Abstract: The Use of 3D Visualization for Understanding Tertiary Deep-Water Clastic Systems: a West Africa Example

Marotta, D.; C. S. Alexander; K. Coterill; K. Hartman; M. Pasley; T. C. Stitelar; and G. Tari - Amoco; L. Binga; Sanangol; and B. Lehner - Shell Angola

Most deep-water Tertiary basins around the world show acoustic impedance differences between sand and shale lithologies. These differences in rock properties enable the interpreter to utilize 3D visualization to illuminate the stratigraphy of these deep-water clastic systems. Unlike traditional 3D methods, the tool allows the interpreter to quickly pick points within a specific geologic feature resulting in a 3D view of the depositional system.

Visualization programs convert each seismic sample of the 3D volume (i.e. stacked, amplitude, coherency, velocity) to a voxel volume where a voxel is defined by crossline and inline spacing sample rate, attribute value and opacity. Assignment of variable opacity allows the user to identify preferred depositional patterns within a large volume of data. Automatic picking of voxels indicates the connectivity of voxels which fall within the user-defined ranges. Several iterations of voxel picking within the same geologic feature can be completed within a matter of minutes. The user has the ability to increase the ranges of the attribute value used for picking until the boundaries of the geologic feature are exceeded The process has the advantage of both speed and flexibility.

The coast of West Africa provides an excellent example for the study of Tertiary deep-water channel/fan deposition from subsurface data. Large scale, high quality 3D seismic, 3D visualization technology and horizon-consistent time slices provide a window into the subsurface for understanding the architecture of ancient deep-water systems.

The West African province is characterized as a passive continental margin. Opening of the South Atlantic began in Early Cretaceous followed by the deposition of Aptian salt that extended from southern Angola to Gabon. The marine incursion was followed by overall deepening in the Late Cretaceous and delivery of deep-water sediments to the slope and basin was enhanced by Tertiary uplift of the African craton. Salt tectonics played a significant role in the modification of the basin topography influencing the architecture and stacking patterns of the Oligo-Miocene fan deposition.

The 3D voxel pick interpretations of these Tertiary deposits give an immediate impression of the morphology of the depositional system and the shape and connectivity of the seismic events. The voxel pick, horizon-consistent time slices and axial traverses along and across true depositional dip provide a new understanding of the depositional evolution of these deep-water fans. These tools aid in the interpretation of the reservoir model and have direct implications on reservoir characterization.

Today, most models for deep-water reservoirs are based on outcrop and modern deep-water fan examples. With the advent of large, high quarry 3D datasets and the application of 3D visualization technology, much information will be learned about ancient deep-water clastic systems. The application of 3D visualization technology will improve deep-water depositional models and enhance the oil industry's and academic world's understanding of deep-water clastic systems.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil