Page 220 - Geochemistry of Oil Field Waters
P. 220

SEDIMENTARY ROCKS                                                     207


            minute  for  10,000 years.  Compaction water is further considered in a sub-
            sequent chapter concerned with the accumulation of  petroleum.
              The  resistance  to  flow  of  bound  interstitial  water  is  greater  than  the
            resistance to deformation of  the sediment framework during the first stages
            of  compaction.  As  pressure  is  increased  and  sediment  compacts,  the sedi-
            ment  framework increases resistance to deformation, which also governs the
            deformation  rate of the bound-water films and the outflow of  the interstitial
            water.  Rosenqvist  (1962) found that bound water has a higher viscosity than
            unbound  insterstitial  water.  Permeability  decreases  with  compaction,  and
            this together  with the increased water viscosity leads to additional resistance
            to expulsion of bound interstitial water during subsequent compaction.
              Porosity  decreases  during  compaction,  and  at  infinite  depth  it  would
            become infinitely small. Porosity and permeability are two important factors
            in  determining the amount of  petroleum and/or water that can be recovered
            from a given reservoir or aquifer (Pollard and Reichertz, 1952; Caraway and
            Gates,  1959). The  porosity  of  an  aquifer  indicates  how  much  fluid  the
            aquifer can hold, and the permeability indicates how fluid can move through
            the aquifer. If  the porosity  is high  but the permeability is low, the reservoir
            may  contain  large  amounts  of  oil,  gas,  or  water,  but  they  cannot  be
            recovered unless special techniques are used to increase the permeability.

            Sediment diagenesis

              Mineralogical  and  chemical  changes occur in the sediments as they com-
            pact.  The  mineralogic  composition  of  recent marine  sediments and ancient
            marine sedimentary rocks are different, as is the composition of the intersti-
            tial  water  in  the  recent  sediments  compared  with  the  waters  in  ancient
            stratigraphic  units.  Chemical  reactions  occur  between  the  sediments  and
            their  interstitial  water  (Chave,  1960). It  has  been  shown  that  chemical
            changes  can  be  measured  in  recent  sediments,  e.g.,  below  the  sediment-
            water interface, changes in pH and Eh result from degradation of  sulfate ions
            by bacteria (Emery and Rittenberg, 1952).
              Numerous chemical inhomogeneities  occur within a single core sample of
            recent  sediments  (Degens  and  Chilingar,  1967). The  magnesium  concen-
            tration in the interstitial w&er decreases slightly with depth, while the con-
            centrations  of  calcium  and  potassium  increase  (Siever  et al.,  1965). The
            interstitial waters from continental shelf sediments have higher chloride con-
            centrations and higher ratios of  Ca/C1, K/Cl, and Rb/C1 than the overlying sea
            water; the ratios of  Li/Cl and Mg/Cl are about the same as in the overlying
            sea  water  except that  the Li/C1 ratios  are higher in the innershelf  samples
            than in the outershelf  samples. The Sr/Cl ratio is higher in the overlying sea
            water,  while  the  pH  and  Eh  values are lower in the sediments than in the
            overlying sea water (Friedman et al., 1968).
              A  detailed study indicates that virtually  no environment exists on or near
            the  earth’s  surface  where  the  pH/Eh  conditions  are  incompatible  with
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