294 Subsurface Geologic Methods 



Bighorn dolomite of Upper Ordovician age yields some of the deepest oil 

 produced at Garland, Wyoming. But for many years, until 1948, the Madi- 

 son limestone of Mississippian age was the oldest proved commercial oil 

 zone in the Rocky Mountain region. In 1948 a Cambrian sand was proved 

 for commercial oil production at Lost Soldier and Wertz, Wyoming, and 

 the search for oil in pre-Mississippian beds was intensified. 



Several wells, particularly in Montana, have penetrated Ordovician 

 and older beds, and a few analyses are available of these older waters, 

 although the number is insufi&cient to warrant generalizing. The Ordovician 

 waters analyzed appear to be of the same general character as Madison 

 waters in the same well, whereas Devonian waters so far analyzed appear 

 to be a more concentrated, more saline type, marked in particular by high 

 calcium content. One typical Devonian water has a concentration of total 

 solids of 19,000 parts per million and a calcium content of 1,096 parts per 

 million; another Devonian water has a concentration of total solids of 

 11,000 parts per million and a calcium content of 1,027 parts per million. 

 The chloride-sulphate ratio was 4:1 in the first water and 2:1 in the 

 second. 



It is believed, however, that these older waters as a whole will not 

 be substantially different from other pre-Triassic waters, and that lime- 

 stone characteristics will be the rule and not the exception. 



Conclusion 



The value of analyses as a means of identification of intrusive waters 

 in well bores has been well established in the Rocky Mountain region. 

 This is the primary purpose of water analyses, and the application of such 

 data to engineering and geologic problems and theory is secondary. It is 

 established definitely that there are sufficient differences in concentration 

 and composition to correlate a water with its reservoir zone so that it can 

 be differentiated from all other waters above or below that zone in a par- 

 ticular well or area. 



The generally dilute nature of the oil-field waters of the Rocky Moun- 

 ain region indicates extensive modification and dilution by meteoric 

 ivaters. Some of the waters encountered seem to indicate little change 

 since deposition, so that it is concluded that for these waters modification 

 and dilution occurred before deposition; other waters seem to indicate 

 extensive modification since deposition. In any respect, the brines com- 

 monly associated with oil in other provinces of the world do not occur in 

 the Rocky Mountain region. 



It is apparent that the oil-field waters in the Rocky Mountain region 

 have been influenced by the petrography of the rocks in which they occur. 

 Secondary characteristics and sulphate are at a minimum in waters of 

 sandstone reservoirs of Cretaceous age or younger, whereas secondary 

 characteristics and sulphate are prominent in the limestones and limy 

 formations of pre-Triassic age. 



