Reservoir properties or attributes such as water saturation, porosity and permeability are determined directly at each cell of a three dimensional grid of a subsurface hydrocarbon reservoir. Input parameters based on original data from petrophysical measurements and other attributes of the reservoir and its subsurface features are generated over a range of uncertainties for the individual cells. The predicted input parameters for the individual cells are provided as processing inputs for petrophysical algorithms. The input parameters at different measures of possible uncertainties for the cells of the reservoir are processed using cluster computers to determine on a cell-by-cell basis a number of possible scenarios or results for the reservoir attributes of interest. The determinations made for the different scenarios can then be assembled as postulated models for sections of interest of the reservoir grid, and the postulated models evaluated to more accurately obtain reservoir attributes over the reservoir, even for areas where no well intersects the reservoir.