A large amount of water is released from coal during the coalification process. Quantitative calculations, derived from a coal dewatering model, reveal that this source of water may be as important as any other sediment-derived water. Based on water resistivity (R_w) calculations and coal compositional changes during coalification, we suggest that this coal-derived water is fresh relative to most formation fluids. The addition of fresh water, in conjunction with the variable stratigraphic distribution of coal beds, and restricted hydrologic communication between lithologic units, implies that variable water resistivities (R_w) could occur and may adversely affect geophysical well log responses and consequent water saturation (S_w) calculation . In coal-bearing rocks, the addition of coal-derived water into the pore system may be a contributing factor to development of abnormally high formation pressures. The adjustment of pore fluids to the addition of this water may produce a potential for dissolution or precipitation of cements in adjacent sandstone reservoirs. The extent to which coal dewatering is effective in relation to these geologic modifications depends on the amount and stratigraphic distribution of coal, stage of coalification, and degree of hydrologic isolation, or impermeability, of the coal-bearing sequence.