FEATURES OF SUBSURFACE EARTH RESERVOIRS OF INTEREST ARE MADE AVAILABLE FOR ANALYSIS AND EVALUATION BY FORMING THREE-DIMENSIONAL, GEOLOGIC BLOCK MODELS BASED ON FIELD DATA. THE FIELD DATA INCLUDE GEOLOGICAL OBSERVATIONS, SUCH AS LITHOFACIES AND POROSITY VALUES OBTAINED FROM WELL DATA AND OTHER SOURCES, AS WELL AS GEOPHYSICAL DATA, USUALLY FROM SEISMIC SURVEYS. THE GEOLOGIC MODELS REPRESENTATIVE OF SUBSURFACE RESERVOIRS SO OBTAINED ARE OPTIMIZED TO MATCH AS CLOSELY AS FEASIBLE GEOLOGIC CONSTRAINTS KNOWN OR DERIVED FROM OBSERVED GEOLOGIC DATA. THE MODELS ALSO CONFORM TO GEOPHYSICALLY BASED CONSTRAINTS INDICATED BY SEISMIC SURVEY DATA. THE MODELED GEOLOGIC LITHOFACIES AND POROSITY ARE CONVERTED INTO ACOUSTIC VELOCITY AND BULK DENSITY VALUES, WHICH ARE THEN FORMULATED AS A SEISMIC RESPONSE WHICH IS THEN COMPARED WITH ACTUAL SEISMIC DATA. A PERTURBATION PROCESS ON LITHOFACIES AND POROSITY CAN BE ITERATIVELY REPEATED UNTIL A REPRESENTATION OF THE RESERVOIR IS OBTAINED WHICH IS WITHIN SPECIFIED LIMITS OF ACCURACY OR ACCEPTABILITY. (FIG.5A)
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