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ABSTRACT: Characterisation of gas migration mechanisms using converted waves: a case history from the N. Sokang block (East Natuna basin, Indonesia)

Ben-brahim, Lotfi, Ronan Petton, Gilbert Guingand, and Suryana Ino , TOTAL INDONESIE, Jakarta, Indonesia

A large undrilled structure is present over Total Indonesie's North Sokang block in the East Natuna Basin (Indonesia). Widespread seismic anomalies were identified on modern 3D data. They are associated with severe seismic quality distortion, anomalous ray paths and intriguing features ('Christmas Tree'shape). Reducing the uncertainties related to the poor imaging over possible drilling locations, the presence of bright spots which needed to be characterised in terms of fluid or lithology effects, and the need to understand some of the seismic features justified the acquisition of a 2D/4C seabed survey. Advances in seismic acquisition technology have enabled the routine recording of shear waves in a marine environment. Processing of PS converted waves is still interpretative and requires continuous input to validate in particular the Vp/Vs ratio based on the correlation of compressional and shear wavefields. The quality of the shear waves is excellent matching favourably the best compressional wave data. Interpretation of these 2D/4C data has enabled seismic amplitude anomaly characterisation, imagery has improved through poor quality areas on the 3D providing a clue as to the origin of a prominent, quite unique, anomalous feature on the conventional 3D data. This is interpreted as a gas migration front through an alternating series of sands and shales.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia