Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

Abstract

Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, nearsurface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone. Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C 1 -C 4 , CO 2 ]). Molecular ratios of LMWHC (C 1 /[C 2 + C 3 ] > 1000) and stable isotopic compositions of methane (δ 13 C = − 53.5‰ V-PDB; D/H around − 175‰ SMOW) indicated predominant microbial methane formation. C 1 /C 2+ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C 1 -C 4 , CO 2 ]) relative to the other gas types and the virtual lack of 14 C-CH 4 indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely. As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C 1 -C 4 , CO 2 )] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of 14 C-CH 4 (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area.

First evidence of widespread active methane seepage in the Southern Ocean, off the sub-Antarctic island of South Georgia

“…For quantification of methane concentrations in 380 the water column, 750 ml of sampled seawater were transferred from the Niskin bottles into preevacuated 1000 ml glas bottles immediately after recovery, followed by gas extraction according to the modification of the vacuum degassing method described by Rehder et al 66 . The extracted gas samples were analyzed with a 6890N gas chromatograph (Agilent Technologies) equipped with a capillary column and connected to a Flame Ionization Detector, as described in Pape et al 55 . 385…”

Data report: gas isotope and conversion ratio of headspace and void gas, IODP Expedition 344

“…Ethane concentration does not show vertical variation with depth at Site U1380, whereas it increases with depth at Sites U1412 and U1413. In general, C 1 /C 2+ ratios are >1000 in biogenic gases and <300 in thermogenic gases (Claypool and Kvenvolden, 1983;Whiticar et al, 1986;Whiticar, 1999;Milkov et al, 2005;Pape et al, 2010;Kim et al, 2011Kim et al, , 2012. In this study, the C 1 /C 2+ ratio is lowest (<300) in lithostratigraphic Unit I at Site U1380 and Unit III at Site U1413.…”

Quantifying in-situ gas hydrates at active seep sites in the eastern Black Sea using pressure coring technique

“…Indications for structure I gas hydrate, i.e. gas hydrates with CH 4 as the prevailing guest molecule, were found at both sites (Pape et al, 2010a;S. Klapp, Univ.…”

“…1), where buried diapiric structures and fault systems form migration pathways for sediments, fluids, and gases (Wagner-Friedrichs, 2007). The Batumi seep area is the largest seep site with an areal extent of 0.5 km 2 characterized by shallow gas hydrates, authigenic carbonates, and vigorous gas venting (Klaucke et al, 2006;Pape et al, 2010a). Pechori Mound has 75 m of relief, steep slopes, and strong seafloor reflection at the top (Wagner-Friedrichs, 2007).…”

“…The samples originate from recently discovered seep sites off Georgia in the eastern Black Sea (Akhmetzhanov et al, 2007;Bohrmann et al, 2007). Despite gas hydrate discoveries in this area (Klaucke et al, 2006;Pape et al, 2010a) little is known about gas hydrate distributions and regional quantities in the Black Sea, the largest anoxic basin at present times (Ross and Degens, 1974). Our results…”