ORGANIC GEOCHEMISTRY IN RELATION TO THE DEPOSITIONAL-ENVIRONMENTS OF MIDDLE JURASSIC COAL SEAMS, DANISH CENTRAL GRABEN, AND IMPLICATIONS FOR HYDROCARBON GENERATIVE POTENTIAL

Middle Jurassic strata from the Harald gas field in the Danish Central Graben include five coal seams. The precursor mires were situated in coastal plain environments, and peat formation occurred during water t able rise related to relative sea level rise. In a sequence stratigrap hic framework, the seams are situated at the transition zone between t he lowstand and transgressive systems tracts or in the transgressive s ystems tract close to a level that can be correlated with the maximum flooding surface. Influence from relative sea level rise on peat accum ulation is reflected in the petrographic and organic geochemical compo sition of the seams. The coals that represent peat accumulation during rapid relative sea level rise are characterized by a higher average h ydrogen index, extractability, hydrocarbon yield, and thermally extrac ted and generated bitumen content (S-1+S-2) compared to the coals that represent peat formation during the initial stages of relative sea le vel rise (slow rate of water table rise). In the latter coals, Pr/Ph r atios are higher, and the content of C-29 steranes is higher and the c ontent of C-27 is lower. With regard to thermal maturity, the coals ar e in the oil window and may possess the potential to generate and rele ase liquid hydrocarbons, This is supported by the com position of oil samples derived from sandstones in the Harald field. Multivariate data analysis shows a positive correlation between S-1+S-2, which is taken to indicate the generative potential of the coals, and in particular the macerals telinite, telocollinite, and cutinite, the microlithotype s vitrite and clarite, and total organic carbon, Such a petrographic c om position is favored in precursor mires characterized by continuousl y waterlogged, anoxic conditions. These conditions are best met in mir es situated on the lower coastal plain during rapid relative sea level rise, The knowledge obtained from multivariate modeling of the data, and sedimentological and sequence stratigraphic interpretations of the coal-bearing strata, may thus make it possible to locate the coals wi th the highest generative potential.