3D Pre-Stack Depth Migration in Permafrost Transition, Point Thomson Field, North Slope, Alaska

By

N.C. Lian (Decipher GeoScience), J.R. Alldridge, J.R. Krebs, and J. Hefti (ExxonMobil)

One of the first full 3D Pre-Stack Depth Migrations (PSDM) on the North Slope of Alaska is in progress in the Point Thomson Field area. The Point Thomson Field lies under the transition zone where high-velocity permafrost thins from onshore to offshore lagoon and thickens under barrier islands. Raypath distortion caused by rapid changes in permafrost thickness and velocity has been a problem for seismic imaging and time-depth conversion for many years. This talk concentrates on the impact of seismic acquisition geometry on processing for PSDM, the critical aspects of time pre-processing for PSDM, and on the approach to velocity model building in this complex environment. Four 3D seismic surveys that image the main field area are being reprocessed through Pre-Stack Depth Migration.Variations in fold (30 vs. 60), offset distribution (10,149 ft vs. >20,000 ft), and azimuth (narrow vs. orthogonal vs. broad) between the four surveys significantly impact the data quality and velocity analysis. The need to distinguish between small-scale statics-related variations and larger-scale velocity model variations requires a dense velocity grid of 0.25 mile for the time pre-processing. The velocity grid for the PSDM velocity model is 0.15 mile in the dip-oriented direction by 0.25 mile in the strike direction. The PSDM velocity model is constrained using data from 23 wells, regional permafrost surfaces and a regional correlation surface along with the seismic data. Close interaction between the project interpreter and the seismic processors during all phases of the project is critical to identifying and solving these difficult data problems.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.