This paper outlines the technology required to correct porosity for variable unflushed gas saturation. Gas effect on density and neutron logs is funda- mental to gas detection. However, in mud drilled wells, a mud filtrate invasion front generally advances radially from the borehole wall and displaces gas within a "flushed zone." Peripheral 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.