Hydrocarbon gases in sediments and mud breccia from the central and eastern part of the Mediterranean Ridge

Microbiologically produced hydrocarbon gases (HCGs) in normal pelagic sediments have lithologically controlled distribution - their content increases sharply in organic-rich sapropels. Generally, the HCG content is low (<10000 nl/l), The amount of heavy HCGs is high (up to 60%, unsaturated HCGs dominate the saturated ones, and ethylene is prevalent. The same features were found in sediment and mud breccia from inactive mud volcanoes. Thermogenic HCG was determined in active mud volcanoes and in a high salinity fluid vent. They are characterized by: high methane concentration, delta(13)C(CH4) = -37.1 to -57.8%, essential ethane contents (2-7%), absence of unsaturated HCG, and the prevalence of iso-over n-butane.