The usual procedure for determining net effective sandstone values to be used in reservoir isopach mapping is to count the net sandstone thickness as measured by logging devices in each well bore penetrating the reservoir sandstone, without applying possible corrections for nonvertical penetration. Application of this procedure is a practical approach to the determination of net sandstone thickness, and the values obtained probably are within the limits of accuracy needed for most reservoir isopach maps. However, certain conditions of bed dip and/or bore-hole deviation preclude the use of this procedure as an accurate method for net sandstone thickness determination. The difference between measured and vertical net sandstone thickness can be significant. As an example, wh re reservoir bed dip is 60° and a well bore drifts directly updip with a 7° angle of inclination, the true vertical net sandstone thickness attributable to the well is 20 percent greater than that observed in the well bore. The use of vertical net sandstone values in reservoir isopach mapping results in a more nearly accurate estimate of recoverable reserves.