Geological and morphological setting of 2778 methane seeps in the Dnepr paleo-delta, northwestern Black Sea

Abstract

The Dnepr paleo-delta area in the NW Black Sea is characterized by an abundant presence of methane seeps. During the expeditions of May-June 2003 and within the EU-funded CRIMEA project, detailed multibeam, seismic and hydro-acoustic water-column investigations were carried out to study the relation between the spatial distribution of methane seeps, sea-floor morphology and sub-surface structures.2778 new methane seeps were detected on echosounding records in an area of 1540 km 2 . All seeps are located in the transition zone between the continental shelf and slope, in water depths of 66 to 825 m. The integration of the different geophysical datasets clearly indicates that methane seeps are not randomly distributed in this area, but are concentrated in specific locations.The depth limit for the majority of the detected seeps is 725 m water depth, which corresponds more or less with the stability limit for pure methane hydrate at the ambient bottom temperature (8.9 8C) in this part of the Black Sea. This suggests that, where gas hydrates are stable, they play the role of buffer for the upward migration of methane gas and thus prevent seepage of methane bubbles into the water column.Higher up on the margin, gas seeps are widespread, but accurate mapping illustrates that seeps occur preferentially in association with particular morphological and sub-surface features. On the shelf, the highest concentration of seeps is found in elongated depressions (pockmarks) above the margins of filled channels. On the continental slope where no pockmarks have been observed, seepage occurs along crests of sedimentary ridges. There, seepage is focussed by a parallel-stratified sediment cover that thins out towards the ridge crests. On the slope, seepage also appears in the vicinity of canyons (bottom, flanks and margins) or near the scarps of submarine landslides where mass-wasting breaches the fine-grained sediment cover that acts as a stratigraphic seal. The seismic data show the presence of a distinct bgas front,Q which has been used to map the depth of the free gas within the sea-floor sediments. The depth of this gas front is variable and locally domes up to the sea floor. Where the gas front approaches the seafloor, gas bubbles were detected in the water column. A regional map of the sub-surface depth of the gas front emphasises this bgas front-versus-seepQ relationship. The integration of all data sets indicates that the spatial distribution of methane seeps in the Dnepr paleo-delta is mainly controlled by the gas-hydrate stability zone as well as by stratigraphic and sedimentary factors. D

Geologic control of natural marine hydrocarbon seep emissions, Coal Oil Point seep field, California

“…Foldand-thrust fault belts are a prominent setting for fluid migration from deeper layers through faults and fractures, allowing accumulation of hydrocarbons in anticlines and other traps (Bonini 2007). In some settings, geologic structures close to the seabed, such as authigenic carbonates (Boetius and Suess 2004) and low-permeability sediment (e.g., clay) layers (Naudts et al 2006), can be important factors controlling flux rates.…”

Anomalous sea-floor backscatter patterns in methane venting areas, Dnepr paleo-delta, NW Black Sea

“…4.1.A.). The region is wellknown for abundant gas seeps, carbonate buildups and shallow gas (Polikarpov et al, 1989;Polikarpov et al, 1992;Egorov et al, 1998;Luth et al, 1999;Peckmann et al, 2001;Thiel et al, 2001;Amouroux et al, 2002;Michaelis et al, 2002;Kruglyakova et al, 2004;Kutas et al, 2004;Popescu et al, 2004;Pape et al, 2005;Pimenov and Ivanova, 2005;Reitner et al, 2005;Schmale et al, 2005;Treude et al, 2005;McGinnis et al, 2006;Naudts et al, 2006). During the 58 th (May-June 2003) and 60 th (MayJune 2004) cruise of R.V.…”

“…(Naudts et al, 2006). The distribution of these seeps is not random, but is controlled by morphology, by underlying stratigraphy and sediment properties, and by the presence of gas hydrates acting as a seal and preventing upward migrating gas to be released as bubbles in the water column (Naudts et al, 2006). This study focuses on a small (3.96 km by 3.72 km) part of the continental shelf, centered on a dense seep and pockmark site at -92 m water depth (Figs.…”

“…shallow gas fronts, enhanced reflections, acoustic blanking) or potential gas or fluid conduits (e.g. faults, diapirs) (Judd and Hovland, 1992;Orange and Breen, 1992;Yun et al, 1999;Garcia-Gil et al, 2002;Van Rensbergen et al, 2002;Krastel et al, 2003;Naudts et al, 2006;Judd and Hovland, 2007). Direct localization of seeps, i.e.…”

“…Direct localization of seeps, i.e. locations of bubble release, is performed by echosounders that detect gas bubbles in the water column due to the acoustic impedance contrast between water and the free gas in the bubbles (Egorov et al, 1998;Artemov, 2006;Naudts et al, 2006;Rollet et al, 2006).…”