Documenting the sand/shale crossover

All types of velocity data, i.e., surface seismic and well log or sonic log data, indicate that velocity in shallow, unconsolidated sands is lower than in associated shales, while velocity in older sands in more consolidated rock sequences is higher than in their associated shales. As the low-velocity sands compact to become the high-velocity member in the sand-shale sequence, sand and shale velocities must become equal or crossover at some age and depth of burial. If the same type of velocity data is used consistently, this crossover occurs at the same depth and age of rocks in the geologic sequence as would be indicated by other types of velocity data, even though the different kinds of velocities would not agree elsewhere. If all rocks in the province are older and compacted, this crossover of sand and shale velocities may not be observed or, if observed, may be very near the surface.