Visualizing, integrating data types positions salt flank
VSP, seismic, well data define a GoM subsalt prospect
Anadarko drilled the Tarantula No. 1 well in the Gulf of Mexico on the flank of a massive salt structure. Although the top of salt is well imaged through 3D poststack time migration, interpretation of the salt flank is ambiguous. A diverse vertical seismic profile (VSP) survey was acquired in this well to locate the salt flank. The comprehensive data sets included:
• A zero offset VSP (Z0) survey
• Two offset VSP surveys (O1 and O2)
• A 3D salt proximity (SP) survey
•Two walkaway VSP lines: north-south (W1) and west-east (W2).
Three-dimensional SP data and a 2D Kirchhoff depth migration of the VSP data defined the salt-sediment boundary. Processors applied tomography to the first break times and the first sediment travel time arrivals to obtain a velocity distribution of the area and to delineate the salt-sediment interface.
The salt boundary offset from the 2D profile reflects seismic energies that can produce a 3D depth migration cube using this 2D dataset and apply a 3C-3D vector depth migration algorithm.
The comprehensive VSP survey resulted in a data-rich processing dataset. The dataset needed a 3D visual interpretation tool to integrate the VSP and surface seismic data. The processing team integrated results from the VSP survey with the surface seismic data using Visus, a PC-based, 3D visual interpretation tool that VSFusion and GeoTomo LLC developed jointly to solve 3D VSP problems.
3D salt proximity and 2D depth migration
The salt exit points derived from the salt proximity survey were superimposed on the north-south surface seismic section to define the salt flank. Preliminary 2D depth migration of the VSP vertical-component data defined the salt flank reflectors.
The reflectors showed a similar dip derived from the SP survey. However, the absolute flank position was significantly offset. Interpreters refined the 2D depth migration image using 3C data rotated to the directions of the maximum reflections.
Performing tomographic inversion using the first sediment arrival times yielded a velocity distribution in the sediment layers. The resulting velocity model with lateral heterogeneity improved the depth migration. A tomographic inversion of the first break times provided the salt-sediment model.
The tomographic image delineates the salt-sediment boundary, which closely matches the salt exit points. The 3D salt exit points, 2D tomography image, and surface seismic data agree reasonably well. Two different methods (SP and tomography) produced a very similar salt flank shape.
3D VSP depth migration volume
The processing team examined vector wavefields of VSP data from three in-line shots. Seismic energies reflected from the salt flank were significantly offset from the 2D in-line profile. Therefore, the 2D migration image is only a projection of the 3D image onto the 2D in-line section. This projection results in a mis-positioning of the salt flank and degradation in image quality.
Processors applied the vector 3C-3D Kirchhoff migration algorithm to migrate the VSP data from three in-line sources using the previously derived sediment model. This processing produced a 3D depth migration image volume. Data from the two walkaway lines produced 3D VSP migration images.
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An enlarged migration image cube shows the sediment reflectors near the wellbore and the salt reflectors at the top and side of the image cube. The image cube is scanned into the center of the volume, and the 3D salt reflection images closely match the salt proximity results.
The result of this effort is an integrated data cube that includes the 3D VSP migration image cube, 3D salt exit points, 2D tomographic inversion image, and the surface seismic profiles. Using different degrees of transparency allows interpreters to see different types of data simultaneously.
The team processed comprehensive VSP data acquired in the Tarantula No. 1 GoM well using diverse VSP techniques to produce images of the salt flank. The use of the various VSP techniques and the 3D visual interpretation tool provides a validation of the results and yields an accurate picture of the salt flank position.•
Editor’s note: This paper was presented at the 2004 SEG convention. A complete list of references is available from Yingping Li at tel: (1) 281-646-2758 or email: [email protected].