This paper outlines the technology required to correct porosity for variable unflushed gas saturation. Gas effect on density and neutron logs in fundamental to gas detection. However, in mud drilled wells, a mud filtrate invasion front generally advances radially from the borehole walla and displaces gas within a "flushed zone." Peropheral to the flushed zone is an "invaded zone" characterized by less thorough gas displacement, and, therefore, a higher residual gas saturation. Since density and neutron tools have depths of investigation which are somewhat shallow, gas effect is significantly influenced by the depth of invasion and the thoroughness of flushing. The depth of invasion is generally shallow in high porosity reservoirs and deep in lower porosities. Data presented within this paper, however, show that this generally accepted logion axiom is true only up to a certain point. When low porosity tight gas sand reservoirs are at a critical minimum water permeability, no invasion takes place. Above this minimum permeability, both time and differential pressure control invasion profile. The consequence of this phenomenon is that the porosity of a very poor quality gas reservoir is overestimated, water saturation is underestimated, and the producible gas zone does not contrast well with the tight zone. Thus, the goal of log analysis- to discern the better zones for completion is not achieved. An iterative mathematical model, involving the density, neutron, and gamma ray response equations, is developed to solve for "Sxo" which in this vase is actually the average saturation of the zone investigated by the density and neutron tools. This "Sxo" is then used to provide more reliable porosity interpretation. This technique is verified by comparing log calculations to core data. Aside from more reliable porosity, interesting spinoffs of the technique are: 1) a more reliable procedure to interpret gas saturation in uninvaded formations (independent of Rw and Rt,1034 ); 2) a new procedure to interpret Rw in uninvaded formations; and 3) a new system to identify invaded and, hence, producible gas zones.