A machine to simulate reservoir production measures of a subsurface reservoir having a hydrocarbon region and a peripheral aquifer region, the reservoir being defined by a plurality of grid cells designated and organized as hydrocarbon grid cells for the hydrocarbon region of the reservoir and aquifer grid cells for the peripheral aquifer region, the plurality of grids cells being partitioned into a plurality of processing sub-domains, each processing sub-domain containing at least a portion of the plurality of grid cells, the machine comprising: a plurality of processor nodes having one or more processors, the plurality of processor nodes being assigned to a processing sub-domain, comprising: a parallel input processing grid data-space , the processor nodes in the parallel input processing data-space organizing the grid cells of the reservoir into input blocks of cell data for processing; an unstructured graph and connection factor data-space , the processor nodes in the unstructured graph and connection factor data-space forming cell geometries for amalgamating aquifer grid cells and defining active grid cells composed of the hydrocarbon grid cells, forming a load-balanced processing network; wherein the processor nodes in the unstructured graph and connection factor data-space perform the steps of (a) determining the presence of vertical columns of cells of the aquifer region ; (b) grouping the vertical columns of cells of the aquifer region into connected grid blocks of amalgamated aquifer cells ; (c) performing load balanced domain partitioning of the cells of the hydrocarbon region and the amalgamated aquifer cells of the aquifer region ; and (d) generating transmissibilities between the cells of the amalgamated aquifer cells of the aquifer region ; a simulation processing data-space for performing reservoir simulation of the active grid cells. a plurality of processor nodes having one or more processors, the plurality of processor nodes being assigned to a processing sub-domain, comprising: a parallel input processing grid data-space , the processor nodes in the parallel input processing data-space organizing the grid cells of the reservoir into input blocks of cell data for processing; an unstructured graph and connection factor data-space , the processor nodes in the unstructured graph and connection factor data-space forming cell geometries for amalgamating aquifer grid cells and defining active grid cells composed of the hydrocarbon grid cells, forming a load-balanced processing network; wherein the processor nodes in the unstructured graph and connection factor data-space perform the steps of (a) determining the presence of vertical columns of cells of the aquifer region ; (b) grouping the vertical columns of cells of the aquifer region into connected grid blocks of amalgamated aquifer cells ; (c) performing load balanced domain partitioning of the cells of the hydrocarbon region and the amalgamated aquifer cells of the aquifer region ; and (d) generating transmissibilities between the cells of the amalgamated aquifer cells of the aquifer region ; a simulation processing data-space for performing reservoir simulation of the active grid cells. (a) determining the presence of vertical columns of cells of the aquifer region ; (b) grouping the vertical columns of cells of the aquifer region into connected grid blocks of amalgamated aquifer cells ; (c) performing load balanced domain partitioning of the cells of the hydrocarbon region and the amalgamated aquifer cells of the aquifer region ; and (d) generating transmissibilities between the cells of the amalgamated aquifer cells of the aquifer region ; a simulation processing data-space for performing reservoir simulation of the active grid cells. <EOS>