Subsurface Laboratory Methods 279 



originally in the sand, as typified by Cole Creek and Big Muddy waters. 

 Representative analyses of all these waters are given in table 9. 



Frontier Formation, 



The Frontier formation has been one of the most productive oil hori- 

 zons in the Rocky Mountain region. It is overshadowed now by pre- 

 Triassic production but nevertheless still is an actual and potential oil 

 producer of large capacity. Where it produces oil the formation ranges 

 in thickness from 370 to 1,200 feet and contains from two to nine beds of 

 sandstone. Where the several Wall Creek sands can be separated, the 

 designation of "First Wall Creek," "Second Wall Creek," et cetera, are 

 given them; where the separate sands cannot be identified, water samples 

 are designated simply as "Frontier." Frontier sands where identifiable 

 in the Big Horn Basin are termed "Torchlight" and "Peay." 



Frontier waters are for the most part solutions of sodium chloride 

 and sodium bicarbonate in varying proportions. Calcium and magnesium 

 occur in small amounts in some waters and are absent in others; in no 

 case is there sufficient calcium or magnesium to give secondary salinity 

 to the water. Sulphate is absent or is present in minor quantities only. 

 Concentrations are quite variable, ranging from about 1,200 parts per 

 million to 50,000 parts per million total solids. 



Representative analyses of a number of the more important Frontier 

 waters of Wyoming are tabulated in table 10. The variation in concentra- 

 tion of these waters is due, it is believed, to several causes, among them 

 and most important being the lenticularity of the sands and their perme- 

 ability development. It is believed that the more dilute and alkaline 

 waters have been modified considerably by meteoric waters, whereas the 

 more concentrated, saline waters are assumed to be connate without any 

 substantial modification by meteoric waters since accumulation. 



It is interesting to note from table 10 that the highest concentrated 

 waters are associated with gas in the Baxter Basin fields. These, together 

 with the Montana-group waters cited above, tend to support the contention 

 of Mills and Wells ^^ that water can become concentrated at depth by the 

 agency of moving and expanding gas. 



Muddy (Newcastle) Sand 



Oil production from the Muddy sand had been small and scattered 

 until the devlopment of the Mush Creek and Skull Creek areas along the 

 eastern edge of the Powder River Basin. Here a twelve- to twenty-foot 

 section of medium- to fine-grained, slightly tripolitic sand with coal inter- 

 calations interbedded with shales, locally called the "Newcastle sand- 

 stone," yields commercial oil production. 



The Newcastle water at Mush Creek is a primary-saline water rang- 

 ing from 10,000 to 15,000 parts per million total solids; secondary char- 



^^ Mills, R. Van A., and Wells, R. C, The Evaporation and Concentration of Waters Associated with 

 Petroleum and Natural Gas: U. S. Geol. Survey Bull. 693, 1919. 



