Subsalt imaging in the Gulf of Mexico

Exploration focus in the Gulf of Mexico (GoM) has shifted from Plio-Pleistocene amplitude plays in the early 1980’s to deep/ultra deep water Miocene/Paleogene subsalt structural plays in the late 1990’s. Accordingly, seismic technologies have evolved from pre-stack time migration and AVO analysis to a variety of pre-stack depth imaging (PSDM) algorithms.
By early 2000, seismic contractors and major oil companies had developed numerous PSDM processing technologies for traditional 3D (narrow azimuth) streamer acquisition. The needs for further improving subsalt structural imaging and stratigraphic interpretation, as well as the high cost of drilling wells in these environments, have encouraged the oil industry to further investigate 1) new and innovative acquisition and processing technology, and 2) more integrated workflows, where geophysical and geological information are merged to provide optimal imaging.
The success of newly developed Wide Azimuth Towed Streamer (WATS) seismic acquisition technology has proven it applicable in the GoM and also for world wide exploration and production. More than 2500 OCS blocks of WATS data are planned to be acquired in 2008 in GoM. This number is up from around 1,700 blocks in 2007 (and less than 100 blocks in 2005).
Given the high cost associated with both drilling and acquisition in this challenging environment, an extensive effort to analyse and improve on the seismic imaging problem by seismic modelling, illumination analysis and depth imaging should be an important and cost effective part of the workflow. We will show examples from a case study that consists of three stages which were in part run concurrently, leading up to a full 3D pre-stack depth imaging of the area.
The first stage of the activity was a modelling exercise to determine the key problems associated with the area. They relate to seismic artefacts, such as base of salt multiples, shear wave mode conversion, or potential mis-picking of the base of salt. The second phase was a post stack depth migration project to quickly see the impact on the data of alterations in salt shape, sediment velocity field, etc. This effort was combined with an illumination study to assess what part of the data one could expect to be able to image. The third and final stage will typically be a full pre-stack depth migration incorporating the knowledge from stages one and two, testing out several scenarios for the interpretation of the salt.
A total of four different salt models were built, and four volumes with PSDM (1000 square km of input data) were generated. The effort produced an image of significantly improved quality, with a much more geologically sound subsalt struc¬tural image. As a result volume and risk estimates have been revised.
In this presentation we will cover both the evolution of seismic acquisition and processing technologies for subsalt targets, as well as the illustration of an integrated workflow for subsalt imaging in the Gulf of Mexico.