ABIOTIC METHANE

Methane (CH4) can be BIOTIC or ABIOTIC.

BIOTIC methane can be microbial (generated by methanogenic microbes, Archaea) or thermogenic (generated by thermal degradation of organic matter or oil cracking). This gas is termed biotic because of its derivation from biologic compounds—mainly lipids and carbohydrates— liberated from marine and terrestrial organic matter, and/or because of the action of microbes. This is the naural gas, often associated to petroleum (oil), that we use as energy source.

ABIOTIC methane is formed by (a) geochemical reactions (gas-water-rock interactions over a wide range of formation temperatures) that do not require the presence of organic matter or microorganisms or (b) by magma degassing.

Abiotic gas does not refer to the theory of abiotic origin of petroleum, meaning crude oil or complex gaseous hydrocarbons, proposed by Russian-Ukrainian scholars and by Thomas Gold (see review in Glasby, 2006; Abiogenic Origin of Hydrocarbons: An Historical Overview, Resour. Geol., 56, 85–98). It refers only to gaseous hydrocarbons.

Reviews and discussions on abiotic gas are provided by Etiope and Sherwood Lollar (2013) and Etiope and Schoell (2014).

The largest abiotic CH4 amounts observed in surface seeps and boreholes are produced in the crust, not in the mantle.

Abiotic gas exists both in the ocean floor and on continental settings, even at shallow depths. In particular, most of abiotic gas has been documented on continents, in ultramafic rock bodies (ophiolites, intrusions, peridotite massifs) and in Precambrian shields (crystalline basement). Generally, these are low temperature geolological environments (not geothermal, not hydrothermal), typically characterized by active (present-day) serpentinization (hydration of olivine). Geochemical geothermometers (e.g., CH4 clumped isotopes) and geothermal gradients indicate that the gas is typically generated at temperatures below 150°C, often <100°C.

Methane origin on Earth

The main chemical reaction invoked for abiotic gas origin is the Fischer-Tropsch Type (FTT). The simplest FTT form is the Sabatier reaction, where CO2 and H2 react on the surface of a catalyst (metal) to produce CH4. This reaction is known to occur over a wide range of temperatures. Classic catalysts, iron, nickel or chromium, are however effective only at high temperatures (>150-200°C). Only special catalysts (e.g., ruthenium) are known to support Sabatier reaction at lower temperatures.

Here below is a map (updated in 2016) of the global distribution of seeps or springs (with hyperalkaline water, pH>9, typical of active serpentinization) with methane in ophiolites, peridotite massifs or intrusions (Etiope, 2017; Procedia Earth Planet. Sci.).

An erroneous paradigm is that abiotic methane is formed by serpentinization, i.e. direct hydration of olivine (and pyroxene) and in aqueous solution. So, many experiments aimed at producing abiotic methane were made expecting CH4 generation simply by hydrating olivine and using hydrothermal conditions (e.g., McCollom, 2016; Abiotic methane formation during experimental serpentinization of olivine. PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1611843113), which are not representative of the low T geological environments where abiotic methane is actually observed. Experts on FTT catalysis indicate, then, that Sabatier reaction does not work in water, especially at low temperatures. Abiotic CH4 synthesis is much stronger in dry condition (unsaturated or dry rock), and abundant H2 is necessary for the reaction.

Laboratory experiments demonstrated that abiotic CH4 can be generated, abundantly and quickly, at low temperatures (<100°C, down to 25°C) by Sabatier reaction in dry phase using a catalyst (ruthenium) that naturally occurs in chromitites, typical rocks of ultramafic massifs, ophiolites and intrusions (Etiope and Ionescu, 2015).

Main papers ON ABIOTIC METHANE

ABIOTIC METHANE IN OPHIOLITES: CHROMITITES ARE THE SOURCE ROCKS (2018)

https://www.nature.com/articles/s41598-018-27082-0

REVIEW PAPERS

Etiope G., Sherwood Lollar B. (2013). Abiotic methane on Earth. Rev. Geoph., 51, 276-299, doi: 10.1002/rog.20011.

Etiope G., Whiticar M.J. (2019). Abiotic methane in continental ultramafic rock systems: Towards a genetic model. App. Geochem., 102, 139-152, https://doi.org/10.1016/j.apgeochem.2019.01.012.

Etiope G., Schoell M. (2014). Abiotic gas: atypical but not rare. Elements, 10, 291-296.

Laboratory experiments Low T abiotic methane by Sabatier reaction (Etiope and Ionescu, 2015, Geofluids)

Abiotic gas in Greece (Etiope et al 2013, Chem. Geol.).

Abiotic gas in Spain (Etiope et al, 2016; Appl.Geoch.)

Abiotic gas in Bosnia-Herzegovina (Etiope et al, 2017; Appl.Geoch.)

Abiotic gas in Portugal (Etiope et al, 2013; Mar.Petrol.Geol.)

Abiotic gas in Italy

Genova (Boschetti et al. 2013)

Elba (Sciarra et al. 2019)