Gas hydrates in the Nile deep-sea fan : a restricted BSR vs widespread fluid venting

Description

The Nile deep-sea fan is the largest Plio-Quaternary depocentre in the Mediterranean Sea, extending over an area of >150,000 km2 within which rapid deposition drives syn-sedimentary collapse tectonics, as well as widespread seafloor venting of mixed thermogenic and biogenic gases. It is thus an ideal setting for the near-seabed accumulation of gas hydrates, which are stable below water depths of about 1100 m in the warm waters of the eastern Mediterranean. Gas hydrates have yet to be sampled on the Nile fan, although their presence has been suggested from semi-published industry seismic and well log data. Here we integrate available industry data with a regional grid of academic geophysical data (seismic and multibeam) acquired by Géoazur, in order to identify a BSR on the central Nile fan, invert it to geothermal gradients, and examine its relationship to fluid venting. The BSR is observed on several intersecting seismic profiles of varying frequency content and offset, as a discontinuous reflection of negative polarity that lies 220-330 ms below seafloor, deepening downslope in water depths of 2000-2500 m. The BSR occurs within a relatively small area (2500 km2), among a larger system of slope-parallel extensional faults. Inversion of BSR depth to temperature gradients was performed using a velocity-depth function for deep-sea sediments, a phase boundary for methane hydrate in equilibrium with seawater of 3.86% salinity (Mediterranean average), and water temperatures from MEDATLAS. The results indicate spatially varying gradients of 27-42˚C/km, up to twice background values reported in the geothermally cool eastern Mediterranean offshore. 'Shallow' BSRs yielding elevated thermal gradients within the Nile fan can be explained in terms of advective heat transfer by upward fluid flux. Upward fluid migration is supported by evidence of widespread seafloor fluid venting, but seafloor data indicate few or no vents in the area of the BSR. In contrast, in an area with many gas vents over 50 km from the BSR, subsurface gas hydrate accumulations are indicated by resistivity log data at two well-sites. If gas and gas hydrates are present across wider areas, why is the BSR so restricted? We hypothesise that these observations can be explained in terms of spatially varying fluid flux, such that away from the BSR a greater flux of fluids rich in dissolved gas leads to rapid gas hydrate formation and the growth of gas chimneys. We intend to test this hypothesis during a forthcoming Franco-Brazilian campaign, focused on understanding the dynamics of the fluid vents. Acknowledgement : this work has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 656821 (SEAGAS project).

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