Garden Banks 260 Field: Dynamic Application of Seismic Migration in a Major Field Development
Authors: Walker D.B. , Pressler R.R. , Checkles S.N. 
Published: October 2008

Abstract
Oil and gas exploration in the Gulf of Mexico has been greatly influenced through the use of seismic amplitude anomalies. However, the spatial position of these anomalies can be changed significantly by varying the migration velocity field. Furthermore, recent developments in ray trace modeling have shown that shooting direction during seismic acquisition can alter the apparent spatial distribution of amplitude anomalies. This was evidenced in the development of the Garden Banks Block 260 Baldpate Field. Drilling results showed that in the initial migration the amplitude anomaly associated with the pay sand was swept 1500' to 2000' beyond the updip reservoir truncation. A comparison of well checkshot velocities from a walkaway VSP against seismically derived velocities indicated severe anisotropy in the area, resulting in over-migration. A series of target oriented post-stack depth migrations were run and evaluated against the ongoing development drilling until an optimum migration was identified. During the course of this evaluation it was further determined that acquisition direction could affect the amplitude image, creating an apparent change in the shape and position of the anomaly.

Introduction
Garden Banks Block 260 lies 110 miles offshore within the Plio-Pleistocene Flex Trend in 1650' of water. Amerada Hess and Oryx Energy drilled the GB 260 #1 discovery well in 1991 to evaluate an Upper Pliocene amplitude associated event on a salt-flank structure which was mapped from a 1989 time migrated 3-D volume (Figs. 1 and 2). The well encountered 180' of net TVD Pliocene pay, with the two major zones having 103' and 64', respectively. The upper pay sand was tied to the primary amplitude anomaly using a VSP and synthetics.
A sidetrack (#1ST1) of the discovery well drilled to test the extreme downdip limit of the anomaly encountered 243' of net TVD pay in the same two major zones. However, a subsequent delineation sidetrack to test the amplitude updip of the discovery well (#1ST2) unexpectedly drilled into salt, which truncated the reservoirs (Fig. 3).
Subsequent evaluation suggested that the seismic velocities were inadequate for proper structural positioning due to abnormally high anisotropy in this area. Overmigration swept the anomaly beyond the reservoir limit and into salt. Remigration of the time data using a velocity field that combined checkshot data with seismic velocities resulted in a migration that appeared to properly position the amplitude anomaly relative to the wells. Unfortunately, steeply dipping reflections associated with salt were now improperly migrated, creating bands of constructive and destructive interference within the amplitude.
In 1995 a new 3-D survey was licensed, and again the data appeared overmigrated. However, recent in-house processing capability now allowed fast and reliable post-stack depth migration. The survey was migrated with revised velocity fields until the best fit to well data was achieved. Amplitude banding was now minimal and salt was better imaged. Depth relationships were improved, resulting in amplitude conformity with structure. Using this migration the subsequent wells, including a critical attic well under salt, encountered the pay sands at the depths and thickness predicted.