Previous Talks

Wednesday, February 11th, 2009

New Velocity Model Building Techniques to Reduce Sub-Salt Exploration Risk

Martin L. Albertin, Stephan Petmecky and Nick Burke (BP)

Abstract

Key aspects of exploration risk, such as trap definition, seal capacity, and hydrocarbon migration are tightly linked to the gross structural geometries provided by 3D pre-stack depth migration. The quality of the seismic depth image is directly related to the accuracy of the underlying velocity model. In most subsalt settings, insufficient angular illumination severely degrades the resolution and accuracy of velocity information derived from the seismic data itself. Because subsalt data quality precludes the reliable use of standard velocity analysis methods, a standard approach for building a starting subsalt velocity model for imaging uses velocity information derived outboard of salt, extrapolated following bathymetry into the subsalt regions. The shortcoming of this method lies in the implicit assumption that effective stress is a function of depth below mudline, and that effective stresses beneath salt are similar to those in outboard basins.

An alternative approach for defining subsalt velocities starts with a model for subsalt effective stress, and uses an empirical effective stress – interval velocity relationship to define the subsalt velocity. We have used three different effective stress modeling approaches in recent imaging projects to build starting subsalt velocity models which better fit trends observed in offset wells. In order of increasing modeling effort, they are: 1) Desalting – an approach for correcting extrapolated velocity models for the expected change in overburden beneath thick allochthonous salt; 2) Structural Modelling – accounting for lateral pressure transfer from deep synclines to structural crests , and 3) 3D Basin modeling, a rigorous approach for determining sub-salt effective stresses which accounts for depositional history, sand distribution and connectivity, variable shale properties, and timing of salt emplacement.

Speaker Biography

Martin L. Albertin is a Geophysical Advisor with BP, in the Deepwater Gulf of Mexico Exploration Business Unit. Martin joined Amoco in 1988, and has worked on depth imaging, subsalt exploration, and pressure prediction projects worldwide. Martin has a BS in Geology from Indiana University of Pennsylvania (1985), and an MA in Geology from The University of Texas at Austin (1989).