Ninth international conference on - Marum

48
Abstracts of oral presentations
zones. Anomalous high backscatter targets were present on some bathymetric anomalies, but also occurred in
areas with no anomalous relief.
The multibeam survey covered 400,000 square kilometers in water depths ranging from 200m to 3500m.
Bathymetric grids (25m bin) and backscatter mosaics (5m pixel) were integrated with all of the other data sets in
order to come up with a suite of potential targets. A final suite of targets was selected for each basin that would
sample a wide range of target types, interrogate targets of interest for petroleum exploration (leads), evaluate
multiple sub-basins, and provide areal distribution even if some parts of the survey area showed fewer or less
charismatic seep targets. 1300 targets were sampled using 6m (and rarely 3 or 9m) piston cores. By accurately
tracking the position of the piston cores in 3D in real time using Ultra-Short Baseline navigation (USBL), we
were able to place the core barrel within meters of the intended target and precisely sample the seafloor feature
of interest. The cores were then subsampled for detailed geochemical analysis. In some of the survey areas,
piston cores on high backscatter features yielded no recovery, or at best, fragments of authigenic carbonate or
outcrop; in other survey areas, piston cores on high backscatter targets yielded near-complete recovery and
excellent geochemistry. As such, we could not predict with whether the first high backscatter core target in basin
would be conducive to recovery. We did find, however, that in any given basin similar features behaved a similar
fashion.
Survey operations began in early December, 2006, and were completed by early April, 2008, with the last
geochemical results received by late April. 11% of the cores contain evidence of migrated liquid petroleum, and
46% of the cores contain thermogenic gas. Over 400 isotope pairs, and 20 biomarker (molecular fingerprinting)
suites provide insight into the maturity and source of multiple gas and oil petroleum systems. Live
chemosynthetic communities were sampled with the piston core in numerous locations; at several of these sites
we re-deployed a box core, using USBL navigation, to sample within meters of the piston core in order to obtain
large quantities of chemosynthetic fauna for later study.
The large number of geochemical hits, both oil and gas, in a project on the scale of this survey is unprecedented.
We attribute the high success rate to a combination of factors: (1) the presence of thermogenic oil and gas
charge, (2) the quality of the high resolution survey program used to identify potential seep targets, and (3) the
quality of the navigation that allowed precise sampling of the targets by the piston core. The distribution of the
oil and gas ‘hits’, and their composition information, are being tied back to the exploration data volume in order
to identify areas where we will go forward with additional exploration activity.
Shallow gas hydrates in an Eastern Black Sea high intensity gas seepage area – quantification by
autoclave technology
T. Pape 1 , A. Bahr 1,2 , F. Abegg 1,2 , H.-J. Hohnberg 1 , S.A. Klapp 1 , and G. Bohrmann 1
1 MARUM – Center for Marine Environmental Sciences, University of Bremen
2 IFM-GEOMAR, The Leibniz Institute of Marine Sciences, Kiel
(tpape@uni-bremen.de / phone +494212183918)
During R/V METEOR cruise M72/3 in 2007, in situ inventories of low-molecular-weight hydrocarbons
(LMWHC) were determined in shallow sediments of one of the largest seep sites in the southeastern Black Sea
(Klaucke et al., 2006). The Batumi seep area is located offshore Georgia, in about 835 m water depth close to the
upper boundary of the nominal gas hydrate stability field. It is characterized by sea bottom features typical for
intense hydrocarbon seepage (e.g., rough topography, authigenic carbonates forming pavements and slabs,
pockmark-like structures).
Controlled on-board degassing of pressurized sediment cores allowed for accurate quantifications of gas and gas
hydrates present in the shallow subsurface. Using our Dynamic Autoclave Piston Corer (DAPC), seven
pressurized cores of up to 264 cm core length were recovered from a seafloor area of about 0.2 km 2 . The mean
gas composition of sub-samples taken during core degassing was strongly dominated by methane (up to 99.97
mol % of ΣC1-C4 LMWHC, CO2) followed by ethane, carbon dioxide and trace amounts of C3 and C4 LMWHC.
Molecular ratios of LMWHC (C1/[C2+C3] > 1,000) and mean stable isotopic compositions of methane (δ 13 C = –
53.5‰ VPDB; D/H = –175‰ SMOW) suggest a gas mixture of biogenic and thermogenic origin. Total methane
concentrations in five pressurized cores strongly exceeded theoretical equilibrium concentrations of dissolved
methane with the hydrate phase of about 93 mM (after Tishchenko et al., 2005), demonstrating the presence of
substantial amounts of shallow-buried gas hydrates.
Pressurized and non-pressurized cores generally contained sediments belonging to the Black Sea stratigraphic
Units 1, 2, and 3. A positive correlation between the thickness of the stratigraphic Unit 3 in pressurized cores and
gas amounts released during degassing suggests preferential precipitation of gas hydrates below Unit 2. This
interpretation is corroborated by computerized X-ray tomographic imaging and visual observations of
conventional gravity cores. Gas hydrates were found to precipitate as distinct, massive layers at the base of Unit