Abstract: A program and method for identifying connectivity of well perforation locations to cells of a reservoir description that meet selected pay criteria. First, a pay indicator is assigned to the cells that satisfy the selected pay criteria. A connectivity indicator is then assigned to the pay indicator-assigned cells that correspond to said well perforation locations. Next, a connectivity indicator is assigned to the pay indicator-assigned cells that are connected, either directly or indirectly through another said pay-indicator-assigned cell, to the well perforation location connectivity indicator-assigned cells. The result is construction of a connectivity index array for the reservoir description that differentiates the connectivity indicator-assigned cells from cells that are not assigned a connectivity indicator. The array may be used to calculate the total volume of the connectivity indicator-assigned cells, thereby estimating the drainage volume, i.e.