Controls on the oil and gas potential of humic coals

Abstract

Humic coals are highly heterogeneous, both with respect to maceral content and chemical composition. Although the oldest known humic coals, albeit gas-prone, are of Devonian age, the majority of humic coals capable of generating and expelling non-volatile oil are of Cretaceous and Tertiary age from Australia, New Zealand and southeast Asia. In contrast, the Middle-Jurassic humic coals of the Sleipner and Hugin Formations in the South Viking Graben of the North Sea are primarily gasprone, but have the capability to expel aliphatic-rich volatile oil. These coals have TOC contents up to 80% and hydrogen indices (HI) ranging from 200 to 400. They are vitrinite-rich, with some containing up to 45% inertinite. Liptinite concentrations are generally around 10–20%, chiefly as spores, pollen and abundant resinite. The main control on the oil potential of humic coals is the concentration of long-chain aliphatic hydrocarbons in the coal matrix. Although the aliphatics have a high H/C ratio, the inference on hydrogen content from the Rock-Eval pyrolysis HI value can be misleading. For HI values below 500 mg HC/g OC there is no relation between HI and the potential of the coals to generate non-volatile oil. Consequently, the HI is a poor indicator of the non-volatile oil potential of humic coals. A more robust indicator is the relative amount of C 8+ aliphatic groups. Also, the “bulk” measure of liptinite content may also prove misleading when assessing the oil potential of coals. High resinite concentrations can lead to an overprediction of the non-volatile oil potential, as they contribute significantly to the HI value but will generate primarily volatile, aromatic-rich oils. Expulsion from the Sleipner and Hugin coals appears to have occurred in a gaseous phase with preferential release of short chain alkanes.