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ABSTRACT: Effects of synsedimentary marine influence on the petroleum generation characteristics of humic coals

Sykes, R., R.H. Funnell, S.D. Killops, and M.J. Dow , Institute of Geological & Nuclear Sciences, Lower Hutt, New Zealand

The potential of some humic coals to generate and expel oil as well as gas relates to their hydrogen-richness and content of long-chain aliphatic groups but the botanical, depositional and early diagenetic factors that give rise to oil-prone coals are incompletely understood. Many perhydrous coals have formed from marine-influenced peats, yet this factor appears to have been overlooked in many previous studies of the petroleum potential of paralic coal-bearing sequences.

In Taranaki Basin, Mangahewa Formation (Eocene) coals have sourced paraffinic oil and gas/condensate fields. The coals were deposited on a coastal plain and comprise vitrinite-rich (>80%) kerogen. Atomic H/C ratios are 0.78-0.95 and plot within or above the relatively perhydrous NZ Coal Band. HI values near the onset of oil generation are 200-400 mg HC/g Corg, indicating kerogens range from mixed oil- and gas-prone to oil-prone.

Variations in coal sulphur and occurrences of dinoflagellate cysts, mangrove pollen and Ophiomorpha indicate frequent marine flooding of the Mangahewa coastal plain and widespread development of brackish conditions. Coal seams that formed in strongly marine-influenced mires contain stronger fluorescing vitrinite, are richer in hydrogen and are thus inferred to be significantly more oil-prone than seams formed in slightly marine-influenced to freshwater mires. Furthermore, their bulk kinetics yield modelled temperatures as much as 25-30ºC lower for the onset and peak of hydrocarbon generation, possibly a result of their higher organic sulphur contents. Studies of the pyrolysates and compositional kinetics of these coals are continuing.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia