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A large amount of water is released from coal and other non-coal carbonaceous lithologies during the coalification process. Calculations of the amount of water released from coal and carbonaceous lithologies in low-permeability Upper Cretaceous rocks in the Green River basin of Wyoming reveal that this source of water is as important as any other sediment-derived water. Based on water resistivity (Rw) calculations and coal compositional changes during coalification, we suggest that this organically derived water is fresh relative to most formation waters. The addition of this water to the pore fluids is of sufficient quantity to create a chemical disequilibrium between the pore fluid and rock constituents, thereby producing a potential for precipitation or diss lution of cements. The addition of fresh water in conjunction with the variable stratigraphic distribution of coal beds and a restricted hydrologic communication between lithologic units implies that variable water resistivities (Rw) could occur that might adversely affect geophysical well-log responses and water-saturation (Sw) calculations. In coal-bearing rocks, the addition of organically derived water into the pore system may, in part, contribute to the development of abnormally high pressures.
The effectiveness of this dewatering process depends on the amount and stratigraphic distribution of coal, stage of coalification, and degree of hydrologic isolation or impermeability of the coal-bearing sequence.
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