Pay maps have long been the starting point for industry geologists when evaluating new plays and trying to identify their potential.  This is especially true with conventional reservoirs, however, as the industry has moved to focus more on unconventional reservoirs it has become clear that the traditional net pay mapping using log cutoffs leaves a lot left to be desired.  Overlaying the most successful wells in the midland Basin with net pay maps based on porosity, density, and gamma ray cutoffs and you see one similarity, little to no correlation.  At the surface this may indicate that rocks don’t matter, but digging deeper it becomes apparent that traditional net pay maps focus on rock properties that are not the primary driver for successful wells.  In fact the rock facies are vitally important.  Petrophysical multi-mineral models have broken down these seemingly homogenous black rocks into highly detailed facies making it possible to tease out the shales that are going to produce the best.  When these petrophysical facies are mapped locally and regionally a much stronger correlation can be seen between rock and production.  While traditional net pay mapping methods have failed to identify the best rock consistently, new petrophysical facies mapping has helped to unlock the secret of shales in the Midland Basin and demonstrate that rock still do matter in the unconventional world.