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 (delta(13)C(1)) of mud volcanoes are 90% v/v and -46.4 parts per thousand, respectively. The other types of seeps, which are independent from mud volcanism, have an average delta(13)C(1) value that is slightly higher (-42.9 parts per thousand). 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 C(1)/(C(2) + C(3)) "Bernard" ratio. Mud volcanoes behave like a "natural refinery" and the origin of gas more isotopically enriched than -50 parts per thousand and with C(1)/(C(2) + C(3)) > 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. (c) 2008 Elsevier Ltd. All rights reserved.