Peat biomass and early diagenetic controls on the paraffinic oil potential of humic coals, Canterbury Basin, New Zealand

Early mature, Pukeiwitahi Formation (Late Cretaceous) coals from Galleon-1 and Endeavour-1 wells in the Canterbury Basin, New Zealand have been analysed by petrographic, bulk chemical and thermal extraction- and pyrolysis-GC techniques and compared with similar coals in the Taranaki Basin to determine the primary controls on their paraffinic oil potential. The Pukeiwitahi coals, which are moderately perhydrous (HI 180–305 mgHC g−1TOC), vitrinite-rich (73–95%) and relatively liptinite-poor (<10%), accumulated in gymnosperm-dominated, planar mires in a temperate, coastal plain environment. Pyrolysate compositions are representative of high-wax, paraffinic–naphthenic–aromatic crude oil-generating coals. Petrographic data and the n-alkyl chain length distributions of extracts and pyrolysates suggest that the long-chain aliphatic hydrocarbons are derived primarily from leaf-derived liptinites (cutinite and liptodetrinite). The paraffinic oil potential of the coals is accordingly dependent on the net amount of leaf biomass in the precursor peats, following partial degradation of the available leaf litter under generally high surface water levels and dilution with other, less paraffinic biomass, particularly wood. The coals in Endeavour-1 generally contain more leaf-derived liptinites than those in Galleon-1 and, thus, have greater paraffinic oil potentials despite commonly lower HI values. Brackish conditions within the depositional environment caused the Galleon-1 and Endeavour-1 coals to be variably enriched in hydrogen but not in aliphatics. HI is, therefore, not a reliable indicator of the paraffinic oil potential of marine-influenced coals.