Summary

This paper presents an integrated, multi-disciplinary approach to exploration in a complex salt basin. The construction of the interval velocity/depth model and the resultant depth migrated seismic data were constrained by gravity/magnetics and, uniquely, by structural restoration of both seismic data and interpretations to ensure consistency between geophysics and geology. This proved invaluable as it placed boundaries on interpretation possibilities in areas where poor seismic imaging persisted. A 1D basin modeling exercise was also performed adding to the complete understanding of the basin. As this work flow allowed for “what-if” scenarios, estimates of uncertainty in image quality, trap, sand distribution, etc. were also provided and used in prospect risking. The project was completed in the same time frame that it would take to complete a conventional pre stack depth migration project yet delivered far more than just depth migrated seismic data.

Introduction

In areas of complex geology, for example imaging in salt basins, pre stack depth migration has become the de facto standard for seismic imaging. Figure 1 shows a comparison between pre stack time migration (PSTM) and pre stack depth migration (PSDM) for a salt body. The colours of the interpretations in Figure 1 represent equivalent geological reflectors. As these reflectors define the trap geometry and reservoir size against the salt overhang, the need for PSDM is apparent. Both Gravity and magnetics and reconstruction require interpretations in depth and therefore when attempting multi-discipline integration, depth is the only domain to use.

In areas of pure exploration, with little or no a priori information, for example from wells, one of the key questions is how reliable are the pre stack depth migration images in defining the salt geometry and sedimentary sequences proximal to the salt?

  • plausible salt model. What is the salt edge geometry? Are salt roots likely?

  • validate the interpretation, including checking the salt budget in the section, are changes in salt area realistic, are horizons correctly picked?

  • constrain topography with time and impact on reservoir thickness and quality, by modelling the changes in geometry through time, identifying depocentres and rates of deposition.

  • document timing of salt windows and impact on migration

  • document trap evolution (size, dip) and secondary Remigration

  • what kind of deposits could be present? Shale or sand? What can be inferred from this about possible velocity anomalies in the model?

An attempt to address this has been undertaken in a pre stack depth migration project from offshore Brasil where gravity and magnetics were used as a constraint on salt geometry. In addition to this, and unique in pre stack depth migration projects, geology, in the form of structural restoration, was used as an additional constraint during construction of the velocity model. Structural restoration allows us to “wind-back” through geological time from the current geological model. Only when a set of interpretations was proven to be valid, was the interpretation valid and included in the model. The ability to do this has several objectives:

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