Three-Dimensional Seismic Imaging of Probable Hydrate Deposits Linked to Chimneys Venting Gases Through Hydrate Stability Zone, From Offshore Mauritania

Abstract

Hydrate deposits within the hydrate stability zone (HSZ) are considered to have a potential source of gas, but their geometries are rarely imaged. Interpretation of a three-dimensional (3-D) seismic dataset, kindly provided by Chariot Oil and Gas, from offshore Mauritania, displays high amplitude features surrounding 15 seismic chimneys. We propose that these chimneys vent gas from immediately below the hydrate bottom simulating reflector (BSR) into the HSZ. The high amplitude zones only occur in specific seismic reflections which are ∼60 m above the BSR, and are restricted to a specific range of water depth (700-900 m). No other similar features can be seen landwards or seawards. These enhanced reflections are interpreted as hydrated sediments perhaps with a contribution of authigenic carbonate for the following reasons: 1) the pressure-temperature (P-T) condition at the depth of high amplitude is suitable for hydrate formation; 2) hydrate-filling pores may noticeably increase the acoustic impedance which is consistent with the high amplitudes we observe; 3) the plume-like geometry of the high amplitude is similar to that observed from CO₂ injection sites (e.g. the Sleipner Field of North Sea), which implies the high amplitude features are gas-derived. The hydrate deposits were sourced from underlying free gas zone (FGZ). This geological process has been seen in offshore southwestern Japan, Oregon, and mid-Norway. Numerical simulation of heat transport in this region may show that between 20 ka and 12 ka ago the increasing pore overpressure caused by hydrate dissociation triggered chimney formation, resulting in the injection of methane-rich gas into the HSZ. Upward vertical fluid migration presumably ceases due to gas freezing and hydrate formation within a succession below the seabed. The buffering role of the permeable near-seabed horizons has not been recognized before and implies that a portion of methane entering the HSZ may be trapped temporarily.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016