272 Subsurface Geologic Methods 



WATER ANALYSIS 



(CHARACTERISTICS OF OIL-FIELD WATERS OF THE 



ROCKY MOUNTAIN REGION) 



JAMES G. CRAWFORD 



The identification and correlation of waters found in drilling and 

 producing oil and gas wells with definite lithologic units have been ap- 

 plications of water analysis of great direct value to the oil-production 

 industry. Correlations are made upon the premises that the composition 

 of the water from a given zone is constant, or nearly constant, throughout 

 the economic life of an oil or gas field and that the water contained in 

 each producing zone has diagnostic characteristics by which it can be 

 distinguished from every other water above or below that zone in the im- 

 mediate vicinity. 



Structural or stratigraphic traps capable of acting as reservoirs for 

 hydrocarbons also act as reservoirs for water. Thus, waters associated 

 with oil or gas are relatively stagnant and usually present an entirely dif- 

 ferent set of properties from surface waters or circulating ground waters 

 in the immediate vicinity. Although waters are identified and correlated 

 by comparison with known samples in the immediate area, it has been 

 found that regional correlations, although somewhat inexact, are possible. 



The variation in composition and concentration of formation waters 

 sampled over a structure should be recognized. Water tables in the Rocky 

 Mountain region are as a rule tilted, and the water yielded on the high 

 side of the structure, although of the same general characteristics, often 

 differs from that of the low side. These variations may be appreciable or 

 may not be particularly noticeable, depending upon the field and area, 

 but analyses must be interpreted with these possible variations in mind. 



It is the purpose of this section to discuss briefly the regional similar- 

 ities and differences of oil-field waters in the Rocky Mountain area, with 

 particular emphasis on correlation with definite lithologic units. 



The writer is indebted to the United States Geological Survey for 

 many of the analyses from which correlations could be made; to the oil 

 companies of the Rocky Mountain region for the many analyses furnished ; 

 to H. E. Summerford for his valuable assistance in the preparation of 

 the geologic information; to J. A. Waatti for preparing the illustrations; 

 and to R. M. Larsen for general and specific criticism. 



Classification of Waters 



The Palmer ^^ system of water classification emphasizes important dif- 

 ferences between waters in geochemical relationship and has been used 

 throughout this paper in the discussion of types of water. The Palmer 

 system groups those radicles that are either chemically similar or geo- 

 logically associated: Sodium and potassium are grouped as alkalies; cal- 



^ Palmer, Chase, The Geochemical Inter pr elation of Water Analyses: U. S. Geol. Survey Bull. 

 479, 1911. 



