Integrated Characterization of Deep-water Natural Hydrocarbon Seeps - First Insights

The occurrence of natural hydrocarbon (HC) at seabed or sub-seabed within the foundation zone represents a significant geohazards for any development in the deep offshore Angola. Indeed, the HC detected on geophysical datasets are commonly associated with Methane Derived Authigenic Carbonates (MDAC), asphalts nodules / seabed mounds and / or oil-clay mixtures, which may affect/ modify the soil geotechnical properties.

This study presents the preliminary results of an integrated geophysical and geotechnical approach, where a site characterized by multiple sub-seabed HC indicators (High Impedance Reflectors HIR) has been selected on high resolution CHIRP sub-bottom (SBP) profiles and has been ground truthed by Cone Penetration Testing (CPT) and continuous sampling boreholes.

The geophysical data discloses eight stacked HIRs, conformable to the stratigraphy and occasionnaly displaced by faults.

Two of the HIRs are characterized by high cone resistance value on the CPT data and are interpreted as representing MDAC levels.

The collected samples have logged using non-destructive techniques including multi-sensor core logging and X-ray imaging. As result, the sample logging detected MDAC or asphalt nodules, clay levels infiltrated with HC, micro-fissuring / fracturing induced by or associated to HC infiltration, several biturbation patterns and significant cracks and voids.