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

Montana group waters, with the exception of those waters associated with gas, are dilute to moderately dilute solutions of sodium salts, sulphate being present in many waters to a considerable extent. Colorado group waters in the Sweetgrass arch fields are moderately concentrated to concentrated solutions of sodium chloride; elsewhere these waters are less saline and moderately dilute. Kootenai waters resemble Colorado waters to some extent, but are ordinarily more alkaline. Ellis waters are alkaline, moderately concentrated, and contain hydrogen sulphide in appreciable amounts. The "Quadrant" formation yields large flows of water in which the sulphates of calcium and magnesium dominate the chemical system. Tensleep waters of central and southern Montana are similar to "Quadrant" wate s of north-central Montana. Madison waters of primary characteristics are found in the Sweetgrass arch fields, and secondary characteristics are present in the Madison waters of the north-central and central fields.

The analyses indicate that all the oil- and gas-field waters of Montana have been considerably modified by the infiltration of meteoric waters.