Methane in the northern Black Sea: characterization of its geomorphological and geological environments

Abstract: Based on hydro-acoustic and geophysical observations, this paper presents an analysis of geomorphological and geological settings of gas methane occurrence on the NW shelf and upper continental slope, in the Sorokin trough and on the Kerch-Taman offshore, in the Black Sea. Gases are associated with seeps, mud volcanoes and gas hydrates. Evidence is given for the thermogenic nature of methane. The gas methane is of mostly abiogenic origin. Small gas releases may be produced by the decomposition of Quaternary organic material near the sea floor through the action of bacteria or biodegradation of redeposited thermogenic hydrocarbons. The origin of carbonate formations is related to degassing sedimentary layers. There is a possible role for deep faults in transporting gas to the sea floor. The gas hydrate stability zone in the Black Sea lies at minimum water depth of 600–650 m with its thickness up to 500 m. In the northern Black Sea intensive outbursts of methane into the atmosphere were first recorded in the scientific literature as early as 1928, one year after a series of destructive Crimean earthquakes (Popov 1928). Some 60 years later the sources of methane were described when Polikarpov et al. (1989) related acoustic anomalies to the leakage of gas. Recently the Black Sea has become a popular target for interdisciplinary studies on its enormous methane resource. It is estimated that the Earth’s largest anaerobic basin – the Black Sea – contains a total of 96 109 kg of methane (Reeburgh et al