Systems and methods for subsurface reservoir simulation

Systems and methods for providing a reservoir simulation are based on data from an unstructured grid using a structured grid reservoir simulator. Exemplary methods comprise obtaining an unstructured grid reservoir model comprising a reservoir model discretized on an unstructured grid. A virtual structured grid is defined for the unstructured grid reservoir model. The unstructured grid is aligned with the virtual structured grid by adding cells to the unstructured grid to make the unstructured grid and virtual structured grid have the same number of cells. The virtual structured grid may be represented in the unstructured grid. Structured grid reservoir simulator input data comprising reservoir model data assigned to the virtual structured grid is prepared based on reservoir model data in the unstructured grid model. A structured grid reservoir simulation is performed using the structured grid reservoir simulator input data to produce a reservoir simulation.