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Abiogenic Petroleum Generated by Serpentinization of Oceanic Mantellic Rocks

J. L. Charlou1, J. P. Donval1, P. Jean-Baptiste2, D. Levaché3, Y. Fouquet1, J. P. Foucher1, P. Cochonat1
1 Géosciences Marines, IFREMER C/Brest, 29280 Plouzané, France
2 LSCE, Centre d’études de Saclay, Gif sur Yvette, France
3 Total, Avenus Larribau, 64018 Pau, France
(contact: [email protected])

Serpentinized peridotites are important components of oceanic lithosphere and their studies have deep impact on both the InterRidge and MARGINS communities, since they are exposed at slow-spreading mid-oceanic ridges, back-arc basins and sedimented margins. Natural hydrocarbons are largely formed by the thermal decomposition of organic matter (thermogenesis) or by microbial processes (bacteriogenesis). The discovery of methane and light hydrocarbons in mid-oceanic hydothermal plumes and hot temperature vents and in other crustal fluids supports the occurrence of an abiogenic source of hydrocarbons. These abiogenic hydrocarbons are generally formed by the reduction of carbon dioxide, a process which is thought to occur during magma cooling and more commonly in hydrothermal systems during water-rock interactions, particularly during serpentinization of ultramafic rocks involving Fischer-Tropsch reactions. In ultramafic-hosted sites, such as Rainbow (36°14’N) on the mid-Atlantic Ridge, the fluids exhibit a mineral and gas composition different on many aspects from basaltic-hosted fluids. Particularly, they contain a very high hydrogen concentration (13 mmol/kg), associated with high concentrations of methane and other light hydrocarbons. In addition, many families of hydrocarbons are identified in the fluids by GC/MS, including linear saturated hydrocarbons, alcohols, aldehydes, ketones, aromatics, and cyclic compounds. These recent results, associated with many other previously found results, confirm that abiogenic petroleum may be produced from crystalline basement, from volcanic structures, from riftogenic zones and probably from sedimented margins. The thermodynamical data suggest that serpentinization of peridotites is possible at temperatures between 400 and 700°C, but the reaction is amplified between 150 and 350°C at moderate pressure. During the cooling of the oceanic lithosphere in continuous extension, the serpentinization process may be very active under the 400°C isotherm and may be occurring in many areas from ridges to sedimented margins since serpentinization and catagenesis processes are active in the same range of pressure and temperature conditions. The global process of serpentinization of mantellic rocks, generating large quantities of hydrogen and hydrocarbons may be considered as a young stage of catagenesis occurring in the rocks together with the complex synthesis of oil-gas deposits involving organic matter in the sedimentary layer. Serpentinization may have influence on petroleum reservoir and solid gas hydrate formation, particularly when fluid circulation is active in the sediment layer, as shown in mud volcanoes and pockmarks. Results obtained from recent cruises on the mid-Atlantic Ridge and on margins will be shown.