Jean-Louis Colliat

Hydrate dissolution as a potential mechanism for pockmark formation in the Niger delta

Received 17 February 2010; accepted 9 March 2010; published 11 August 2010. [1] Based on acquired geophysical, geological and geotechnical data and modeling, we suggest hydrate dissolution to cause sediment collapse and pockmark formation in the Niger delta. Very high‐resolution bathymetry data acquired from the Niger delta reveal the morphology of pockmarks with different shapes and sizes going from a small ring depression surrounding an irregular floor to more typical pockmarks with uniform depression. Geophysical data, in situ piezocone measurements, piezometer measurements and sediment cores demonstrate the presence of a common internal architecture of the studied pockmarks: inner sediments rich in gas hydrates surrounded by overpressured sediments. The temperature, pressure and salinity conditions of the studied area have allowed us to exclude the process of gas‐hydrate dissociation (gas hydrate turns into free gas/water mixture) as a trigger of the observed pockmarks. Based on numerical modeling, we demonstrate that gas‐hydrate dissolution (gas hydrate becomes mixture of water and dissolved gas) under a local decrease of the gas concentration at the base of the gas‐hydrate occurrence zone (GHOZ) can explain the excess pore pressure and fluid flow surrounding the central hydrated area and the sediment collapse at the border of the GHOZ. The different deformation (or development) stages of the detected pockmarks confirm that a local process such as the amount of gas flow through faults rather than a regional one is at the origin of those depressions.

Pockmark formation and evolution in deep water Nigeria: Rapid hydrate growth versus slow hydrate dissolution

In previous works, it has been suggested that dissolution of gas hydrate can be responsible for pockmark formation and evolution in deep water Nigeria. It was shown that those pockmarks which are at different stages of maturation are characterized by a common internal architecture associated to gas hydrate dynamics. New results obtained by drilling into gas hydrate-bearing sediments with the MeBo seafloor drill rig in concert with geotechnical in situ measurements and pore water analyses indicate that pockmark formation and evolution in the study area are mainly controlled by rapid hydrate growth opposed to slow hydrate dissolution. On one hand, positive temperature anomalies, free gas trapped in shallow microfractures near the seafloor and coexistence of free gas and gas hydrate indicate rapid hydrate growth. On the other hand, slow hydrate dissolution is evident by low methane concentrations and almost constant sulfate values 2 m above the Gas Hydrate Occurrence Zone.

Analysis of submarine slumping in the Gabon continental slope

The Gabon continental slope is selected as a case study for slope-stability analysis because of evidence of previous slide activities. Different types of data were collected from the continental slope in the Gulf of Guinea off west Africa during Guiness and ZaiAngo surveys. The offshore investigation was carried out using swath bathymetry and associated imagery, deep-towed high-resolution subbottom profiles, side-scan sonar images, observation from remotely operated vehicle Victor, and Kullenberg cores. These data reveal different examples of seafloor instabilities commonly related to fluid-escape features. These slides occur on the continental slope at low declivities, showing that slope gradient has a secondary role on the marine slope instability with respect to external triggering mechanisms such as fluid flow, earthquake, shallow gas, and gas hydrates. One case of mass slide with small downslope displacement was studied on the Gabon slope.In this work, a pseudothree-dimensional slope-stability analysis (Sultan et al., 2001) was undertaken. Three scenarios of instability were tested to identify the possible trigger mechanism of the observed slide instability: (1) under static gravity loading, (2) under earthquakes, and (3) under upward fluid flow. Simulation results show that static stability of the area is satisfactory. However, the stability is very sensitive to fluid escape. These results agree with sonar images showing seepage features aligned along the upslope limit of the observed slide.

Surconsolidation apparente et pression osmotique dans un sédiment marin

This paper concerns the study of the over-consolidation state of a marine sediment extracted from the continental slope in the Gulf of Guinea. Special attention was devoted to the physicochemical phenomena. The study was carried out 1) at a microscopic level using the theory of the diffuse double layer [6, 8, 12] and 2) at a macroscopic level using an elastoplastic model considering the suction as a stress state variable [1]. These models have provided a physical explanation as well as an adequate description of the phenomenon of over-consolidation.

Gas hydrate occurrences and seafloor deformation: investigation of strain-softening of gas-hydrate bearing sediments and its consequence in terms of submarine slope instabilities

Published laboratory geotechnical data by Masui and co-authors showed that increase in gas hydrates content tend to increase the peak shear strength and stimulate strain softening of the host sediment. Therefore, development of shear strains may lead to an important degradation of the shear strength (strain softening). In the present work, the strain softening of gas hydrate-bearing sediments was implemented in a 3D slope stability model (SAMU-3D). This was done by adding to the classical limit analysis method a shear strain field compatibility equivalent to the velocity field compatibility. Examples of slope failures related to strain softening behavior documented in the literature were used to test the model formulation. The developed model was then used to assess the stability of a steep flank of a shale-cored anticline in the eastern part of the offshore Niger delta. Numerical modeling showed that the formation of gas hydrates in the shallow sedimentary layers could considerably affect the factor of safety of the studied slope. The present work showed clearly that the strain-softening behavior of gas hydrate-bearing sediments has relevance for the stability of submarine slopes.

Characterization of Soft Deepwater West Africa Clays: SHANSEP Testing is Not Recommended for Sensitive Structured Clays

With the development of deepwater fields offshore West Africa, in water depths currently ranging between about 600 and 1,500 metres, geotechnical characterization of the sediments has become an issue since the properties of the soft Gulf of Guinea clays differ from those of the equivalent Gulf of Mexico or North Sea clays. In particular, the highly plastic (plasticity index Ip over 80 %) West Africa deepwater clays have a very low unit weight, are highly compressible, and have a relatively strong natural structure. As part of a joint research effort carried out within the CLAROM framework in France, a high quality triaxial testing programme was carried out on clay specimens from an oilfield located in 1,300 metres of water. The paper describes the laboratory test procedure and the results obtained, comparing the results of SHANSEP testing with the results of other triaxial tests at low confining stresses representative of the in situ conditions. With the SHANSEP testing procedure, the micro-structure of the soft West Africa clay is altered by the reconsolidation stresses in the over-consolidated range through large consolidation deformations. The paper shows that the SHANSEP procedure provides undrained shear strength values that are similar to those of a disturbed or reconstituted sample. High quality triaxial testing under confining stresses representative of the in situ stress level appears as the only way to provide geotechnical parameters that are representative of the in situ clay strength. Therefore, the SHANSEP triaxial testing procedure is not recommended for the soft sensitive structured clays from deepwater West Africa sites.