Reflectance Retardation (Suppression) and Source Rock Properties Related to Hydrogen‐enriched Vitrinite in Middle Jurassic Coals, Danish North Sea

Abstract

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.

Interaction of coal and oil in confined pyrolysis experiments: Insight from the yield and composition of gas hydrocarbons

“…demonstrated in a study of confined pyrolysis on coal that the maturation of the residual solid kerogen was substantially accelerated due to the removal of hydrocarbons from the reaction system, confirming that hydrocarbons significantly retard the maturation of the residual solid kerogen. Furthermore, the results of the present study and by are consistent to the phenomenon of retardation of vitrinite reflectance which was documented by numerous previous studies (e.g., Hutton and Cook, 1980;Newman and Newman, 1982;Price and Barker, 1985;Raymond and Murchison, 1991;Petersen and Rosenberg, 1998).…”

“…Suppression of reflectance of vitrinite has been widely documented (e.g., Hutton and Cook, 1980;Newman and Newman, 1982;Price and Barker, 1985;Raymond and Murchison, 1991;Petersen and Rosenberg, 1998). It has been suggested that the reflectance suppression is principally due to absorption by vitrinite clasts of aliphatic-rich fluids, emanating from different exinitic macerals, at different stages during coalification (e.g., Raymond and Murchison, 1991).…”

The petroleum generation potential and effective oil window of humic coals related to coal composition and age

“…Diessel (1992) has likewise shown that coals formed during marine transgressions plot towards or above the upper limit of the bright coal band in the Seylers chart. The elemental data in the present study are derived from bulk humic coals, but Petersen and Rosenberg (1998) studied handpicked vitrinite concentrates. The concentrates of per-hydrous vitrinite from thermally mature petroleumgenerating Middle Jurassic coals in the Danish North Sea plotted close to or above the bright coal band and showed an average increase of ∼22% in HI compared to the concentrates of 'normal' vitrinite.…”

“…However, the increasing recognition of the ability of humic coal and Type III kerogen source rocks to generate and expel liquid petroleum has stimulated research, which has provided considerable new insights into the petroleum generative potential and generation and expulsion characteristics of these source rocks (for a thorough review, see Wilkins and George, 2002). Several studies have demonstrated that the depositional conditions influence not only the overall organic composition of coals, but also the chemical composition of the huminite/vitrinite, for example, by enhancing the hydrogen content in humic organic matter deposited in marine-influenced mires (Diessel, 1992;George et al, 1994;Petersen and Rosenberg, 1998;Petersen et al, 1996Petersen et al, , 1998aSykes, 2001).…”

Physical and chemical environments of abnormal vitrinite reflectance evolution in the sedimentary basins

“…The type of the organic matter, the environment in which the organic matter forms (consisting of inorganic geochemical environment), and major and minor elements may catalyze and/or retard the OM evolution. Many papers have been published on the types of organic matter that retard maturation (Hunton and Cook, 1980;Zhao and Wang, 1990;Goodarzi et al, 1994;Petersen and Rosenberg, 1998).…”