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EXTENDED ABSTRACT: Interaction between Submarine Channels and Mass-Transport
Deposits in an Outer Thrust and Fold Belt of the Western Deepwater Niger
Eddy Lee, Efthymios Tripsanas, R. Craig Shipp, and Larry Gibson
Shell International Exploration and Production Inc., 200 N. Dairy Ashford Rd., Houston, Texas 77079
The objectives of this presentation are to use 3D volume visualization tools to analyze visually and interactively complex geologic elements in multiple seismic data volumes to enhance seismic data interpretation. The study area is located in the western deepwater Niger Delta and is characterized by a series of imbricated, seaward thrusts and folds formed by the downdip movement of strata as a result of sediment loading and extension at the shelf margin (Fig. 1). Deepwater channels and mass-transport deposits were identified using 3D seismic volume visualization, which allows fast volume scanning and extraction of seismic attributes, and multi-volume or attribute rendering for use in geologic interpretation.
The seismic data presented in this study includes near- and far-angle stack and “ dissemblance” volumes. Shell proprietary interpretation software was used for attribute analysis. To highlight channel depositional patterns and relationships, seismic data are best visualized, classified, and analyzed along time surfaces. Time slices are extracted and displayed using a dissemblance volume that was created by measuring seismic data discontinuities. This attribute is useful for highlighting structural and stratigraphic discontinuities that best images channels and faults (Fig. 2). Amplitude scanning software is used to analyze visually and interactively seismic data volumes and to display amplitude attribute as a cube or slabs (stacks of time slices). Amplitude threshold levels (color bar editing) are set so that insignificant reflections are turned transparent and only key features are shown as voxelized images (Fig. 3). Multi-volume corendering was also used to reveal the relationships between different attributes at the same surface. Figure 4 demonstrates amplitude variations along with channel and masstransport deposit distribution. High-amplitude reflections (red color in Figure 4) are identified in channel cutoffs.
Miocene to Holocene sedimentation in the study area is characterized by alternating periods of channel formation and mass-transport deposition.
Channels tend to exhibit higher-amplitude reflections and have linear to meandering paths that can easily be identified using seismic attribute extractions. These channel pathways were controlled by the seafloor morphology, associated with the underlying thrust and fold, and masstransport deposits. In Figure 3, two mass-transport deposits appear to have been diverted by an anticlinal fold in the study area, resulting in the development of a paleodepression that “captured” a channel system. The most recent channel system developed after a period of significant mass-transport deposition that covered almost the entire study area and caused an abrupt change in the channel position (approximately perpendicular to older channels) (Fig. 2). The recent channel avulsion just northwest of the study area may also contribute to this significant change in channel orientation.
3D seismic volume visualization has increased the understanding of the complex nature of channel migrations that have been modified by the interaction with mass transport deposition and the growth of underlying thrust and fold structures. Combined with traditional structural and stratigraphic interpretation, the geological history of channel systems can be further visualized and evaluated.Figure 2. Dissemblance time slice showing channels and mass-transport deposits at a time surface of 3600 msec. Dissemblance slices works best on highlighting structure and stratigraphic discontinuity, such as channels and mass-transport deposits.
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