Nonbiodegraded aromatic condensate associated with volcanic supercritical carbon dioxide, Otway Basin: Implications for primary migration from terrestrial organic matter

Abstract

Liquid CO 2 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 CO 2-rich fluid (∼ 98% CO 2, ∼ 1% CH 4, ∼ 0.5% N 2) from the Late Cretaceous Waarre Sandstone is accompanied by a small amount of heavy (15° API gravity), low sulphur (0.2%), aromatic condensate. Gasoline-range hydrocarbons (C 5C 10) comprise less than 2% of this petroleum. However, there is no evidence of alteration by water-washing or biodegradation whichmight 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 CO 2 in its origin. In this case, supercritical volcanic CO 2 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 CO 2 in the mobilisation of liquid hydrocarbons from terrestrial organic matter in major petroleum provinces like the Cooper Basin, central Australia.