Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System in the Western Black Sea

Stephan Ker (1) , Yannick Thomas (1) , Vincent Riboulot (1) , Nabil Sultan (1) , Céline Bernard (2, 3, 1) , Carla Scalabrin (1) , G. Ion (4) , Bruno Marsset (1)

1 GM - Unité Géosciences Marines
2 ENSTA Bretagne - École Nationale Supérieure de Techniques Avancées Bretagne
3 STIC - Département STIC [Brest]
4 GeoEcoMar - National Institute for Marine Geology and Geo-ecology

A comprehensive characterization of the gas hydrate system offshore the western Black Sea was performed through an integrated analysis of geophysical data. We detected the Bottom Simulating Reflector (BSR), which marks, in this area, the base of gas hydrate stability. The observed BSR depth does not fit the theoretical steady-state base of gas hydrate stability zone (BGHSZ). We show that the disparity between the BSR and predicted BGHSZ is the result of a transient state of the hydrate system due to the ongoing re-equilibrium since the Last Glacial Maximum. When gas hydrates are brought outside the stability zone due to changes in temperature and sea level, their dissociation generates an increase in interstitial pore pressure. This process is favorable to the re-crystallization of gas hydrates and delays the upward migration of the hydrate stability zone explaining the anomalously deep BSR. The BSR depth, which is commonly used to derive geothermal gradient values by assuming steady state conditions, is used here to derive the maximum excess pore pressure at the base of the gas hydrate stability zone. Derived excess pore pressure values of 1-2 MPa are probably the result of the low permeability of hydrate-bearing sediments. Higher pore pressure values derived at the location of a fault system could cause hydro-fracturing enabling the free gas to cross the gas hydrate stability zone and emerge at the seafloor, forming the flares observed in close vicinity to where the shallow gas hydrates were sampled.

Mots clés: Gas hydrate; BSR; Excess pore pressure; Black Sea

Domaines: Géophysique [physics.geo-ph]

Stephan Ker, Yannick Thomas, Vincent Riboulot, Nabil Sultan, Céline Bernard, et al.. Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System in the Western Black Sea. Geochemistry, Geophysics, Geosystems, 2019, 20 (1), pp.442-459. ⟨10.1029/2018GC007861⟩. ⟨hal-02123994⟩