3-D geostatistical velocity modeling: Salt imaging in a geopressured environment

Abstract

The success of salt imaging requires accurate delineation of salt boundaries and faults around the salt. It is essential to define accurate salt boundaries for drilling updip hydrocarbon potentials at salt boundaries and to delineate small faults around the salt due to the compartmentalization of hydrocarbon traps by these faults around the salt. However, the success of salt imaging largely depends on whether an accurate 3-D velocity model can be built. An accurate 3-D velocity model is required for seismic imaging and accurate depth prediction. A reasonable way to build such a model is to make sure that it incorporates geologically feasible lateral and vertical velocity variations and all available well information. The integration of well information is particularly important when dealing with geologic complications such as geopressure zones, reef build-ups, and salt. A method for building 3-D velocity models incorporating geopressure zones and the development of an algorithm and methodology to integrate well data are discussed in this article along with an application of this technology in the Gulf of Mexico. Overpressure conditions are common in the Gulf of Mexico in areas of increasing water depths in which the percentage of shale over sediment becomes higher and the top of geopressure becomes shallower. Salt movement may have induced the geopressure. Successful salt imaging in these areas requires identification and modeling of often complicated geopressure zones. Seismic imaging and time-to-depth conversion in these areas are handicapped by severe raypath distortions. Because the velocities in geopressure zones can be very slow compared to velocities in surrounding rock, it is often difficult to image even top boundaries of the salt unless the geopressure zones are incorporated accurately into velocity models. Solid evidence of geopressure zones can be obtained from sonic and resistivity logs …