Strong heat flow variability in an active shallow gas environment, Dnepr palaeo-delta, Black Sea

Abstract

The Dnepr palaeo-delta on the north-western continental slope of the Black Sea is a prolific seepage zone characterised by a highly variable heat flow ranging from 24 to 88 mW/m(2). New thermal data were collected at 33 closely spaced stations in order to better understand the apparent relation between heat flow variability and seepage features. Strong relief gradients and associated landslide shaping may explain first-order heat flow variability but, locally, thermal parameters appear to be controlled by fluid and gas migrations. High heat flow anomalies are found at sites where faults and diapirs offer pathways for warm fluid flow from deeper sedimentary levels. Low heat flow is most strongly expressed at ridge crests near seepage sites but is also found where seeps are absent. Reduced heat flow and nonlinear temperature-depth variations are interpreted to result from natural or induced gas ebullition of saturated shallow gas covered by a thin, relatively impermeable sedimentary seal. The presence of sealed gas pockets, in particular at ridge crests, is supported by methane pore-water analysis and a shallow gas front widely observed in the study area

Active venting sites on the gas-hydrate-bearing Hikurangi Margin, off New Zealand: Diffusive- versus bubble-released methane

“…Colder sediment temperature in comparison with the near-bottom water temperature is opposite to what is expected at seepage sites. The negative temperature anomaly can be explained as a remnant of cold water infiltration induced by earlier gas bubble release through a process of recharge, as discussed by Poort et al (2007). This could be plausible, since near-bottom water temperatures were indeed similar or colder in comparison to the anomalous sediment temperature measurements, ca.…”

Geological control and magnitude of methane ebullition from a high-flux seep area in the Black Sea—the Kerch seep area

“…In contrast, seeps on the middle slope at intermediate water depths have gained considerable less attention. For instance, the gas emissions at the broad Dnepr and Danube paleo-fan at the northwestern Black Sea margin are well-known from a multitude of studies (Luth et al, 1998;Michaelis et al, 2002;Lüdmann et al, 2004;Naudts et al, 2006Naudts et al, , 2008Naudts et al, , 2009Artemov et al, 2007;Poort et al, 2007;Popescu et al, 2007). More than two thousand gas emission sites have been identified at the continental shelf and upper slope during a detailed hydroacoustic survey (Naudts et al, 2006).…”

Regional and local geothermal conditions in the northern Black Sea

“…These data are listed in Table 2 and have also been incorporated in Poort et al (2007). The majority of the thermal stations are located on the top and the flanks of the sloping ridge structure where seepage is widely spread.…”

“…Offshore thermo-probe measurements are made at water depths larger than 300 m because at shallower depths seasonal bottom water temperature variations are too large (Kutas et al 1999;Poort et al 2007). Sea bottom temperature in the Black Sea changes from 8.66 ± 0.03°C at 300 m water depth to 9.1 ± 0.1°C at 2,200 m water depth.…”

“…3). Such anomalous feature can be explained by the presence of a gas pocket very close to the surface (Poort et al 2007). Probe penetration through the sealing crust and into the gas pocket will result in gas ebullition (by fracturing of the seal) and pressure release in the gas pocket.…”