1. A method of evaluating a reservoir, comprising: providing a three dimensional reservoir framework having a plurality of cells; assigning one or more constant reservoir property values to some or all of the cells to provide a first three dimensional reservoir model; on a computer, performing a flow simulation on the first three dimensional reservoir model; making an assessment of the reservoir using the first three dimensional reservoir model; when it is determined that the first three dimensional reservoir model is to be updated, updating the first three dimensional reservoir model by populating some or all of the cells with one or more variable reservoir property values to provide a second three dimensional reservoir model; on a computer, performing a flow simulation on the second three dimensional reservoir model; making an assessment of the reservoir using the second three dimensional reservoir model; comparing the assessment of the reservoir made using the first three dimensional reservoir model with the assessment of the reservoir made using the second three dimensional reservoir model; when it is determined that the second three dimensional model is to be updated, updating the second three dimensional reservoir model by populating some or all of the cells with one or more rock-type fraction, porosity and permeability property values to provide a third three dimensional reservoir model; on a computer, performing a flow simulation on the third three dimensional reservoir model; making an assessment of the reservoir using the third three dimensional reservoir model; comparing the assessment of the reservoir made using the second three dimensional reservoir model with the assessment of the reservoir made using the third three dimensional reservoir model; when it is determined that the third three dimensional model is to be updated, updating the third three dimensional reservoir model by populating some or all of the cells with net-to-gross values directly derived from seismic data to provide a fourth three dimensional reservoir model; on a computer, performing a flow simulation on the fourth three dimensional reservoir model; making an assessment of the reservoir using the fourth three dimensional reservoir model; comparing the assessment of the reservoir made using the third three dimensional reservoir model with the assessment of the reservoir made using the fourth three dimensional reservoir model; and when it is determined that the fourth three dimensional model is to be updated, updating the fourth three dimensional reservoir model by populating some or all of the cells with net-to-gross values directly derived from production data to provide a fifth three dimensional reservoir model.