1. A method for making production forecasts for a field containing one or more oil or gas reservoirs, comprising: (a) developing a three-dimensional earth model of the field; (b) developing a geologic model of at least one reservoir, based at least in part on the earth model, said geologic model being a cellular-based representation of at least one reservoir, each cell being assigned a value for a plurality of properties describing distribution of pore space, fluid types and amounts in place; (c) developing a reservoir simulator based at least in part on the geologic model, said simulator being capable of predicting production rates; (d) obtaining simulated historical production data from the reservoir simulator; (e) comparing the simulated production data with actual production data; (f) adjusting the earth model to reduce any differences between simulated and actual data; (g) repeating steps (b)-(f) using the adjusted earth model, adjusting the geologic model and simulator consistent with the adjustment to the earth model; and (h) using the adjusted earth model to make production forecasts for the field.