Light hydrocarbon gases in shallow sediments in the northern North Sea

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

“…Accordingly, oil biodegradation processes will yield secondary-formed methane and washing of C 2+ compounds that eventually will be resulted in dry characteristics of gas mixtures. Similar scenario was suggested by Brekke et al (1997) and is consistent with Blinova et al (2003), studied neighboring MV structures in the Sorokin Trough. Additional evidence for the subsurface oil biodegradation is anomalously light d 13 C values of C 2 detected in all studied MVs ( Table 2).…”

“…Subsurface biodegradation of oil (Dimitrakopoulos and Muehlenbachs, 1987;Brekke et al, 1997;Pallasser, 2000;Larter and di Primio, 2005) and gas (Connan and Coustau, 1984;James and Burns, 1984) is yet poorly known, although it is a common process in the hydrocarbon reservoirs. Most of the world's petroleum is biodegraded A.…”

“…Stadnitskaia et al / Marine and Petroleum Geology 25 (2008) 1040-1057 (Roadifer, 1987). Recent studies of gas and oil biodegradation processes demonstrate increasing evidence that dry-gas fields developed from the microbial degradation of oil (Dimitrakopoulos and Muehlenbachs, 1987;Brekke et al, 1997;Pallasser, 2000;Head et al, 2003;Larter and di Primio, 2005). Our HCG data show that apart from the Kazakov MV, studied locations are characterized by relatively dry HCGs with a small (although varied in content and composition) admixture of C 2+ components (Table 1) and by microbial-like d 13 C signatures of C 1 .…”

The biogeochemistry of sorbed methane in marine sediments

“…The reproducibility of both the alkaline and acidic extraction was low (Table 2) but similar to previously reported results (e.g., Brekke et al, 1997;Abrams, 1996a). Though the tested extraction protocols differed strongly with respect to methane yields, carbon and hydrogen isotopic compositions of the released methane were similar for both methods and for repeated extractions (Table 2).…”

“…Concentrations of sorbed methane typically range from 1 to 100 lmol kg À1 wet sediment while individual higher VHCs reach up to 6 lmol kg À1 (e.g., Horvitz, 1981;Whiticar and Faber, 1989;Whiticar and Suess, 1990;Whiticar et al, 1995;Brekke et al, 1997). Molecular ratios of C 1 vs. C 2+ gases are below 100 (e.g., Faber and Stahl, 1984;Whiticar and Suess, 1990) and with d 13 C values ranging from À45& to À22&, stable carbon isotopic compositions of methane suggest thermogenic sources (Horvitz, 1981;Stahl, 1983, 1984;Whiticar and Faber, 1989;Whiticar and Suess, 1990;Whiticar et al, 1995;Abrams, 1996b;Brekke et al, 1997;Knies et al, 2004). Exceptionally high concentrations of predominantly thermogenic methane (up to 331 lmol kg À1 wet sediment) were observed in the northern North Sea and Arctic Ocean (Brekke et al, 1997;Knies et al, 2004) and high concentrations of biogenic methane (up to 4.5 mmol kg À1 wet sediment) were found in the Japan Trench (Ijiri et al, 2009).…”