Changes in oil-water contact (OWC) depth across a field are often a signature of reservoir compartmentalization as a result of faulting or stratigraphic barriers, such as shales, but may equally well be caused by an underlying hydrodynamic aquifer. In particular, the pressure-depth data obtained from an aquifer whose flow is changing over time can look very similar to that obtained from a compartmentalized reservoir. Misunderstanding which of these mechanisms causes the observed changes in OWC across the field may result in poor estimates of oil in place and reduced recovery. To address this problem, an analytic expression is presented to estimate the time taken for a steady state tilted OWC to be established once an aquifer starts flowing. A comparison with simulations of hydrodynamic aquifers in homogeneous, compartmentalized, and heterogeneous reservoir models shows that this expression can be used in combination with the one derived by M. K. Hubbert, for the steady state tilt of the OWC, to clarify whether a reservoir contains barriers or baffles to flow or may simply have not yet reached equilibrium.