Characterization of natural gases in Japan based on molecular and carbon isotope compositions

Abstract

In this paper, the origin, maturity, migration, biodegradation and mixing of natural hydrocarbon gases in Japan have been interpreted using molecular and carbon isotope compositions. No indications of abiogenic gases have been found, the gases being classified as microbial, thermogenic or mixed microbial ⁄ thermogenic. However, secondary alteration (mixing, biodegradation, fractionation in migration processes) has had a major but variable impact on the composition of natural gases. Biodegradation, especially, has altered both molecular and isotopic signatures. Thus, the prime control of isotopic and molecular characteristics in gases is due to genetic phenomena, but secondary effects must be taken into account when attempting to understand the origin and distribution of gas. Where the extent of secondary alteration is small, carbon isotope compositions of thermogenic hydrocarbons are controlled largely by maturity. An isotope model developed by Berner and Faber was applied successfully to natural gases in northeast Japan. Besides maturity estimation, the application of the model enables detection of biodegradation, mixing between microbial and thermogenic gases, and mixing among gases with different maturities. The carbon isotope compositions of carbon dioxide also provide information for their genetic origins.

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

“…Reservoir gas associated with anaerobic biodegradation processes has a typical fingerprint characterized by large isotopic separations between successive n-alkanes and high C 2 /C 3 (ethane/ propane) and iC 4 /nC 4 (iso-butane/normal-butane) ratios due to preferential degradation of C 3 and n-alkanes (Pallasser, 2000;Waseda and Iwano, 2008;Milkov and Dzou, 2007). These features have been reported for some submarine mud volcanoes, such as in the Black Sea (Stadnitskaia et al, 2008).…”

Gas Seepage along the Edge of the Aquitaine Shelf (France): Origin and Local Fluxes

“…When a submarine seep is linked to a petroleum system, the gases are primarily of thermogenic origin, resulting from the thermocatalysis of deeply buried organic matter [43,[46][47][48][49]. However, the two aforementioned gas types derive from organic matter; therefore they belong to biotic gases [43,44,[48][49][50][51]. They represent by far the biggest hydrocarbon sources on earth.…”

Thermogenic Methane with Secondary Alteration in Gases Released from Terrestrial Mud Volcanoes

“…Considering that CO2 is a minor component of gases released from mud volcanoes, the concentration and isotopic composition of CO2 can largely be affected by the reaction. The isotope effect of oil biodegradation results in the increase in  13 C of residual CO2, which can easily exceed +10‰ (Pallasser, 2000;Waseda and Iwano, 2008). Occurrence of oil biodegradation is suggested by the high C2/C3 and i-C4/n-C4 ratios (Pallasser, 2000;Waseda and Iwano, 2008) and/or by the presence of H2 gas.…”

“…The isotope effect of oil biodegradation results in the increase in  13 C of residual CO2, which can easily exceed +10‰ (Pallasser, 2000;Waseda and Iwano, 2008). Occurrence of oil biodegradation is suggested by the high C2/C3 and i-C4/n-C4 ratios (Pallasser, 2000;Waseda and Iwano, 2008) and/or by the presence of H2 gas. For example, all these characteristics are identified in the gases released from mud volcanoes in China, which show a large variation in  13 CCO2 as described above .…”