Pitfalls in Seismic Amplitude Interpretation: Lessons from Oligocene Channel Sandstones

Hari Lal*, S.K. Biswal, and J.L. Nangia
ONGC, India
*[email protected]

An high-amplitude 3-D seismic anomaly in an Upper Oligocene clastic sequence depicted a channel depositional signature and was interpreted as a bright spot generated from gas-sands. Upon drilling, however, it was found water-bearing. The post-drill appraisal, based on the interpretation of pre-stack time-migrated data using 3-D visualization techniques, reconfirmed the bright spot and large channel. The geostatistical cross-plots between net-sand-thickness and seismic amplitude showed a direct proportionate relationship. Impedance versus lithology cross-plots showed varying inter-relationships. Gas-bearing sandstones have higher impedance in some wells and lower impedance in other wells with respect to the enclosing shales. The bright spot was produced by thick, high-impedance porous sandstones deposited in a fluvial to estuarine channel. The amplitude-versus-offset (AVO) responses of water-sands in a new well did not show an AVO anomaly, whereas gas-bearing sands in an existing well showed a good class-II anomaly. The interpretation pitfalls could have been avoided by more studies like AVO.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain