Subsurface Laboratory Methods 285 



mation waters depend to some extent at least upon the petrography of the 

 rocks. 



Representative Morrison and Sundance waters are tabulated in table 

 12. 



Triassic 



Small amounts of oil have been produced from the basal part of the 

 Moenkopi formation of Triassic age in Utah, and oil production has 

 come from stray sandstone beds in the Chugwater formation of Permian 

 and Triassic age at Grass Creek and Hamilton Dome, Wyoming, but for 

 the most part Triassic beds have been dry. Water analyses of Triassic 

 age have been few and scattered, but the Chugwater waters of Wyoming 

 that have been sampled are highly concentrated solutions of sodium sul- 

 phate and sodium chloride averaging from 40,000 to 60,000 parts per 

 million total solids; alkalinity is negligible in these waters, the bicarbonate 

 usually averaging about 200 parts per million. 



Triassic beds are the dividing line between the post-Triassic primary 

 waters and the pre-Triassic secondary waters of the Rocky Mountain 

 region. Most of the waters in zones younger than Triassic contain few, 

 if any, secondary characteristics; in contrast, secondary characteristics 

 dominate the waters of formations older than Triassic. 



Permian 



Permian oil production in the Rocky Mountain region is limited prin- 

 cipally to the Embar formation in Wyoming, consisting of porous dolomite, 

 limestone, and chert. Much of the exploration work in the Rocky Moun- 

 tain region since 1940 has been in the Embar and older formations, and 

 these now outrank the post-Triassic beds in productivity. 



Embar waters range from 1,800 parts per million total solids at 

 Dallas Dome to 38,000 parts per million total solids at Neiber Dome, but 

 the average concentration ranges from 4,000 to 7,000 parts per million 

 total solids. With but few exceptions these waters are solutions of sodium, 

 calcium, and magnesium sulphates in varying proportions. Sulphate 

 salinity exceeds chloride salinity, and the bicarbonate ion usually is rela- 

 tively low; most of these waters carry hydrogen sulphide. 



The Embar water at Neiber Dome is very unusual in that there is a 

 large amount of alkalinity present as the bicarbonate radicle. This is 

 decidedly out of line in a post-Triassic water, but there have not been sufl&- 

 cient samples from this structure for postulations concerning its source. 



Typical Embar waters of Wyoming are tabulated in table 13. 



Pennsylvanian 



The marine Pennsylvanian beds include the important oil-producing 

 Tensleep sandstone of the Rocky Mountain region, the Amsden forma- 

 tion, and their equivalent of eastern Wyoming, the Minnelusa formation. 



