The Value of Geophysical Tools in the Subsalt Play of the Gulf of Mexico
Authors: Abriel William L. 
Published: October 2008

Relatively expensive geophysical tools, like seismic depth imaging, provide critical value in fundamental decisions in the subsalt Gulf of Mexico play. For example, seismic imaging tools like post-stack and pre-stack 3D depth migration can add value for subsalt lease acquisition and disposition, prospect identification, prospect risk analysis, drill site selection, and reserves determination. The physical reason why these tools are important is that depth imaging is more than a multiplication of rock velocity and time seismic images. For instance, complex 3D topology and rugosity of the surface at both the top and base of the salt can significantly alter the location of reflected subsalt energy. The images below salt are most affected by large ray-bending at the interface with the sediments. Depth images of the subsalt can be different enough from time images that prospects for drilling can disappear or new ones appear elsewhere. In addition, models and live data experiences both show that the expensive geophysical process of prestack 3D depth migration has high value for reservoir identification and reserves analysis below salt. Apart from seismic imaging itself, other related geophysical tools are also important. Computer simulations of the energy illumination (ray-tracing) of the subsalt can be used to validate structures, explain lost data zones, and design appropriate re-acquisition parameters. Other simulations (e.g., forward modeling) can explain noisy problems and potential solutions during data processing. Finally, 3D physical models can answer fundamental questions about subsalt seismic imaging techniques that are not possible to unravel in live data.

Introduction

Depth imaging has become an offshore technology of high use in the Gulf of Mexico. This is due to the advancement of the subsalt play both on the continental shelf and in the deep water. Although 3D post-stack depth imaging was deployed sooner in the plays of the North Sea, 3D pre-stack depth imaging was not considered a cost viable tool until applied in the Gulf of Mexico subsalt.4 The reasons for the success of 3D prestack depth imaging are attributed both to its cost and its technical viability. When the data processing cost was more than 30% of the drilling expenditures, 3D pre-stack depth imaging projects were not considered appropriately priced for the value added. As quality has improved and price lowered, the application of the technology has become significantly more widespread. Estimates of the depth imaging business in the Gulf of Mexico suggest that the processing portion of this technology is being employed by the industry at an annual cost that exceeds $80,000,000. Central to the technical success of 3D pre-stack depth imaging has been the recognition of its quality over post-stack imaging. Favorable comparisons of 3D prestack and poststack imaging have been made several times in the literature. In these cases the pre-stack processing is shown to be useful when the salt bodies are 3 dimensional in their topology of the top and base salt. In most parts of the Gulf of Mexico, there is a high contrast between the velocity of the salt and the surrounding sediments. This creates a situation where the seismic energy bends significantly at this boundary and creates imaging requirements more difficult than in non-salt areas.