Abstract
The aim of this paper is to present the methodology implemented to assess the geohazards potentially affecting a subsea development offshore Azerbaijan and the main findings. The site investigated is located on the continental slope of the Caspian Sea in water depths ranging from 50m to 600m.
A preliminary geohazards assessment has been made from the available 3D exploration seismic and revealed large scale geological features, including a giant mud volcano, regional scale Mass Transport Complexes (MTC) and a large slope failure scar at seabed. Nevertheless, the resolution of the 3D dataset was of insufficient quality to address the drilling hazards with enough confidence and a serie of high resolution geophysical site investigations has been recommended. They include a dense 2D high resolution seismic survey and a high resolution Autonomous Underwater Vehicle (AUV) survey, aiming at characterizing seabed and near-seabed geohazards. These geophysical site investigations have been complemented by a dedicated geotechnical survey in order to provide preliminary geo-mechanical parameters and to build a proper geological model. All these surveys have been achieved very early in the development project in order to have sufficient flexibility to adapt the intended subsea field layout and to minimize the exposure to geohazards.
The high resolution dataset revealed an unsuspected complexity in the recent shallow geological history of the site, resulting from the close interaction of tectonics, mud volcanism, high sedimentation rates, mass transport deposits and gaseous fluid upward migration. The gaseous fluid migration is currently active and is at the origin of many geohazards, with gas accumulation at shallow depths, seepages at seabed along faults, mud volcano and pockmarks, and with occurrence of methane derived authigenic carbonates (MDAC). The gaseous fluid migration and accumulation may also be related to the occurrence of slope failures observed at shallow depths within the soil.
