The primary goal of this study was to determine the effect of presence of gas hydrates on the performance of thermal recovery methods such as steamflooding of heavy oils and tar sand bitumens. In order to achieve this goal, three models have been developed. The first model provides a fully implicit, r-z, finite difference simulation of hydrate dissociation for a reservoir containing only layered hydrates. The second model is a fully implicit, r-z, finite difference reservoir simulator which allows prediction of performance of a steamflood for a reservoir containing layered hydrates in conjunction with a zone of heavy oil/tar sands. For the third model, it is considered that the reservoir contains uniformly distributed packs of gas hydrates within a heavy oil or tar sand zone. The report provides detailed equations, numerical solution approach for all the three models. A user friendly, interactive, menu-driven computer software for the fundamental hydrate dissociation model was developed and the program listing is provided. This model has been fully validated with other finite element type models. The results of the fundamental hydrate dissociation model are provided, and discussed. The other two models have also been computer coded. 24 refs., 19 figs., 3 tabs.