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 different age and properties and on a tri maceralic coal associated with one of the perhydrous group are discussed. S uch a study provides information about the effect of natural hydrogen enric hment on vitrinite structure at the molecular level. Moreover, the influenc e of the different conditions in the sedimentary environment on the chemica l structure of the vitrinite is also discussed. This influence is inferred through differences in the distribution and relative amount of phenolic com pounds found in the pyrolysates from two samples of two different coal-beds in the same basin but formed under different paleoenvironmental conditions , For vitrinites with a high H/C atomic ratio, despite having a strong perh ydrous character, their pyrolysates exhibit the highly phenolic signature t ypical of lignin-derived material with only minor aliphatic compounds. Thus , the major chemical structural elements in these vitrinites are simple phe nols with a high contribution of para alkyl-substituted derivatives. Howeve r, there is no parallel relationship between the evolution of the oxygenate d functionalities and the reflectance values. From the results obtained a c oalification 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 str ucture of vitrinites have, therefore, affected the normal evolution of the lignin and contributed to the special properties of this type of materials. (C) 2000 Elsevier Science Ltd. All rights reserved.