Origin of hydrocarbon gas in southeast of Caspian Sea (Gorgan plain) mud volcanoes based on molecular and isotopic composition of carbon and hydrogen

Mud volcanoes are common phenomena in tectonic compression and high sedimentation settings from terrestrial and offshore areas which are of great importance for research on tectonic activities, sedimentary environments and oil and gas accumulation. In this study, three onshore mud volcanoes Gharniarigh, Sofikama and Naftlicheh in Gorgan plain are described and the origin of hydrocarbon gases (as free and dissolved gas in sediment) is investigated. The gases molecular composition of all three mud volcanoes show high amount of methane (90.68 to 99.74 vol.%) and minor concentration of ethane, propane as well as iC4 in Gharniarigh and Naftlicheh. The origin of released gases from Sofikam (δ13CCH4=-74‰), Gharniarigh (δ13CCH4=-52.2‰) and Naftlicheh (δ13CCH4=-52.2‰) based on the molecular composition and isotope ratios of carbon and hydrogen are distinguished the bacterial carbonate reduction, mixed (thermogenic-biogenic) and mixed (thermogenic-biogenic), respectively. Although the minor amount of ethane (358 ppmv) in Sofikam gas indicates minor thermogenic contribution. The carbon isotopic composition of CO2 )δ13CCO2=.14‰( show anaerobic biodegradation process in Naftlicheh mud volcano. The radiogenic isotope ratios 3He / 4He represents crustal source for emitted gases. The presence of thermogenic gases in mud volcanoes of Gorgan plain can indicate the hydrocarbon potential of this area.