NONBIODEGRADED AROMATIC CONDENSATE ASSOCIATED WITH VOLCANIC SUPERCRITICAL CARBON-DIOXIDE, OTWAY BASIN - IMPLICATIONS FOR PRIMARY MIGRATION FROM TERRESTRIAL ORGANIC-MATTER

Liquid CO2 of magmatic origin is produced commercially from a petroleum exploration well, Caroline-1, in the Otway Basin, South Australia. The well is located near a 4500-year-old maar volcano. Production of this CO2-rich fluid (approximately 98% CO2, approximately 1% CH4, approximately 0.5% N2) from the Late Cretaceous Waarre Sandstone is accompanied by a small amount of heavy (15-degrees API gravity), low sulphur (0.2%), aromatic condensate. Gasoline-range hydrocarbons (C5-C-10) comprise less than 2% of this petroleum. However, there is no evidence of alteration by water-washing or biodegradation which might otherwise explain its high specific gravity and unusual aromatic-intermediate composition. Naphthalene and the methylnaphthalenes are the dominant individual components. Aspects of the condensate's biomarker geochemistry are consistent with its derivation from bacterially-reworked terrestrial organic matter deposited in an oxic aquatic environment. The methylphenanthrene index of the oil (MPI-1 = 0.59) converts to a calculated vitrinite reflectance of 0.62%. Mature land-plant-derived crude oils and condensates usually have a primary paraffinic composition. The highly aromatic character of the Caroline-1 crude may reflect the involvement of CO2 in its origin. In this case, supercritical volcanic CO2 appears to have stripped aromatic and less abundant saturated hydrocarbons from marginally-mature, poor-quality Type III-IV kerogen in the reservoir rock. This enigmatic condensate occurrence highlights the potentially important role played by CO2 in the mobilisation of liquid hydrocarbons from terrestrial organic matter in major petroleum provinces like the Cooper Basin, central Australia.