Print this pageThe Characterization of CO2 Origins in the Neuquen Basin (Argentina): Mantle Fluids Influence for Oil Maturity and Gas Composition
CO2 may occur in association with petroleum and hydrocarbon gas at very high concentrations, representing a risk for exploration. When the proportion of CO2 is above 5%, its carbon isotopic signature indicates a mineral origin, without any clue to distinguish between carbonate decomposition and a mantle contribution. Only the associated noble gas isotopes may indicate a crustal or mantle origin. The Neuquen Basin is a foreland sedimentary basin, associated with numerous volcanic intrusive and extrusive formations. It has been possible, using both carbon isotopes of hydrocarbon gas compounds and associated noble gases, to characterize the influence of mantle fluids in the petroleum systems. CO2 which may present concentrations close to 100% in the gas phase, is demonstrated to be of mantle origins. The mantle fluids influence in the basin increases smoothly from the South to the North, and is not directly related to the occurrence of volcanic rocks. Natural lead isotopes in the oil matrix present also a clear mantle contribution, indicating that the mantle fluids invade the sedimentary formations as hydrothermal fluids. The system CO2/noble gases may be modelled with a simple mixing between a crustal and a mantle end-member respectively. On the opposite, the same mixing model does not fit the chemical and isotopic signatures of the system hydrocarbons/noble gases. This implies uncommon chemical reactions generating light hydrocarbon compounds with a non-organic origin. The mantle fluids contribution modified the proportions and amounts of hydrocarbon compounds, demonstrating the effect of abiotic chemical reactions associating organic molecules and alteration products of volcanic rocks as natural hydrogen.
These new informations demonstrate that the invasion of sedimentary rocks with allochtonous gas molecules occurs with an advective influx of deep hot extra-sedimentary fluids, and not only through a diffusive contribution, as it could be imagined when only helium isotopes were used. This has several consequences on the non-hydrocarbon gas risk in exploration, on the composition and amounts of hydrocarbon gases, and on the thermal history of petroleum systems.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil