A method for estimating reservoir quality from a reflection seismic survey includes determining seismic interval velocity (or their inverse, interval transit time “ITT”) with respect to depth from the reflection seismic survey. A normal compaction trend of the seismic interval velocity with respect to depth is determined. A fractional amount of sand and a fractional amount of shale at at least one depth is determined based on deviation of the seismic interval velocity (or ITT) at the at least one depth from the normal compaction trend.