Assessment of the petroleum generation potential of Lower Carboniferous coals, North Sea: evidence for inherently gas-prone source rocks

The petroleum generation potential of Lower Carboniferous coastal plain coals in the Danish Gert-2 and Norwegian 2/11-9 wells in the Central Graben, North Sea, has been assessed in order to determine the source-rock quality of the coals in connection with deeper plays. The coals are vitrinite-rich, have H/C ratios from 0.75-0.79, and hydrogen index (HI) values from 171-219, which are very close to the HI _max values attained during thermal maturation. The thermal maturity of the Gert-2 coals is close to the start of the effective oil window (oil expulsion), c. 0.90-0.95% vitrinite reflectance. The proportion of aliphatic chains >C ₁₈ in the coals is negligible or very low: the C _19-35 aliphatic chains constitute only c. 60% of the total amount of C _12-35 aliphatic chains in the coal structure. Aliphatic moieties are characterized by shorter-chain aliphatics and isolated CH ₂. The generate liquid petroleum is trapped in the coals and the saturated hydrocarbon fraction is dominated by shorter-chain aliphatics. Upon solvent extraction, the measured HI values of the Gert-2 coals can, on average, be reduced by 30%, indicating that the trapped petroleum contributes to the measured generation potential. The Lower Carboniferous coals are thus inherently gas-prone, a feature they have in common with Carboniferous coals in general and which is not maturity related. Inefficient expulsion of the saturated hydrocarbons may enhance the gas-proneness by intra-source rock oil-to-gas cracking. In a deep exploration context in the N Sea area, Lower Carboniferous coal/coaly sections can therefore only constitute a gas source rock.