Mapping Pays and Estimating Net-Pay Thickness from 3-D Seismic Attributes in Low Acoustic Impedance Gas Sandstone Reservoirs

Hari Lal
GEOPIC, ONGC, Dehradun, India

3-D seismic attribute anomalies were mapped and predicted to be gas bearing in Dahanu area of Mumbai Offshore Basin, India. Post-stack-migrated 3-D data was used in the initial study. A well was drilled and the mapped anomalies were found gas bearing at multilevel in sandstone reservoirs of Upper Oligocene Daman Formation. To find the aerial extension, thickness and reserves, reprocessed 3-D pre-stack-migrated (PSTM) data of 600 km² area was interpreted. Log and seismic signatures of pay sands were identified and matched with help of synthetic seismograms. The gas sands were found to have low acoustic impedance enclosed in high impedance shales and thus producing high negative amplitude (troughs). Seismic attribute analysis and spectral decomposition were used for mapping the geometry and aerial extent of pay sands. For estimation of net sand thickness and their lateral variability, thickness of amplitude in milliseconds was computed by counting the number of samples falling within specified amplitude range in the given time interval. Amplitude and thickness of amplitude showed high positive correlation. Amplitude thickness in ms was multiplied by interval velocity to get the thickness in metre. After calibrating the estimated thickness at well, final thickness map was prepared.

The thickness map was found quite accurate as it matched with thickness at well which was not included in calibration. Thickness estimation based on spectral decomposition tuning frequency was also attempted but the method based on amplitude thickness was found more quick and accurate. The method demonstrated in the study may be applied in case of low acoustic impedance reservoirs producing amplitude anomaly.

Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery