Abstract
Numerous hydrocarbon seep sites at the continental shelf, slope, and in the deep water basin are known to feed the Black Sea water reservoir of dissolved methane. In this study, we identified the likely sources of gas and oil that are emitted at four sites located on the continental slope offshore Georgia in the Eastern Black Sea at 830 to 1,140 m water depth – an area with gas seepage only (Batumi seep area) and three areas of coupled gas and oil seepage (Iberia Mound, Colkheti Seep, and Pechori Mound). The geochemistry of bulk parameters, organic fractions and individual hydrocarbon biomarkers in near-surface sediments and of gas/oil expelled from the seafloor was analyzed and jointly interpreted to assign most likely hydrocarbon source rocks in the studied region. Presence of oleanane in shallow oil-impregnated sediments and oil slicks attests that the source rock at all sites is younger than Mid Cretaceous in age. We conclude that hydrocarbons ascending at all the four seepage areas originate from the Eocene Kuma Formation and/or the Oligocene–Lower Miocene Maikop Group, which are considered the principal hydrocarbon sources in the Eastern Black Sea region. Distributions of crude oil biomarkers in shallow sediments suggests moderate to heavy biodegradation. C1/C2+ ratios (10–4,163) along with stable C and H isotopic ratios (δ13C–CH4 ‒46.3 to −53.1.3‰ V-PDB; δ2H–CH4 ‒159 to −178‰ V-SMOW) indicate gas mixtures of oil-associated thermogenic and secondary microbial light hydrocarbons that are discharged from the four seep sites. Light hydrocarbons discharged at the Batumi Seep area are characterized by significant enrichments of methane, but have almost similar δ13C–CH4 values if compared to the other study sites. Such methane enrichments likely result from a comparably higher degree of petroleum degradation and associated formation of secondary microbial methane.
Highlights
The Black Sea basin hosts about 12 km thick, partially organic-rich sediments, which promote oil and gas formation in the deep subsur face
Sediment and gas samples from four gas and oil seepage sites in the Eastern Black Sea were investigated in order to gain information about the depositional environment, the burial depth, the degree of maturity of organic matter sourcing hydrocarbons present in shallow deposits, and potential chemical alterations during migration
Large amounts of extractable organic matter spanning from 650 ppm (Batumi seep area) to 150,000 ppm (Pechori Mound) of sediment weight showed that Colkheti seep and Pechori Mound sediments were heavily impregnated by oil (Reitz et al, 2011)
Summary
The Black Sea basin hosts about 12 km thick, partially organic-rich sediments, which promote oil and gas formation in the deep subsur face. Vola tile hydrocarbons in shallow sediments and C15+ hydrocarbons in nat ural sea surface oil slicks above one of the seep sites were evaluated with respect to potential modifications during migration through the sedi ment and passage through the water column. These data are useful for future explorations at these seepage sites and similar geological settings and might be of prime interest for a better understanding of deep active hydrocarbon systems in the area
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