A simple two-dimensional flow model is used to illustrate the effects of different porosity and permeability distributions and retort geometries on sweep efficiency during modified in-situ oil shale retorting. Results of these case studies, two emphasizing different retort geometries and porosity distributions and one a study of nonuniform flow around a single, large block, are presented. The most interesting is a simulation of the flow field and resultant retort front movement for Occidental Petroleum Company Retort 5. A porosity/permeability distribution was chosen based on reported tracer data. The results of the model calculation indicate a sweep efficiency of about 55%. From these data an estimate can be made of the amount of oil degradation from burning, cracking, and coking. 17 figures.