278 Subsurface Geologic Methods 



cent of the system. The concentrations vary from about 1,000 to 2,000 

 parts per million total solids, and the alkaline earths and sulphate are 

 absent or negligible. The Cat Creek field is one of the areas in the Rocky 

 Mountain region in which a dilute to moderately dilute water is found 

 associated with commercial oil production. 



Three typical Colorado-group waters are tabulated in table 9. 



Shannon Sandstone 



Although the Shannon sandstone has been tested in many wells, the 

 only fields now producing from it are Cole Creek and Big Muddy, Wyo- 

 ming. At Cole Creek the Shannon sandstone is found at a depth of approx- 

 imately 4,500 feet. A concentrated, saline water averaging about 18,000 

 parts per million total solids is produced with the oil from edge wells. The 

 water averages about six percent secondary alkalinity, and sulphate is 

 present in quantities of 50 to 500 parts per million. This water, although 

 more concentrated and with little higher alkalinity, resembles equivalent 

 water at Big Muddy. 



The top of the Shannon sandstone in the Big Muddy field occurs at 

 a depth of about 900 feet and consists of about thirty feet of alternating 

 lenses of buff to gray sandstone and sandy shale carrying both oil and 

 water. The water is a solution of sodium chloride varying from 9,000 to 

 15,000 parts per million total solids, with about three percent secondary 

 characteristics; bicarbonate is relatively low, ranging from 450 to 700 

 parts per million. 



It is interesting to note the variation of the Shannon waters along 

 the western edge of the Powder River Basin. The Shannon sandstone in 

 the Billy Creek gas field yielded a balanced water, i.e., a water in which 

 alkalinity and salinity occupy about fifty percent each in the chemical 

 system, with no secondary characteristics or alkaline earths; concentra- 

 tions ranged from 2,000 to 3,000 parts per million total solids. The sand- 

 stone in this field is 900 to 1,300 feet below the surface and is fed by 

 fresh, sulphate-free alkaline water from the nearby Big Horn Mountains. 



The influence of secondary-saline surface water can be seen in the 

 Salt Creek area, where the Shannon sandstone forms an escarpment on 

 the east and west sides of the Salt Creek uplift. Here the feed is the 

 gypsum-impregnated waters of Castle Creek, Teapot Creek, and Salt 

 Creek, and the Shannon formation waters encountered during drilling 

 were practically identical to these surface waters. 



Saline waters of relatively high concentration are associated with 

 oil production from the Shannon sandstone at Cole Creek and Big Muddy, 

 and it is concluded that they have not been influenced to any extent by 

 surface-water infiltration. 



Thus, it is possible in this one area, trending northwest-southeast 

 along the western edge of the Powder River Basin, to observe the effects 

 of surface-water infiltration of two different types upon the connate water 



