text: Mud volcanoes are often associated with petroleum deposits and tectonic subduction zones and orogenic belts ; hydrocarbon gases are often erupted . They are also often associated with lava volcanoes ; in the case of such close proximity , mud volcanoes emit incombustible gases including helium , whereas lone mud volcanoes are more likely to emit methane .
simplified: Most of the gases given off by mud volcanoes are methane .