Subsurface Laboratory Methods 277 



characteristic? usually predominate, and the available reanalyses are so 

 scattered that generalization is difficult. 



The Two Medicine and Eagle sandstones at Cut Bank yield waters of 

 dilute to moderate concentration — from 800 to 8,000 parts per million 

 total solids — consisting principally of sodium sulphate and sodium bicar- 

 bonate. Calcium and magnesium are absent or are present in small quanti- 

 ties only, and chloride seldom exceeds 100 parts per million. The Eagle 

 waters are more dilute than the Two Medicine waters and average about 

 1,200 parts per million total solids. Both waters show ground-water 

 characteristics and are usually shut off with a single string of casing. 



In contrast, the Judith River waters encountered in the gas-producing 

 Cedar Creek anticline are saline with concentrations ranging from 8,000 

 to 15,000 parts per million total solids; chloride varies from 4,000 to 

 10,000 parts per million; sulphate is absent or is present only in neg- 

 ligible quantities; bicarbonate is low, averaging about 200 parts per mil- 

 lion; and calcium and magnesium are present in relatively small but defi- 

 nite quantities. These waters are associated with natural gas, and their 

 higher-than-average concentrations are ascribed to the evaporative effect 

 of the gas. It has been found that as a general rule throughout the Rocky 

 Mountain region waters associated with gas are more concentrated than 

 waters associated with oil. 



The extreme difference in composition between the surface-water 

 type at Cut Bank and the chloride-saline type in gas-producing fields is 

 shown in the ionic statements in table 9. 



Colorado Group 



The Colorado group in Montana includes, besides dark fissile shale, 

 sandy and sandstone members that yield oil or gas at several localities. 

 In Wyoming the Colorado group includes the Shannon sandstone, the 

 important oil-producing Frontier formation, and the Muddy (Newcastle) 

 sandstone. 



The Blackleaf sandy member of the Colorado shale yields gas with 

 showings of oil in a number of Montana fields. The waters encountered 

 in this member at Border-Red Coulee, Cut Bank, Kevin-Sunburst, and 

 Bowdoin are essentially solutions of sodium chloride ranging from 6,000 

 to 16,000 parts per million total solids. Sulphate is negligible in these 

 waters, and secondary characteristics are low but persistent, ranging from 

 50 to 200 parts per million calcium and a trace to 100 parts per million 

 magnesium. Primary salinity averages ninety percent, primary alkalinity 

 six percent, and secondary alkalinity four percent of the chemical system. 

 It is interesting to note that these waters resemble Judith River water as 

 found in the Cedar Creek anticline; thus the concentrations and salinity 

 can be ascribed to their association with gas. 



First Cat Creek sandstone waters at Cat Creek are moderately dilute 

 and balanced, the alkalinity and salinity each averaging about fifty per- 



