Evidence of subsurface anaerobic biodegradation of hydrocarbons and potential secondary methanogenesis in terrestrial mud volcanoes

New silica clathrate minerals that are isostructural with natural gas hydrates

“…However, the alteration of marine carbonates is less likely because the host rocks are tuffaceous and rarely contain carbonate minerals, except for the calcic concretions of diapiric blocks and veins. Furthermore, 13 C-enriched CO 2 (δ 13 C > + 5‰) is generated by isotopic fractionation in mud volcanoes by secondary methanogenesis following anaerobic biodegradation of thermogenic hydrocarbons 43,44 . This origin of CO 2 is consistent with the thermogenic character of the gas composition of chibaite.…”

Geochemical Dynamics of the Natural-Gas Hydrate System in the Sea of Marmara, Offshore Turkey

“…Hence, it is unlikely that the gas bubbles may come from directly those natural-gas fields.  13 C-enriched CO 2 , with values above +5‰, are typically due to secondary methanogenesis following hydrocarbon biodegradation occurring in petroleum reservoirs or during secondary and tertiary migration (Etiope et al, 2009b). The secondary methanogenesis increases the  13 C of residual CO 2 (previously produced by anaerobic oxidation of heavy hydrocarbons) which may easily exceed +10‰ (Etiope et al, 2009b;Pallasser, 2000).…”

“… 13 C-enriched CO 2 , with values above +5‰, are typically due to secondary methanogenesis following hydrocarbon biodegradation occurring in petroleum reservoirs or during secondary and tertiary migration (Etiope et al, 2009b). The secondary methanogenesis increases the  13 C of residual CO 2 (previously produced by anaerobic oxidation of heavy hydrocarbons) which may easily exceed +10‰ (Etiope et al, 2009b;Pallasser, 2000). Petroleum biodegradation process is a phenomenon that was neglected in the past, but today it is considered to affect a large fraction of conventional oil reserves (Dimitrakopoulos and Muehlenbachs, 1987;Head et al, 2003;Jones et al, 2007).…”

Mud volcanoes in the South Caspian basin: Nature and estimated depth of its products

“…Based on the isotopic composition of helium [2] argon and carbon in methane [6,9] the Azerbaijan mud volcanoes have a sedimentary origin and they are products of different stages of a single process of hydrocarbon generation within the sedimentary succession. The chemical and isotopic signatures of MV gases are predominantly thermogenic with some admixture of microbial methane [14]. Based on the isotopic composition of the C 1 to C 4 , that the hydrocarbon gases on the Regnano mud volcano (northern Italy) have a thermogenic origin and they were formed by thermal cracking (>150˚C) [24].…”

“…OPEN ACCESS Some suggest that the process takes place wholly within the sedimentary succession [1,14,21], whereas others envisage a connection with the deeper processes operating within the mantle [3]. Therefore results of estimation of the depth occurrence of the MVs products in SCB are presented in this paper.…”