Ore. Geochem. Vol. 18, No. 5, pp. 611~27, 1992 0146-6380/92 $5.00 + 0.00 Printed in Great Britain. All rights reserved Copyright © 1992 Pergamon Press Ltd Nonbiodegraded aromatic condensate associated with volcanic supercritical carbon dioxide, Otway Basin: implications for primary migration from terrestrial organic matter* DAVID M. McKIRDyI*I" and ALLAN R. CHIVAS 2 1Department of Geology and Geophysics, University of Adelaide, Adelaide, S.A. 5001 and 2Research School of Earth Sciences, The Australian National University, A.C.T. Canberra 260 I, Australia Abstract--Liquid CO2 of magmatic origin is produced commercially from a petroleum exploration well, Caroline-l, in the Otway Basin, South Australia. The well is located near a 4500-year-old maar volcano. Production of this CO2-rich fluid (~ 98% CO2, ~ 1% CH4, ~ 0.5% N2) 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 (Cs-Cl0) 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 methylnaph- thalenes 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-I 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 Ill-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. Key words--volcanic carbon dioxide, aromatic condensate, naphthalene, dimethylnaphthalenes, biomarkers, terrestrial organic matter, non-marine source rocks, primary migration, Otway Basin, Cooper Basin INTRODUCTION Many field and laboratory studies have demonstrated that injection of high-pressure CO2 can efficiently displace oil from petroleum reservoirs during sec- ondary recovery operations (e.g. Holm and Josendal, 1974; Gardner et al., 1981). More recently, the use of supercritical (high density-high pressure) CO2 has been investigated as a means of routinely extracting hydrocarbons from oil shales and petroleum source rocks (Kesavan et al., 1988; Monin et aL, 1988; Hopfgartner et al., 1990). Carbon dioxide, at lower concentrations in aqueous and hydrocarbon-rich fluids, may also be involved in the natural, subsurface migration of petroleum. Experimental work by Bray and Foster (1980) drew attention to the role CO2 dissolved in pore water might play in the primary migration of oil from source rocks. Appreciable volumes of CO2 are gener- ated concurrently with gaseous and liquid hydrocar- tAuthor to whom correspondence should be addressed. *This paper was presented in part as a poster at the 13th International Meeting on Organic Geochemistry, Venice, Italy, September 1987; and orally at the Australian Organic Geochemistry Conference, Hobart, April 1989. bons during catagenesis of humic (woody and coaly) organic matter. For example, the amount of COs released by coal increases from ~ 10 to ~ 50 l/kg over the maturation range corresponding to the oil gener- ation window (vitrinite reflectance, VR = 0.6-1.35%) (Hunt 1979, Fig. 5-7; based on data from Karweil, 1969). The ability of this gas to mobilise oil from rocks apparently is not related to the enhanced solubility of C5+ hydrocarbons in CO2-charged pore water. Rather, as pointed out by Durand (1983), the COs lowers the interfacial tension between oil and water, thereby increasing the relative permeability of the source rock to oil. In other words, the cogener- ation of CO2 and liquid hydrocarbons in a source rock containing land-plant-derived Type III kerogen facilitates the expulsion of the latter in a separate oil phase (of. Jones, 1980). Inorganic sources of COz in natural gas accumu- lations include the leaching and thermal decompo- sition of impure carbonate rocks or minerals at temperatures in excess of 150°C (Hunt, 1979). These high temperatures may result either from deep burial of the carbonates (Colombo et al., 1969; Koncz, 1983), or their intrusion by igneous bodies (Holm- quest, 1965). In other cases, seeps and large subsur- face accumulations of COs can be genetically linked 611