292 Subsurface Geologic Methods 



from 3,000 to 4,000 parts per million total solids. Secondary salinity often 

 dominates the chemical system, and even when it does not dominate it is 

 appreciable. Like Embar waters, sulphate is the principal negative radicle, 

 and bicarbonate is usually low. 



The dilute Tensleep waters in table 14, Dallas, Derby, and Lander, 

 are artesian even though associated with oil. The Popo Agie River is ap- 

 parently the source of these waters, and it is surmised that the Tensleep 

 sandstone in these fields is subjected to an active, vigorous natural water 

 drive; it is estimated that about four barrels of water are produced for 

 each barrel of oil. Dilute water also is associated with oil at Black Moun- 

 tain, Wyoming. 



MiSSISSIPPIAN 



The important Mississippian oil-producing zone is the Madison lime- 

 stone and its Black Hills equivalent, the Pahasapa. It was the oldest sedi- 

 mentary formation in the Rocky Mountain region to produce oil until the 

 recent discovery of oil in a Cambrian sandstone at Lost Soldier, Wyoming. 

 The Madison limestone yields oil in Montana and in the Lost Soldier- 

 Wertz area and the Big Horn Basin of Wyoming and has yielded showings 

 of oil in other parts of the region. (See table 15.) 



Madison waters of Montana are somewhat variable. Chloride salinity 

 dominates in the fields around Sweetgrass arch and Sweetgrass Hills, and 

 sulphate salinity is predominant in the central and north-central fields. 

 The average concentration is about 5,000 parts per million total solids, and 

 calcium and magnesium are present to some extent in all Madison waters, 

 although less pronounced in the chloride-saline type. 



Madison waters from Wyoming fields, with the exception of one or 

 two extremely dilute waters, seem to be more uniform than equivalent 

 Montana waters. As a rule there is more secondary salinity in Madison 

 waters than in Embar or Tensleep waters, and Madison waters usually are 

 more dilute. The average concentration of Madison waters is about 2,500 

 parts per million; sulphate usually dominates the negative ions, although 

 there are a few waters, such as Torchlight, in which chloride dominates. 

 Bicarbonate usually is low, averaging less than 500 parts per million, and 

 in this respect the Madison waters resemble the average Tensleep water. 



Although there is a marked similarity between pre-Triassic waters, it 

 is not difiicult usually to distinguish among Embar, Tensleep, and Madi- 

 son waters in the same field or area. There are sufficient differences in con- 

 centration, alkalinity, or sulphate-chloride ratio to correlate each water 

 with its lithologic unit and make identification relatively easy. 



Devonian and Older 



Showings of oil have been found in pre-Cambrian crystallines, Cam- 

 brian strata, Ordovician strata, and Devonian strata at widely separated 

 localities in the Rocky Mountain region. Some geologists believe that the 



