Journal of Petroleum Geology,
vol. 21(3), July 1998, pp. 247-263
REFLECTANCE RETARDATION (SUPPRESSION) AND SOURCE ROCK PROPERTIES
RELATED TO HYDROGEN-ENRICHED VITRINITE IN MIDDLE JURASSIC COALS, DANISH NORTH SEA
H. I. Petersen* and P. Rosenberg*
Middle Jurassic high volatile bituminous A coals from
the Danish North Sea may yield vitrinite reflectance values retarded by 0.10-0.14 %Rr.
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. S1(vitr),
S2(vitr) and HIvitr 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 measurable
difference in Ea 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.