Geological Methane

Abstract

Natural emissions of CH4 are not only produced by contemporary biochemical sources such as wetlands, termites, oceans, wildfires and wild animals, and fossil CH4 (that is geologically ancient, radiocarbon-free CH4) is not emitted only by the fossil fuel industry. Beyond CH4 from the biosphere and CH4 from anthropogenic sources, a third CH4 ‘breath’ exists – earth’s degassing. The term ‘degassing’, in general, makes one think of volcanoes and geothermal manifestations (eruptions, fumaroles, mofettes, hydrothermal springs, either on land or on the seafloor) that release carbon dioxide, water vapour and sulphur gases, but this is only a partial vision of earth’s degassing. Earth also exhales hydrocarbons, especially in geologically ‘cold’ areas, such as sedimentary basins where large quantities of natural gas migrate from shallow or deep rocks and reservoirs to the surface along faults and fractured rocks. The phenomenon is called ‘seepage’ and the gas is almost totally CH4, with low quantities (hundreds of ppmv to a few per cent) of other hydrocarbons (mainly ethane and propane) and non-hydrocarbon gases (CO2, N2, H2S, Ar and He). Gaseous hydrocarbons are produced by geologically ancient microbial activity, in shallow and low-temperature sedimentary rocks, and by thermogenic processes in deeper, warmer rocks. Therefore, seepage is a natural source of fossil CH4. Until recently, geological seepage has generally been neglected or considered a ‘minor source’ for CH4 in the scientific literature (for example Lelieveld et al, 1998). The Second and Third Assessment Reports of the Intergovernmental Panel on Climate Change (Schimel et al, 1996; Prather et al, 2001) only considered gas hydrates as geological sources of methane. Gas hydrates, or CH4 clathrates as they are sometimes called, are ice-like mixtures of water and CH4 trapped in oceanic sediments (for example Kvenvolden, 1988). The majority of this gas escaping from melting deep-sea hydrates is dissolved in the seawater column and does not enter the atmosphere. However, global emissions of CH4 to the atmosphere from hydrates have been reported to be roughly 3Tg y–1 (Kvenvolden, 1988) to 10Tg y–1 (Lelieveld et al, 1998),highly speculative values since they result from misquotations not supported by direct measurements. Studies conducted during the last ten years have made it clear that other geological CH4 sources, much more important than gas hydrates, exist; and there has been a growing consensus regarding the importance of marine (offshore) seepage, independent from gas hydrates, as a global contributor of CH4 to the atmosphere (for example Judd et al, 2002; Judd and Hovland, 2007). Experimental flux data, acquired since 2001, have provided more and more evidence for large emissions from continental (onshore) gas manifestations, including macroseeps and diffuse microseepage from soils (Etiope et al, 2008; Etiope, 2009, and references therein). Geothermal emissions are subordinate, but worth considering globally, while volcanoes appear not to be substantial CH4 contributors (Etiope et al, 2007a). At present, it is clear and unambiguously understood that geological emissions are a significant global source of CH4; and today, earth’s degassing is considered the second highest natural source for CH4 emissions after wetlands (for example Etiope, 2004; Kvenvolden and Rogers, 2005; Etiope et al, 2008). A new global estimate for geological sources has finally been acknowledged by the IPCC in its Fourth Assessment Report (Denman et al, 2007). Also, geological seepage has been considered as a new source for natural CH4 in the Emission Inventory Guidebook of the European Environment Agency (EMEP/EEA, 2009) and in the new US Environmental Protection Agency report on Natural Emissions of Methane (US EPA, 2010).