The genesis of coal from the viewpoint of coal geology

Abstract

The genesis of the microscopic constituents of coal (macerals) and of maceral associations (microlithotypes) representing various coal components is discussed in light of recent advances that have been made in the study of coals and modern peats. Peat microscopy especially promoted the understanding of humification and biological gelification, both of which are decisive for the genesis of huminities/vitrinites in brown coals and hard coals, respectively. Microbial activity during peatification has been testified by chemists, microbiologists, and petrologists, and also recently by electron-microscopical observations matter and later, influences the chemical, physical and technological properties of vitrinites. Geochemical gelification transform the huminites of brown coals into the vitrinites of bituminous coals and is probably caused by the onset of bitumen generation in coal. In this process, the hydrogel brown coal is transformed to the bitumogel bituminous coal. “Coal as a source rock for oil” has become an important research subject for coal petrologists (and oil exploration!). The formation of more or less liquid bitumen and its later cracking to gaseous hydrocarbons leads to “coalification jumps” of the macerals, especially liptinite and vitrinites and to the generation of secondary macerals like exsudatinite (bitumen) and micrinite (dead carbon). Bituminization in coal has been revealed by the use of fluorescence microscopy that also showed the presence of new liptinite macerals (bituminite, fluorinite and exsudatinite) and suggests a higher proportion of algal material in coals than was thought earlier. The genesis of various inertinites will probably become an increasingly important subject of research, especially with regard to “rankinertinites” which seem to attain their high inertinitic reflectance during early coalification. Besides the origin of macerals, the origin of various coal facies in terms of maceral associations is considered in relation to the paleogeographic depositional milieu, the peat-forming vegetation, paleoclimate, water and nutrient supply, acidity, marine and calcareous influence, and fire, with examples from Euramerican Carboniferous coals, Gondwana coals and Tertiary brown coals. Vegetation and water and oxygen supply (Eh-conditions) are most important for the formation of coal facies. Vegetation has been reconstructed with the help of paleobotany, including palynology, cuticular analysis and wood anatomy (in brown coals) and with the study of petrified peats preserved in coal balls. The microscopical investigation of botanically recognizable plant remains isolated from coals or accompanying rocks has been especially informative. However, the results of palynological and cuticular analyses should be evaluated with caution considering their varying resistance and, in the case of palynomorphs, their possible extrapaludal origin. Microscopical studies of modern peats derived from different types of vegetation growing in different hydrological environments have been very helpful in interpreting coal facies, especially in brown coals. The much disputed origin of brown coal lithotypes is considered in detail. For hard coals there seems to be agreement that coal layers rich in vitrite and liptinite-poor clarite were deposited in wet forest swamps, that microlayered liptinite-rich clarites were subaquatic humic muds, and that inertinite-rich durites were oxidized peats. The origin of liptiniterich durites is controversial; they are either subaquatic muds or oxidized peats the latter perhaps from raised bogs. A recent trend is the assumption that may of not all mineable coal seams have been deposited in raised bogs under ombrogeneous, oligotrophic conditions. Although raised bogs with a dense arborescent vegetation are known from Borneo, it seems not probable that most seams (which were deposited in foredeeps or steadily sinking graben zones) are of ombrogeneous origin although a succession from relatively wet and eutrophic to relatively dry and oligotrophic conditions may be true for some seams. But, as in modern peats, striking changes of seam facies are commonly caused by hydrological events, for example, inundations as a consequence of crevasses or of fires that burnt the peat. The origin of the inertinite-rich Gondwana coals is discussed in relation to their specific climatic conditions and a comparison with sub-arctic peats is recommended. Coals influenced by marine and calcareous conditions are characterized by high contents of sulfur and pyrite, as well as hydrogen-rich vitrinites with a relatively low reflectance and high fluorescence, all due to a strong bacterial activity in peats of low acidity. Proposals for further work will be made in the conclusions [see Discussion].