Seepage in Serpentinised Peridotites and on Mars

Abstract

Thus far, this book has mainly addressed the classical seepage of natural gas of biotic (microbial and thermogenic) origin in sedimentary rocks. However, it is also known that methane and other light hydrocarbons can be abiotically produced (i.e., by chemical reactions that do not directly involve organic matter). Abiotic production may occur over a wide range of temperatures and pressures and within a variety of geological systems. Two main classes of abiotic CH 4 generation processes can be distinguished, magmatic and gas-water-rock interactions. For details, the reader is referred to the review of Etiope and Sherwood Lollar (2013). Here, it is important to keep in mind that while magmatic or mantle-derived CH 4 is abiotic, not all abiotic CH 4 is mantle derived. Magmatic and high temperature hydrothermal processes in volcanic and geothermal systems produce a gas mixture that is mainly composed of carbon dioxide (CO 2 ), while abiotic methane is a very minor component. Indeed, field observations suggest that the majority of abiotic gas on Earth's surface is produced by low-temperature, gas-water-rock reactions. Of particular interest are the Fischer-Tropsch Type (FTT) reactions, the mechanism most widely invoked for explaining large quantities of abiotic CH 4 seeping to the surface. In the sub-chapters that follow, only CH 4 -rich abiotic gas seeping to the surface is considered. Such seepage generally occurs in serpentinised ultramafic rocks (peridotites). Serpentinisation (the hydration of olivine and pyroxene minerals, which leads to the formation of H 2 and the subsequent production of CH 4 through FTT synthesis) is considered a fundamental step in the origin of life, representing the primordial passage from inorganic to organic chemistry (Russell et al. 2010). Serpentinisation can also be a source of methane in atypical petroleum systems where hydrocarbon reservoirs are formed by or are adjacent to igneous rocks (Sect. 7.1.2), and a potential source of methane on other planets, such as Mars (Sect. 7.2).