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 table rise related to relative sea level rise. In a sequence stratigraphic framework, the seams are situated at the transition zone between the lowstand and transgressive systems tracts or in the transgressive systems tract close to a level that can be correlated with the maximum flooding surface. Influence from relative sea level rise on peat accumulation is reflected in the petrographic and organic geochemical composition of the seams. The coals that represent peat accumulation during rapid relative sea level rise are characterized by a higher average hydrogen index, extractability, hydrocarbon yield, and thermally extracted and generated bitumen content (S₁+S₂) compared to the coals that represent peat formation during the initial stages of relative sea level rise (slow rate of water table rise). In the latter coals, Pr/Ph ratios are higher, and the content of C₂₉ steranes is higher and the content of C₂₇ is lower. With regard to thermal maturity, the coals are in the oil window and may possess the potential to generate and release liquid hydrocarbons. This is supported by the composition of oil samples derived from sandstones in the Harald field. Multivariate data analysis shows a positive correlation between S₁+S₂, which is taken to indicate the generative potential of the coals, and in particular the macerals telinite, telocollinite, and cutinite, the microlithotypes vitrite and clarite, and total organic carbon. Such a petrographic composition is favored in precursor mires characterized by continuously waterlogged, anoxic conditions. These conditions are best met in mires 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 with the highest generative potential.