1. In a method of utilizing reservoir simulator software to determine subsidence resulting from the withdrawal of fluids from a subterranean reservoir by solving a set of failure criterion equations which comprise the residuals and any derivatives, to determine a set of rock plastic displacements, the improvement comprising an equilibration technique for initialization of stresses and pore volumes comprising the steps of applying traction boundary conditions at the surface of a gridded reservoir and thereafter finding a rock state that satisfies the equation 
 V e NTGφ e =V 0 NTGφ 0 =PV 0 where V e is the bulk volume after application of the traction boundary conditions, NTG is the user-specified net-to-gross ratio, φ e is the porosity after application of the traction boundary conditions, V 0 is the initial bulk volume which is directly calculated from the Cartesian or corner point geometry, and φ 0 is the initial porosity.