Evaluation of dissolved light hydrocarbons in different geological settings in Romania

The paper presents the first systematic geochemical study on the content of natural gas (methane, heavier alkanes, CO2), and isotopic composition of methane dissolved in natural springs in Romania, from various tectonic and geological settings. The dissolved methane concentrations in the investigated areas range between 0.08 and 16,359 μg/L. Based on isotopic analyses, the origin of methane in the gas mixture released by the investigated springs has been inferred. The highest concentrations of methane have been measured in the Transylvanian Basin and Carpathian Foredeep. Ethane in relatively high concentrations has been detected in Herculane and Calimanesti-Caciulata areas. Very high concentrations of dissolved CO2 were determined in the Eastern Carpathians volcanic chain area. Lower amounts of dissolved CO2 were detected in the Carpathian Foredeep, Southern Carpathians, Transylvanian Basin, and Pannonian Basin. The measured values of δ¹³C-CH4 range between − 67‰ and + 4‰. The methane from the investigated areas is biotic, microbial in the Transylvanian Basin, Pannonian Basin, and Moldavian Platform, and thermogenic in the Carpathian Foredeep. We report for the first time the presence of a presumably abiotic methane component in Romania, from geothermal sources, in the areas of Ciomadul Volcano and Herculane area.