Tabular and graphic analyses of waters from the oil- and gas-producing zones of Wyoming are presented and discussed.

Post-Chugwater waters are essentially solutions or sodium salts, whereas in pre-Chugwater waters calcium and magnesium sulphates ordinarily predominate.

Frontier waters are solutions of sodium chloride and sodium bicarbonate in varying proportions. Waters from the "Dakota group" are solutions of sodium salts, but contain appreciable quantities of sulphate. Most Sundance waters are solutions of sodium salts similar, except for concentration, to Frontier and "Dakota group" waters. Chugwater water is exceptionally concentrated, ranging from 30,000 to 50,000 parts per million. Embar waters contain hydrogen sulphide and are the youngest subsurface waters in which secondary salinity is prominent. In most of the Tensleep waters secondary characteristics dominate the chemical system with secondary salinity ordinarily occupying 40-70 per cent. Madison waters commonly have a somewhat higher secondary salinity than Tensleep waters in the same ar a, but otherwise resemble them.

The analyses indicate that all the oil-field waters of Wyoming have been considerably modified by the infiltration of surface water.