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General Studies |
1 Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 OEX, UK
2 Organic Geochemistry Unit, Delft University of Technology, Faculty of Chemical Technology and Materials Science, De Vries van Heystplantsoen 2, 2628 RZ Delft, The Netherlands
3 Division of Marine Biogeochemistry, Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB, Den Burg, Texel, The Netherlands
A combination of microscopy and chemistry is used to characterize high molecular weight components of plant material which contributed to Carboniferous coals and coal-bearing strata. Material was selected from coals, paper coals, coal balls, oil-shales and shales and included cuticles from stems and leaves, periderm from stems and rooting systems, spore walls, algal cell walls and ‘resin’ rodlets. Chemical analyses were undertaken using Curie-point pyrolysis-gas chromatography (-mass spectrometry) (Py-GC-(MS)) and 13C solid state Nuclear Magnetic Resonance (NMR). Light microscopy and scanning electron microscopy were used to ensure a detailed understanding of the plant material which was analysed chemically. This study has emphasized those plants, plant tissues and organs which are known to have been dominant or major contributors to Carboniferous coals and coal-bearing sequences: arborescent lycophyte periderm (Diaphorodendron stems and Stigmaria rhizomorphs); pteridosperm cuticles (medullosan (e.g. Alethopteris) and Karinopteris), lycophyte stem cuticle (Eskdalia) and arborescent lycophyte spores (megaspores and microspores). Several algal cell walls (Tetraedron, Tasmanites and Gloeocapsomorpha) and the ‘resin’ rodlets derived from medullosan pteridosperm petioles have also been analysed. Results show that all of these elements (except resins) contain (or are dominated by) highly resistant, highly aliphatic macromolecules which have the potential to yield, upon catagenesis, n-alkanes which are found in crude oils. Resins potentially contribute cyclic hydrocarbons to crude oils. Combining all this evidence it is concluded that Carboniferous coals are oil prone and that explanations for the absence of oil-pools derived from such coals must be sought in geological or exploration factors and not in the nature of the coals themselves. A review of coal-forming floras through time, and of suggested terrestrially sourced oils and their parent coals, leads us to conclude that high molecular weight components of higher plant materials have potentially made a major contribution to oils sourced from coals and coal-bearing strata over a long period of geological time (Devonian onwards). Cell walls of freshwater algae have made a comparable contribution since the Ordovician. The oil-generating potential of each coal or organic-rich sediment will depend upon a combination of depositional environment and the floristic and chemical composition of the source vegetation and its component plants and plant parts.
