PLATE, KAREN L.¹ and ROBERT C. BARTMAN^2
1Landmark Graphics Corp., Houston, TX
²Pennzoil, Houston, TX

Abstract: Accurate Conversion of Seismic Horizon Time Values to Depth Requires Validation

The accurate conversion of mapped seismic time values to depth remains one of the biggest challenges to successful exploration for additional hydrocarbon reserves and the optimized exploitation of existing reserves. Simple vertical stretch to depth or volumebased calculations are frequently inadequate where structure is complicated or rock velocities change rapidly such as when salt is present. Methods that allow for the direct conversion of mapped time grids that utilize existing well data yield results that appear very accurate because the resultant velocity model forces the wells to tie the seismic values exactly. However, there is no way to account for geologic or velocity variation away from the well control. A preferred workflow for depth conversion allows for the validation of the vertically stretched depth result using forward ray-tracing. The power of this method lies in the ability to "back-check" the velocity volume by image ray tracing through the depth model. Areas where the image ray-tracing shows significant lateral displacement can suggest inaccuracies in the velocity field or the vertical stretch technique. Further work to update the velocity field or a more sophisticated time to depth methods would then be required.

We will describe a case study that utilized new integrated technology to depth-convert two reservoir horizons in the Eugene Island 330 Field, Gulf of Mexico. This integrated approach resulted in a remarkable increase in accuracy that was validated through the use of horizon time values.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas