Geology as Constraints in Velocity Model-building for Overpressured Carbonates

The recent gas discoveries in Central Luconia have been found in highly overpressured carbonate pinnacles. These structures have steep flanks and are buried by a layer of overpressured shales, where both vertical and lateral velocity variations exist due to the anisotropic effects and overpressures of the shales. Single time-depth functions usually underestimate the possible GRV of these structures. The result from using an integrated 3D seismic velocity approach is heavily dependent on having 3D seismic coverage and good seismic velocities.

An alternative depth-conversion approach is being proposed, whereby a hybrid VoK approach is being used to address the slower velocities in the overpressured shales. The anisotropy effects of the shales where studied using empirical data and its conclusion verified via a quick forward modeling exercise. In addition, the overall shape of the structure and internal facies of the carbonate pinnacle are used as QC constraints in verifying the results of the depth conversion exercise.

This alternative approach is highly-favored as it not only addresses the velocity variations from the overpressured shales, but also conforms to the subsurface geological model. The resulting depth structure of the carbonate pinnacle is both geologically and volumetrically consistent.