During the preliminary stage of a field development project, a regional multiscale near-surface geological study is carried out in order to assess geohazard processes with regards to their potential impacts for subsea installation (flowlines, wellhead, templates…) and drilling activities. This integrated approach, essential to assess a new subsea field development project, especially in a very challenging province, needs several types of geophysical and geotechnical surveys, from large to small scale extent. Each survey brings particular informations associated with a specific uncertainty and resolution.

This typical process has been implemented to assess the geohazards potentially affecting an offshore subsea development of the South Caspian Basin (SCB-Azerbaijan). The site investigated is located on the continental slope of the Caspian Sea in water depths ranging from 50m to 600m.

A preliminary regional study, based on a 3D seismic dataset, 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 dataset was of insufficient quality to address the drilling hazards with enough confidence and series of hydrographical and geophysical site investigations have been recommended. They include a dense 2D high resolution seismic survey and a high resolution Autonomous Underwater Vehicle (AUV) survey, aimed at characterizing the drilling hazards, the seabed and near-seabed hazards. These geophysical site investigations have been complemented by a dedicated geotechnical survey in order to provide preliminary geo-mechanical parameters and a proper geological modeling.

This integrated study revealed an unsuspected complexity in the recent geological history of the site, resulting from the close interaction of tectonics, mud volcanism, high sedimentation rates, mass transported deposits and gaseous fluid upward migration.

The gaseous fluid migration is currently active and is at the origin of many geohazards, with shallow gas accumulation at shallow depth, seepages at seabed along faults, mud volcano and pockmarks, and occurrence of methane derived authigenic carbonates (MDAC). They may also be related to the occurrence of slope failure and collapse depressions observed at shallow depth within the soil.

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