likelihood proxy for the likelihood function is constructed reservoir model parameters are sampled utilizing the usable proxy for the likelihood function and utilizing the probability models to determine a set of retained models forecasts are estimated for the retained models using a forecast proxy finally computations are made on the parameters and forecasts associated with the retained models to obtain at least one of probability density functions cumulative density functions and histograms for the reservoir model parameters and forecasts the system carries out the above method and the program storage device carries instructions for carrying out the method application_number US 96028807 A description the present invention provides a method to calibrate numerical models of subsurface oil and gas reservoirs to measurements related directly and indirectly to the production and or injection of fluids from and or into the reservoirs further the present invention provides a method for estimating the uncertainty associated with future performance of the oil and gas reservoirs after the calibration of the numerical models probabilistic descriptions can be obtained which are conditional to observed data related to the movement of fluids within the subsurface for both the mathematical models used to represent actual oil and gas reservoirs and for the predictions of future performance computed using such models both model description and predictions are ideally conveyed by way of approximated probability density functions pdf 39 s conditioned to the observed data the probabilistic description of both the reservoir model and predictions.