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 retardation is related to the vitrinite composition. Vitrain (assumed to be pure 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 yielding 'true' reflectance values, the vitrinite from three of the samples 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 hydrogen enrichment is inherited from oxygen-deficient, waterlogged and marine-influenced conditions in the precursor mires. FTIR spectra demonstrate 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 HI_vitr values 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 energies for the vitrinites are centred at 260 and 268 kJ/mole. It is not possible to detect a mwasurable difference in E_a between the perhydrous and 'normal' vitrinites. This may suggest more-or-less similar generation characteristics, or it may indicate that Rock-Eval kinetics do not replicate Nature in this regard.