Generation and Interpretation of Multiple Seismic Data Volumes for Fluid and Lithology Discrimination in the Mahogany Field, Offshore East Coast Trinidad

Rosemarie T. Ramkhelawan¹, Charlie Baynes², Mick J. Casey¹, Samuel R. Johnson¹, and Jennifer L. Nelson^1
1 BP America Inc, Houston, TX
² BP Trinidad and Tobago LLC, Port of Spain, Trinidad and Tobago

Negative seismic amplitudes from fullstack reflectivity data have been used very successfully to identify and map hydrocarbon intervals in the Trinidad Mahogany Field. As infill drilling continues as part of field development, resolving uncertainties due to reservoir heterogeneities, off-structure amplitudes and strong amplitudes from low saturation gas zones, becomes increasingly important. Generating impedance volumes tuned to lithologies and fluids is key to the successful identification of remaining reserves. Understanding the reservoir properties from the exploration and development wells is fundamental in calibrating the seismic data to rock and fluid properties.

In this study, seismic reflectivity data sets, including fullstack, angle stacks, AVO intercept and gradient, have been inverted to relative impedances, using a coloured inversion approach. In addition, fullstack reflectivity data has been spectrally blued to enhance frequencies, which allow for more detailed reservoir mapping. Coherencies and spectral decomposition volumes are generated to image reservoir geometries. Outside the core field area, brine sands are strongly visible on fullstack impedance data. Inside the core field area, the strong acoustic signature from the gas-filled reservoirs, masks all other responses and it is difficult to image the aquifer. Analogues from outside the core field, applied to inside the core field are important, as well as generating impedance datasets tuned to lithology, so that wet sands become visible.

No single seismic volume has all the answers. It is in the combination and interpretation of multiple reflectivity and impedance datasets, and coherency and spectral decomposition volumes, which allow the reservoir story to be better understood.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005