Terrestrial methane seeps and mud volcanoes: A global perspective of gas origin

A global database of gas composition and methane stable isotopes of 143 terrestrial mud volcanoes from
12 countries and 60 seeps independent from mud volcanism from eight countries, was compiled and
examined in order to provide the first worldwide statistics on the origin of methane seeping at the
earth’s surface. Sixteen seep data were coupled with their associated subsurface reservoirs.
The surface seepage data indicate that at least 76% of the mud volcanoes release thermogenic gas,
with only 4% biogenic and 20% with mixed character. The average (201 data) of methane concentration
and methane carbon isotope ratios (δ to the power of 13 C1) of mud volcanoes are 90% v/v and -46.4‰, respectively. The
other types of seeps, which are independent from mud volcanism, have an average δ to the power of 13 C1 value that is
slightly higher (-42.9‰). Gases from mud volcanoes are generally lighter (more methane, less ethane
and propane) than their associated reservoir gases, suggesting a molecular fractionation during
advective fluid migration. Other types of seeps, especially "dry" seeps, maintain the reservoir C1/(C2 +
C3) "Bernard" ratio. Mud volcanoes behave like a "natural refinery" and the origin of gas more
isotopically enriched than -50% and with C1/(C2 + C3) >500 should be attributed to a thermogenic source, rather than partial oxidation of biogenic gas. Some data that appear biogenic in the "Bernard diagram" can be explained by molecular fractionation of mixed gas. Consequently, the "Bernard"
parameter may be misleading when applied to mud volcanoes since it does not always reflect the
original gas composition. The mechanisms of the molecular advective segregation should be studied
quantitatively by specific models and experiments.