Natural and artificial diagenesis of coal macerals

Abstract

Non-condensed structures in vitrinites and sporinites, prepared from high- to medium-volatile bituminous coals, have been studied both by extraction and degradation. Additionally, seven concentrates of alginites, resinites and cutinite were studied. Analyses of extractable vitrinite and sporinite alkanes showed that their yields increased markedly between 83.3% and 83.9% C vitrinite (d.a.f.), and reached maximum values at 85.4% Cvitrinite (d.a.f.)n-Alkane distributions showed increasingly lower average molecular weights with increasing rank, and the CPI's (n-C23 to n-C33) decreased to almost unity at 83.9% Cvitrinite (d.a.f.). This occurred while individual homologues progressively increased in concentration. Pyrolytic and oxidative degradation of the extracted and saponified maceral residues showed that n-alkyl chains with more than 20 carbon atoms, attached peripherally to the "kerogens", generally had even-dominated distributions. Thus, during natural diagenesis of these materials. the generation of long-chain n-alkanes showing an even-carbon-number preference may be the cause of the observed decreasing CPI values. The branched/cyclic alkanes of vitrinites and sporinites also s~owed progressive distributional changes with increasing rank, manifested by relative decreases in the yields of polycycloalkanes and increases in the yields of lower molecular weight constituents, particularly the Cl4 to C20 acyclic isoprenoid alkanes. Hopane-type triterpanes with 27. 29. 30 and 31 carbon atoms have been identified among the higher molecular weight constituents, and gc retention data suggest a remarkable constancy in the distributions of these compounds between the two macerals. Pyrolysis showed that further quantities of these compounds could be produced after extraction of the macerals. The extractable alkanes of alginites and cutinite were also composed of complex mixtures of normal and branched/cyclic alkanes, and iso- and anteiso-alkanes were identified in one alginite fraction. Resinite alkanes were unique in type, showing restricted gc distributions confined generally to the sesqui- and diterpane regions. Small yields of carboxylic acids were obtained by saponification of both extracts and residues. Fatty acids from all the macerals except resinite contained palmitic and stearic acids as prominent constituents, and, in general, the distributions of fatty acids could not be related to those of the alkanes obtained by extraction or pyrolysis. Carboxylic acids from resinites were again unique in type, and analysis of these compounds from one of the resinites has shown that they consist of diterpenoid acids, including agathic acid, pimaric acid and isomers, and dehydroabietic acid. Straight-chain, saturated ∝, w-dicarboxylic acids, with up to 28 carbon atoms, were found only in alginite and cutinite. These macerals were deposited in more fresh-water environments than vitrinite and sporinite, and the presence of dicarboxylic acids in fossil lipid mixtures may be indicative of an early phase of aerobic decomposition during diagenesis. Oxidative degradation of extracted and saponified sporinite, alginite and cutinite residues showed that polymethylene chains in the "kerogens" generally contained up to 15 carbon atoms j although chains with up to 24 carbon atoms were detected. In sporinites, the amounts decreased with increasing rank, and were scarcely detectable above ca. 85% Cvitrinite (d.a.f.). Loss of these linear structures at higher ranks may be associated with the approaching coalification break of sporinites in medium- to low-volatile bituminous coals.