HYDROCARBON GASES IN SEDIMENTS AS INDICATORS OF "DEGASSING PIPES" RELEASING DEEP HYDROCARBON FLUIDS ON THE BLACK SEA BOTTOM

The Black Sea is the largest hydrocarbon reservoir on Earth. According to the theory of global fluidogenesis, deep fluids enriched with methane and its homologues move from the mantle to the seafloor through “degassing pipes.” The latter are usually formed along tectonic ruptures that create weakened zones in the sedimentary cover allowing filtration streams of fluids to rise and saturate bottom sediments.

This presentation is devoted to use of geochemical analysis of hydrocarbon gases in sediments to locate the outlets of "degassing pipes" on the seafloor. The study was performed as part of the ODESSA project at 46 stations sampled within one of the largest structures on the northwestern Black Sea shelf, Pradneprovskaya, a promising place for hydrocarbon extraction. 46 samples were studied by a variety of methods (e.g., Yanko et al., this volume). Gaschromatographic analysis of hydrocarbon gases was done in the Laboratory of Marine Geology, Geochemistry and Paleontology of Odessa I.I. Mechnikov National University. The distribution of hydrocarbon gases was found to be uneven, and three anomalous areas can be distinguished. According to our working hypothesis, these areas are the outlets of "degassing pipes" on the seafloor. Methane and its saturated homologues (alkanes) were identified and mapped, showing that their concentrations inside and outside the "degassing pipes" differs by orders of magnitude. One can divide the entire data set into three subsets corresponding to the central, marginal, and outer parts of the "degassing pipes."

A comparative analysis of the average content of hydrocarbon gases in each part show a greater prevalence of methane over other gases; most of the alkanes showed the same distribution in the marginal and outer parts of the "degassing pipes"; in the central part, the content of butane is completely opposite that of other parts, and there is a slight decrease in iso-butane toward the margin. All this permits the use of a butane index (ratio of iso-butane to butane) at each investigated point to delineate the pipe outlets, while methane-alkane indices can be used for general characteristics as well as detection of endogenous hydrocarbon sources quite accurately.