PTA is a Reservoir Engineering tool when we infer liquid contacts from pressure derivative buildup pattern deflections. Remember, analytical equations solve for transmissivity (kh/µ); net pay (h) and viscosity (µ) are assumed to be constant to derive perm’ (k). So, it’s actually the change in viscosity (µ) that PTA picks up. Gas over water (non-active) generates a signature and dramatic secondary unit slope warning of possible water coning issues (which occurs due to excessive drawdown and high velocity rates). Active aquifer drive appears as pseudo steady-state, the pressure derivative dropping off the screen. Oil over water is less of a change in viscosity, so the secondary slope can be less than 1:1. In very high gas deliverability zones, like WCSB Nisku or Leduc active aquifers, it is possible to calculate volumetric or material balance hydrocarbon reserves from the deflection.