Secondary porosity in sandstones is created by subsurface dissolution of grains or cement by pore water that is undersaturated with respect to one or more of the major mineral phases. Such undersaturated pore water may be derived from: (1) meteoric water driven by a hydrostatic head; (2) compactional pore water containing CO₂ released from maturing kerogen; (3) clay minerals reactions including the transformation of kaolinite and smectite to illite; and (4) reactions between clay minerals and carbonate releasing CO₂. Calculations of the CO₂ generated from different types of kerogen suggest that few basins will generate enough CO₂ to produce large-scale leaching in thicker sandstones. Dissolution of minerals and removal of alumin m and silica in solution requires that very large volumes of pore water flow through the sandstone. Because leaching often enlarges primary pore space, it is very difficult to estimate the percentage of the pore space that is secondary. Leaching and formation of secondary pore space may also be accompanied by reprecipitation of other minerals so that the net gain in porosity is less than the observed secondary pore space.