Velocity Studies in the Deepwater Gulf of Mexico: Keathley Canyon and Walker Ridge Areas

Two different, yet related, velocity studies were undertaken in and around the Keathley Canyon and Walker Ridge areas of the Gulf of Mexico. The first is a compilation of wellbore-salt-body interval velocities (Vint) from 55 wells exhibiting interval compressional-wave velocity variation from 13,966 ft/s to 18,535 ft/s with mean velocity of 14,920 ft/s and a standard deviation of 726 ft/s. The velocities vary significantly with latitude. Five different Vint zones have been identified with each having specific-associated mineralogies within a latitude range. In the mid-latitude zones, sylvite and small amounts of clastics, with traces of both anhydrite and gypsum, are found within the salt, yielding salt Vint variation from 14,388 ft/s to 14,965 ft/s. The salt Vint in the southern limits of the study area is higher than 15,000 ft/s and associated with more gypsum. The northern-most wells are anhydrite-rich and exhibit the highest velocities. The Vint are relatively uncorrelated to and insensitive to factors such as wellbore temperature, depth, and pressure. Composite medium modeling of the salt-body compositions shows that various mineral and lithologic inclusions within the salt body can explain the observed velocity variations. The second study is a 3D velocity model constructed using high resolution 2D seismic data with 15 km offsets and 22 seconds (40 km or 130,000 ft) of record, constrained by sonic logs from 94 wells, 34 VSP or borehole seismic velocity surveys, and 38 calculated time versus depth tables derived from other borehole data. All forms of sonic information were transformed into Vint and loaded into the CGG VelPro velocity-modeling application. Comparison of the resultant 3D velocity model with available constraints shows that regional geological trends expressed in the model faithfully reproduce the observed borehole Vint profiles at 18 locations where the seismic velocity control is in close proximity to measured borehole-velocity data. Zones of over-pressure and of Cenozoic limestone are discernible within the velocity model. The resultant 3D “cube” of Vint values covering Keathley Canyon, Walker Ridge, and a portion of Green Canyon evidences distinct details due to well control.