Using cone penetration tests to differentiate between gas hydrates and carbonate concretions in the Niger Delta ///

This article presents a new method to characterize free gas, gas hydrate and carbonate concretion occurrence, which are considered high-risk factors for subsea developments. This method is based on the combination of 3D seismic data and geotechnical site characterizations using piezocone Cone Penetration Tests (CPT). The calibration of P-wave velocity anomalies was completed using in situ geotechnical testing carried out during two oceanographic surveys in 2003 and 2004. Comparisons between in situ testing using the piezocone, recovered cores, and predicted gas and gas hydrate distribution based on compressional wave velocity have shown that 3D seismic data is a valuable tool in identifying heterogeneous areas, but using in situ CPT testing is essential to discriminate between gas hydrates and carbonate concretions. Furthermore, in situ compressional wave velocity (Vp) measurements have clearly demonstrated the co-existence of gas hydrate and free gas in the study area that was only previously suspected from seismic data.

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