Fluid migration observed from seismic evidence in different hydrate provinces - Nicaragua vs. Black Sea
Bialas, J. M.; De Batist, M. ; Naudts, L.; Poort, J. ; Talukder, A.R.; Klaeschen, D. (2006). Fluid migration observed from seismic evidence in different hydrate provinces - Nicaragua vs. Black Sea. Eos, Trans. (Wash. D.C.) 87(48)
In: Eos, Transactions, American Geophysical Union. American Geophysical Union: Washington. ISSN 0096-3941; e-ISSN 2324-9250
Comparisons of observations from the Black Sea and the Nicaraguan margin demonstrate the existence of large amounts of gas in the sedimentary column. Differences in how the gas could be observed imply that quite different mechanism either support the discharge or keep the gas at depth. High resolution seismic profiles reveal a Bottom-Simulating-Reflector (BSR) signature underneath the slope of the Nicaraguan continental margin. The BSR can be observed with varying strength underneath mound structures. The distribution of the mounds in part is related to faults cutting through the sediment column. Obviously such faults have been active as pathways for the discharge of fluids. Despite intensive observations during recent cruises no active discharge could be located. Hence the existence of the BSR indicates that a certain amount of gas remains in the subsurface, and is sealed down there. Seismic data recorded in the area of the Dnepr palaeo delta, Black Sea, shows a continuously strong amplitude, reaching from the foot of the slope up to shallow water depth. The top of the gas hydrate stability (TGHS) field is found at 700 m water depth in this region. At this depth the strong amplitude event is observed to step from about 300 m below seafloor (bsf) at greater depth to a few meters below the seafloor at shallower water depth. As this event does not cross any sediment layers it is referenced as gas front. Numerous active bubbling gas expulsion locations are found at shallow water depth in this region, while only a few locations have been found, which discharge gas below the TGHS. Analysis of the gas discharge in the shallow water indicates that it does not origin from greater depth. Consequently the slope sediments in this region act as a perfect sealing for the gas at greater water depth. The observation of the gas front demonstrates that the sealing is independent from the hydrate stability field. Large gas flares in the water column and transparent parts of seismic record sections indicate the existence of a certain amount of gas in the area of the Sorokin Trough in the Black Sea as well. Other than in the previously describe provinces no indication of a gas front image or a BSR has been found here. Combined interpretation of multichannel seismic data, wide angle seismic observations and high frequency echo sounder records show a clear relation ship of gas discharge locations and the conduit of the Dvureshenskii mud volcano. Comparisons of seismic images along the Sorokin Trough do show, that the mud volcanoes are underlain by updoming diapir features. Geochemical analysis at the Dvureshenskii indicates that fluids origin from about 3 km depth (Aloisi et al.) while heat flow indicates a source at 300 600 m depth. As gas flares are observed above the neighbouring diapir top as well it can be assumed that the distribution of mud volcanoes is dense enough to discharge the entire region form upward migrating fluids. Here the tops of the diapirs act as migration path for the gas. Although clear conduits and mound structures are imaged at the Nicaraguan margin and within the Sorokin Trough their existence itself is not enough to stimulate gas expulsion. Both regions, Nicaragua and the Sorokin Trough are within a compressional tectonic regime and hence different processes of sealing and activation of discharge must be expected. The observations described above indicate that a discharge is dependent on fracturing and other forces.
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