Large-scale areal coalification studies in foredeeps and backdeeps of ancient and recent orogens suggest estimations of crustal thicknesses and relations to plate tectonics.