Sources of hydrocarbon gases in mud volcanoes from the Sorokin Trough, NE Black Sea, based on molecular and carbon isotopic compositions

Abstract

Molecular and stable carbon isotope properties of hydrocarbon gases (methane through pentanes and sometimes hexanes) from seven sediment cores collected from five mud volcanoes (MVs) in the Sorokin Trough (NE Black Sea) suggest that these gases are initially derived from the comparable hydrocarbon pools and are likely initial products of non-microbial oil cracking processes. Our results state that dry characteristics of gas and 13C-depleted signatures of methane are result of a high admixture of secondary microbial gas formed due to subsequent microbial anaerobic degradation of redeposited hydrocarbons in the shallow reservoirs. The wet gas components in all MVs and gas hydrates are related to each other. The compositional variations in C 2+ content appear to result from a complex of secondary processes such oil cracking in the deep subsurface, migration and mixing of resulted gaseous and liquid hydrocarbons, biodegradation of possibly redeposited hydrocarbons forming shallow reservoirs, and additional alterations of hydrocarbon gases in the surface sediments due to currently active microbial processes, such as AOM, C 2+ consumption, etc. Our data show that the most unaltered gas is in the mud breccia from the Kazakov MV. The gas mixture possibly represents the original properties of the hydrocarbons trapped in the deep subsurface of the Sorokin Trough. Analysis of the hydrocarbon gas data, complemented with published maturity characteristics of organic matter from Maycopian rock clasts and mud breccia matrix implies that the original source of gases is likely to be located below the Maycopian Shale Formation.