Origin, migration and alteration of gases from surface seeps and mud volcanoes in the Niigata basin

Abstract

Gases were sampled from 7 surface gas seeps including 2 mud volcanoes in the Niigata oil and gas basin. Based on the gas compositions and isotope compositions, the origin, maturity, migration, and alteration of gases were studied. A seep gas from the paddy field is interpreted to be microbial. The other 6 gases are thermogenic, or the thermogenic gases were generated in the peak to late oil generation stage. Some gases show high C₁/(C₂+C₃), mixtures of thermogenic and microbial. The methane, ethane, and propane carbon isotope compositions suggest that suggesting molecular fractionation during the migration to the surface. The ¹³C-enriched propane and heavy δ¹³C values (>+2 ‰) of CO₂ in some gases, especially in gases from the mud volcanoes, indicate active processes of the secondary microbial methanogenesis accompanied by biodegradation of petroleum. The microbial processes could proceed during the residence time in the mud chambers, which are estimated to exist several hundred meters below the mud volcanoes.