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Success Factors for Effective Subsurface Imaging of Overthrust Belt Legacy Seismic Data

Ritchie, Walter 1
1 Fusion Petroleum Technologies Inc., Houston, TX.

In recent years 3-D pre-stack depth migration (PSDM) has changed industry’s perspective on subsurface imaging of seismic data in areas of complex geology enhancing industry’s ability to more fully exploit the hydrocarbon potential of difficult geologic areas. There is also an abundance of legacy seismic data sets available to industry today for re-evaluation but many of these existing surveys does not always fully lend itself to the more rigorous re-examination demanded by depth imaging methods. This paper discusses a seismic reprocessing program intended to assess deep gas potential in a mature producing province using the PSDM method and how challenges presented by these vintage seismic 3-D data in a complex structural regime were overcome. The targeted area was a major gas field discovered in 1978 that has been producing large quantities of gas in the Overthrust Belt of Utah/Wyoming.

To overcome deficiencies in the legacy data, a “keep it simple” modeling strategy was adopted with strong adherence to the well-derived interval velocity trends except where strongly and consistently contra-indicated by the seismic. This approach was especially helpful in reducing the high variability experienced in the initial velocity models and provided guidance in areas of no/low well control. With this strategy in place, the PSDM process proceeded rapidly through a series of iterations to produce the final PSDM volume. The final volume based on the geology-constrained modeling approach exhibited good image quality and closely tied key formation top data. In addition to exhibiting good reflector continuity in the primary zones of interest and tying available well data, confidence in the accuracy of the volume was re-inforced by the stability of the deeper section where the reflection energy in the final depth volume closely matched the known regional attitudes of these beds. The velocity model-building solution developed for this project using well-derived interval velocities as a constraint allowed this greatly reduced parameter space to be explored systematically with a small number of pre-stack depth migration iterations. The method successfully overcame the limited velocity-resolving power of the legacy data and provided good convergence on the best-fit model that met project objectives in this difficult subsurface imaging area. Thus, with an awareness of key success factors, subsurface depth imaging using legacy data sets is a very practical option.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009