The objective of the work performed under this contract was to obtain accurate measurements of the porosity and permeability of western tight gas sands and eastern gas shale under pressures representative of the actual underground reservoir. The results of these measurements were then correlated with the pore structure inside the rocks. The approach was to use proven core analysis facilities at IGT to make these difficult measurements, and to study the pore structure by means of optical and scanning electron microscopy. Results obtained under this contract indicate that gas flow through microdarcy tight sand is controlled by narrow, slot-like cracks between abutting quartz overgrowths on sand grains. The characteristic width of these cracks under reservoir net stress is approximately 0.1 micron. Because of the small size and linear configuration of these ''slot pores,''gas permeability in western tight sands is extremely sensitive to water content, with gas permeability effectively dropping to zero at water saturation of 40% to 60%. This is generally less than the saturation levels required for water to become a mobile phase in the rock, so many tight sands cease production due to water problems without any water ever flowing into the wellbore. The results of the eastern gas shale study indicate that the permeability of Devonian shale is strongly stress-dependent. Gas permeability in the shale matrix is adversely affected by the presence of mobile liquid hydrocarbon in the shale pores. Because of the small pore size in shale, mobile liquids block flowpaths by capillary action and strongly reduce gas permeability. 17 refs., 36 figs., 13 tabs.