REFLECTANCE RETARDATION (SUPPRESSION) AND SOURCE-ROCK PROPERTIES RELATED TO HYDROGEN-ENRICHED VITRINITE IN MIDDLE JURASSIC COALS, DANISH NORTH-SEA

Middle Jurassic high volatile bituminous A coals from the Danish North Sea may yield vitrinite reflectance values retarded by 0.10-0.14 %R-r . The coals are low in liptinite content, and the reflectance retardat ion is related to the vitrinite composition. Vitrain (assumed to be pu re vitrinite) was hand-picked from eight samples and characterised by means of element analysis, Fourier Transform infrared spectroscopy and Rock-Eval pyrolysis. In contrast to the vitrinite from samples yieldi ng ''true'' reflectance values, the vitrinite from three of the sample s yielding retarded reflectances is perhydrous in nature (5.57-5.79 wt -% (d.a.f.) H) and has H/C atomic ratios of 0.80-0.83. Vitrinite from another reflectance-retarded sample is enriched in sulphur The hydroge n enrichment is inherited from oxygen-deficient, waterlogged and marin e-influenced conditions in the precursor mires. FTIR spectra demonstra te that the perhydrous vitrinites have a lower aromaticity as measured by aromatic hydrogen vibrations in the region 900-700 cm(-1) compared to the ''normal'' vitrinites. This suggests a reduced maturation rate of the hydrogen-enriched vitrinite. S-1(vitr), S-2(vitr) and HIvitr v alues from the vitrinite concentrates indicate that the vitrinite is a significant contributor to the overall generative potential; this is particularly the case for the perhydrous vitrinite. Activation energie s for the vitrinites are centered at 260 and 268 kJ/mole. It is Mot po ssible to defect a measurable difference in E-a between the perhydrous and '';normal'' vitrinites. This may suggest more-or-less similar gen eration characteristics, or it may indicate that Rock-Eval kinetics do not replicate Nature in this regard.