Molecular characterisation of vitrinite in relation to natural hydrogen enrichment and depositional environment

The Py-GC/MS results of the study carried out on two groups of vitrinites (perhydrous and non-perhydrous) of di€erent age and properties and on a trimaceralic coal associated with one of the perhydrous group are discussed. Such a study provides information about the e€ect of natural hydrogen enrichment on vitrinite structure at the molecular level. Moreover, the in ̄uence of the di€erent conditions in the sedimentary environment on the chemical structure of the vitrinite is also discussed. This in ̄uence is inferred through di€erences in the distribution and relative amount of phenolic compounds found in the pyrolysates from two samples of two di€erent coal-beds in the same basin but formed under di€erent paleoenvironmental conditions. For vitrinites with a high H/C atomic ratio, despite having a strong perhydrous character, their pyrolysates exhibit the highly phenolic signature typical of lignin-derived material with only minor aliphatic compounds. Thus, the major chemical structural elements in these vitrinites are simple phenols with a high contribution of para alkyl-substituted derivatives. However, there is no parallel relationship between the evolution of the oxygenated functionalities and the re ̄ectance values. From the results obtained a coali®cation pathway where hydrogenation processes predominate over thermal ones is proposed. The presence of resin-like substances and/or oils (which are two of the causes of natural hydrogen enrichment) in the molecular structure of vitrinites have, therefore, a€ected the normal evolution of the lignin and contributed to the special properties of this type of materials. # 2000 Elsevier Science Ltd. All rights reserved.