Transfer of hydrocarbons from natural seeps to the water column and atmosphere

Abstract

Results from surface geochemical prospecting, seismic exploration and satellite remote sensing have documented oil and gas seeps in marine basins around the world. Seeps are a dynamic component of the carbon cycle and can be important indicators for economically significant hydrocarbon deposits. The northern Gulf of Mexico contains hundreds of active seeps that can be studied experimentally with the use of submarines and Remotely Operated Vehicles (ROV). Hydrocarbon flux through surface sediments profoundly alters benthic ecology and seafloor geology at seeps. In water depths of 500–2000 m, rapid gas flux results in shallow, metastable deposits of gas hydrate, which reduce sediment porosity and affect seepage rates. This paper details the processes that occur during the final, brief transition — as oil and gas escape from the seafloor, rise through the water and dissolve, are consumed by microbial processes, or disperse into the atmosphere. The geology of the upper sediment column determines whether discharge is rapid and episodic, as occurs in mud volcanoes, or more gradual and steady, as occurs where the seep orifice is plugged with gas hydrate. In both cases, seep oil and gas appear to rise through the water in close proximity instead of separating. Chemical alteration of the oil is relatively minor during transit through the water column, but once at the sea surface its more volatile components rapidly evaporate. Gas bubbles rapidly dissolve as they rise, although observations suggest that oil coatings on the bubbles inhibit dissolution. At the sea surface, the floating oil forms slicks, detectable by remote sensing, whose origins are laterally within ∼1000 m of the seafloor vent. This contradicts the much larger distance predicted if oil drops rise through a 500 m water column at an expected rate of ∼0.01 m s−1 while subjected to lateral currents of ∼0.2 m s−1 or greater. It indicates that oil rises with the gas bubbles at speeds of ∼0.15 m s−1 all the way to the surface.

Methane hydrate stability and anthropogenic climate change

“…The Gulf of Mexico is a leaky oil field (Macdonald et al, 1994(Macdonald et al, , 2002(Macdonald et al, , 2004MacDonald et al, 2002MacDonald et al, , 2004Milkov and Sassen, 2000, 2003Sassen et al, , 2003Sassen and MacDonald, 1994). Natural oil seeps leave slicks on the sea surface that can be seen from space.…”

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

“…Recent studies have shown that methane transfer from marine and lacustrine seeps to the atmosphere is only effective when methane is transported by bubbles released in relatively shallow water (< 100 m water depth) (Leifer and Patro, 2002;MacDonald et al, 2002;Schmale et al, 2005;McGinnis et al, 2006). In most cases, even if a bubble reaches the surface with a significant size, most of the methane is dissolved into the water column and replaced by other stripped gases, particularly oxygen (in oxic conditions) and nitrogen.…”

Shallow gas off the Rhône prodelta, Gulf of Lions

“…Gas bubbling into the water column induces a distinctive signature on singlebeam or side scan systems (Hornafius et al, 1999;Hovland and Judd, 1988;MacDonald et al, 2002). On high-resolution seismic records, regions of anomalous gas concentrations in the sub-surface may be associated with anomalous seismic character (originally proposed by Løseth et al, 2003;Heggland, 1998;Hovland et al, 1999;and Schüler, 1952; note that anomalous seismic character may also be caused by variations in attenuation and resonance).…”