3D Prestack Depth Migration of the Neptune Structure, Deepwater Gulf of Mexico
Authors: Purnell Nigel P. , Anderson Alan , Kapoor S. Jerry 
Published: October 2008

In late 1995 BP and BHP completed an exploratory well on the Neptune structure, which is located about 150 miles offshore Louisiana within the Deepwater Mississippi Fan Fold Belt at the edge of the Sigsbee escarpment. A 3D seismic survey was subsequently acquired in early 1996. The desire to drill an early appraisal well on the structure within 14 months plus the need for sub-salt 3D pre-stack depth imaging presented us with very little time to produce a product of sufficient quality to meet our interpretation requirements for well placement. This paper is a case history which describes the process undertaken by BP, BHP, and Western Geophysical to produce a quality 3D pre-stack depth image in a shortened time-frame. The project demonstrated the potential of a wellfocused and committed team to work together to produce extraordinary results. A team building exercise at the beginning of the project which enabled all to have a shared commitment to both the process and the outcome was a key success factor. Also, joint interpretation by BP/BHP/Western with a multidisciplinary review team enabled us to have a single agreed upon dataset which included input from many different areas of expertise such as structural geology, velocity modeling, and migration algorithm development. This shared vision has allowed us to build upon the initial project and produce further improvements in data quality since the initial fast-track results were delivered in late 1996.

Introduction

In 1995 BP and BHP drilled an exploratory well on the Neptune structure. Subsequent to the completion of drilling, BP/BHP contracted Western Geophysical to acquire a proprietary 3D survey over approximately 30 Gulf Of Mexico (GOM) blocks (23 sq. km./block) surrounding the Neptune structure. Seismic acquisition took place between January and April of 1996.

The planning for this survey took place in late 1995 with the basic acquisition and conventional processing parameters selected by reviewing existing 1994 2D proprietary, and older spec. data. Some of the impediments to good seismic quality in the area are rapidly varying waterbottom, steep dips at the target and poorly imaged subsalt data as illustrated by Purnell et al.1. 2D pre-stack depth migrations and 3D raytracing were undertaken to determine shooting orientation, migration aperture halo, and optimum offset ranges necessary to properly image both the subsalt and outside of salt portions of the structure. As a result of these studies, we acquired the new 3D survey with a 6 km maximum offset, a 12 km migration aperture as reported by Albertin et al.2, and in a NW-SE orientation.

During the 2D pre-stack depth migration processing, BP/BHP discovered that the shallow sediment velocity field was much more complex than expected. These shallow anomalies and the complexity of the salt bodies largely invalidated 2D methods. To define the structure's size and shape rior to any further drilling, 3D pre-stack depth migration was necessary.

Our desire to drill a second well on the structure in mid-1997 plus the need for 3D pre-stack depth imaging presented us with very little time to produce a product of sufficient quality to meet our interpretation requirements.