Abstract: Carbon Dioxide Generation from Coals in On-shore Taranaki Basin, New Zealand, and Implications for Petroleum Expulsion

Stephen D. Killops, Richard G. Allis, Robert H. Funnell

Elevated CO₂ levels (ranging to >40% in Kapuni field) in gas/condensate accumulations within the on-shore area of the Taranaki basin can be quantitatively accounted for by decarboxylation of coals over the lignite to early high-volatile bituminous coal ranks. The highest concentrations are found within sandstone reservoirs immediately vertically adjacent to Eocene coal units which appear to have realized about 50% of their CO₂ genetic potential. The gas exists as a supercritical fluid under reservoir conditions and appears to be generated in significant amounts, comparable by mass to liquid hydrocarbon generation. Volumetrically, CO₂ may exceed liquid hydrocarbon generation by up to a factor of 2 to 3, depending on the efficiency of removal mechanis s, such as dissolution in pore water and carbonate precipitation. General kinetic modeling of coal maturation at heating rates similar to thermal history of the Taranaki basin (2-3°C/m.y.) suggests the onset of CO₂ evolution occurs at around 75°C, whereas significant oil generation begins by 105°C, and CH₄ by 135°C (defined by 10% generation levels). For perhydrous New Zealand coals, CO₂ generation probably begins some 10-20°C lower. It is proposed that the CO₂ has aided oil expulsion from coals, both directly, because CO₂ generation overlaps with the early phase of oil generation, and indirectly by pressure-induced microfracturing of source rock during the relatively rapid generation of large amounts of CO2 prior to the main phase of oil generation.