Geophysical Evidence of Gas Hydrates Associated with Widespread Gas Venting on the Central Nile Deep-Sea Fan, Offshore Egypt

• Main Authors: Praeg, Daniel, Migeon, S., Mascle, J., Unnithan, V., Wardell, N., Geletti, R., Ketzer, J, M
• Other Authors: Géoazur (GEOAZUR 7329), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de la Côte d'Azur, Université Côte d'Azur (UCA)-Université Côte d'Azur (UCA)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD France-Sud ), Université Pierre et Marie Curie - Paris 6 - UFR de Médecine Pierre et Marie Curie (UPMC), Université Pierre et Marie Curie - Paris 6 (UPMC), Jacobs University = Constructor University Bremen, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Pontifícia Universidade Católica do Rio Grande do Sul Brasil = Pontifical Catholic University of Rio Grande do Sul Brazil = Université catholique pontificale de Rio Grande do Sul Brésil (PUC-RS), H2020-MSCA-IF-2014-GF-656821, EC SEAGAS (656821)
• Format: Conference Object
• Language: English
• Published: HAL CCSD 2017
• Subjects: [SDE]Environmental Sciences; [SDE.ES]Environmental Sciences/Environment and Society; Methane hydrate
• Online Access: https://hal.science/hal-02155718

International audience Deep-sea fans are favoured settings for the formation of gas hydrates, due to high sedimentation rates and organic matter content that promote the upwelling of methane-rich fluids. Gas hydrates have not been sampled on the Nile fan, but geophysical evidence of their presence is known to the Egyptian hydrocarbon industry. Here we use academic data to document a bottom-simulating reflection (BSR) on the central Nile fan, and examine its relationship to evidence of seabed fluid seepage. We use reprocessed multi-channel seismic profiles acquired from 1973-2002, of varying offset (0.3-4 km) and peak frequency content (10 1-10 2 Hz), together with sonar imagery and subottom profiles (10 3-10 5 Hz). The regional methane hydrate stability zone (RMHSZ) is modeled using a phase boundary for methane hydrate in equilibrium with bottom water of 3.86% salinity (Mediterranean average) and gridded inputs for bathymetry, bottom water temperatures and geothermal gradients. A BSR is observed across the central Nile fan in water depths of 2000-2500 m, as a discontinuous reflection of negative polarity at depths of 220-330 ms below seabed. BSR 'patches' vary in extent and character, but are mainly of low amplitude and, on higher frequency data, may be associated with reduced amplitudes or blanking in the overlying succession. The BSR is observed both in stratified intervals and within interbedded unstratified mass-transport deposits (MTDs, up to 200 ms thick). The depth below seabed of the BSR is comparable to the base of the modeled RMHSZ (converted to travel-time at sediment velocities of 1.6-1.8 km/s). The presence of gas hydrates in the lower part of the RMHSZ is indicated by published resistivity logs from two exploration wells, here located in areas lacking a BSR. We infer that on the central Nile fan gas hydrates are present within a sub-seabed interval up to 250 m thick, that is in places associated with a weak BSR recording basal accumulations of free gas. Previous work shows the central Nile fan is ...