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Sedimentary Marine Organic Matter Diagenesis under Methanogenic Conditions: A New Model for Biogenic Gas Quantification

Azdine Ravin¹, Françoise Behar², Patrick Hatcher³, and François Baudin¹
¹Université Pierre et Marie Curie, Paris, France
²Total, Paris, France
³Old Dominion University, Norfolk, VA, USA

After its deposition on the sea floor, sedimentary organic matter (SOM) is subjected to intense diagenesis that affects both its quality and concentration in the first hundred meters of sediment. This alteration is due to changes in physicochemical conditions of the sediment and also to the activity of microorganisms growing within the porous medium. Methanogenesis is the final step of this degradation provided by Archaea that produce methane (CH4) as a waste. The study of methanogenesis in sediments has so far been conducted mainly through thermodynamic calculations and analysis of chemical species dissolved in pore waters, including CO2 and CH4. It has been shown that these methods had large errors due to losses occurring during the handling of cores. There are also numerous studies on the rate of methanogenesis through measurements of carbon isotopic fractionation by methanogens and laboratory simulations. Despite this abundant literature, the precise effects of methanogenesis on SOM are poorly understood and interactions with the mineral medium are virtually unexplored due mainly to the highly insoluble character of the SOM and the lack of appropriate structural analysis tools.

The aim of this study is to describe the early diagenesis of a recent marine SOM under methanogenic conditions so as to determine the most bio-reactive structural motifs (whose concentration decreases with depth) and those with a slower degradation kinetics, considered refractory. Once the labile compounds and the process and stoichiometry for their degradation are established, we’ll be able to determine the total volume of CH4 produced using the existing equations for methanogenesis.

To achieve this goal we choose a natural sedimentary series from a core drilled off Namibia during the course of Leg 175 of the Ocean Drilling Program (ODP; Wefer et al., 1998). This area is known to be one of the most productive regions in the world and is characterized by organic-rich sediments with high organic carbon content (3-4%; Wefer et al., 1998; Giraudeau et al., 2002). This site appears to be suitable for “natural diagenetic experiments”, allowing us to not only characterize the impact of methanogenesis on natural organic matter but also to apprehend the interactions with the mineral matrix and the changes in the geological parameters (temperature, porosity, permeability, etc.).

AAPG Search and Discovery Article #120098©2013 AAPG Hedberg Conference Petroleum Systems: Modeling the Past, Planning the Future, Nice, France, October 1-5, 2012