1. A machine implemented iterative method of well log analysis to determine the volumes of anhydrite (or gypsum) in three mineral combination lithology utilizing well log measurements of gamma-gamma density, neutron and acoustic properties of earth formations in the vicinity of a well borehole comprising the steps of: (a) obtaining a gamma-gamma density versus neutron porosity crossplot from the respective logs at a corresponding depth in the well borehole; (b) determining .rho..sub.map (the average density of rock matrix) and .DELTA.t.sub.map (the average matrix acoustic travel time) by solving said crossplot for a two mineral lithology; using the .rho..sub.map and .DELTA.t.sub.map, then determining the volume of anhydrite (or gypsum) present from the relationship given by EQU (.rho..sub.map (1-Va)+.rho..sub.a .multidot.V.sub.a -.rho..sub.b)/(.rho..sub.map -.rho..sub.ef)=[.DELTA.t-(.DELTA.t.sub.map)(1-V.sub.a)+.DELTA.t.sub.a .multidot.V.sub.a)]/(.DELTA.t.sub.f -.DELTA.t.sub.map) where V.sub.a is the volume of anhydrite in the formation; .rho..sub.a is the density of anhydrite; .rho..sub.b is the bulk density from the density log response; .rho..sub.ef is the electron density of the formation fluid; .DELTA.t is the acoustic travel time from the acoustic log response; .DELTA.t.sub.a is the acoustic travel time of anhydrite; and .DELTA.t.sub.f is the acoustic travel time in the formation fluid; (c) using the V.sub.a just computed, recompute the two mineral lithology volume for V.sub.l (volume of limestone) from the relationship EQU V.sub.l =V.sub.l .multidot.(1-V.sub.a), and; repeating steps (b) and (c) using new values of .rho..sub.map and .DELTA.t.sub.map in an interative manner until the corrected values of V.sub.a do not vary from repetition to repetition by more than a predetermined amount, thereby arriving at a value of V.sub.a.