In this talk will discuss a novel approach for rapid field-scale performance assessment of shale gas and oil reservoirs. The proposed approach is based on a high frequency asymptotic solution of the diffusivity equation in heterogeneous reservoirs and serves as a bridge between simplified analytical tools and complex numerical simulation. The high frequency solution leads to the Eikonal equation which is solved for a ‘diffusive time of flight (DTOF)’ using the Fast Marching Method. The DTOF generalizes the concept of depth of investigation to heterogeneous and fractured reservoirs and provides an efficient semi-analytic method to calculate drainage volume, pressure depletion and well performance in unconventional reservoirs. More importantly, in a manner analogous to streamline simulation, the ‘diffusive time of flight’ can also be used as a spatial coordinate to reduce the 3-D diffusivity equation into a 1-D equation which can be solved numerically accounting for the relevant physics related to shale gas and oil reservoirs. The speed and versatility of our proposed method makes it ideally suited for high resolution reservoir characterization through integration of static and dynamic data. The major advantages of the proposed approach are its simplicity, intuitive appeal and computational efficiency. We demonstrate the power and utility of our method using field examples.