Page 248 - Geochemistry of Oil Field Waters
P. 248

23 2                                        ORIGIN OF OILFIELD WATERS


            diagenesis.  It  has  been  shown  that  a  tendency  exists  for  potassium  to be
           adsorbed and fixed by clay minerals, mica, and potassium feldspar in normal
           low-temperature processes  (White,  1965; Khitarov  and  Pugin, 1966; Grim,
            1952).
              The data in Tables 7..III-XIV  indicate that the concentration of  calcium
           in oilfield waters generally is enriched relative to sea water. Cation exchange
           reactions with clays accounts for some of this enrichment:

              2Na+ (solution) + Ca (clay) + Ca+* (solution) + 2Na (clay)

              Collins (1972) found that the ratio Na/(CL + Mg) tends to decrease as the
           dissolved solids concentration increases in some oilfield waters from the East
           Texas  Basin.  This depletion of  sodium with respect to calcium plus magne-
           sium  was  attributed  to diagenesis of  the waters  and  it  correlated  with  an
           index of  base exchange (Schoeller, 1955), indicating that the alkali metals in
           the waters exchanged with alkaline earth metals on the argillaceous minerals
           to decrease  the dissolved alkali  metals  and  increase  the  dissolved alkaline
           earth metals.
              Fig.  7.8  is a  plot  of  the calcium  concentration  in  some  oilfield  waters
           taken  from  the  Smackover  formation.  All  of  these  waters  are  enriched in
           calcium relative to the evaporated sea water.
              Krejci-Graf  (1963) found  that  solutions  predominantly  concentrated  in
           chloride  can force an exchange  of  calcium and bromide from clay minerals
           for sodium and chloride from the solution.  If this type of reaction occurred
























                                                                3

           Fig. 7.8. Relationships  of  the  concentrations  of  chloride  to  calcium  in  an  evaporite-
           formed brine to oilfield brines taken from the  Smackover formation in five states of the
           United states.
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