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.