Page 252 - Origin and Prediction of Abnormal Formation Pressures
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224                                  H.H. RIEKE, G.V. CHILINGAR AND J.O. ROBERTSON JR.

            after  sedimentation.  Kryukov  and  Zhuchkova  (1963)  and  Sayles  and  Manheim  (1975)
            reported  on  the  chemistry  of  fluids  during  burial.  Ancillary  experimental  compaction
            studies  of peat  and  the  mobilization  of  major  inorganic  ions  were  reported  by  Bailey
            et  al.  (2000)  on  three  different  naturally  occurring  peats,  i.e.,  Cladium,  Rhizophora,
            and  Cyrilla.  Berry  (1959,  1973),  Stephenson  et  al.  (1994),  Kouznetsov  et  al.  (1994),
            Chilingar  et  al.  (1996),  and  Khilyuk  et  al.  (2000)  relate  pore-fluid  chemistry  and  the
            movement of fluids to tectonic  activity.
               Cannon  and Craze (1938)  stated that uplift and erosion are not the probable  causes of
            abnormally high pressures in the U.S. Gulf Coast Basin.  Rowaik (1975),  Magara (1978),
            and Luo  and  Vasseur (1995),  however,  pointed  out that uplift  and erosion  are important
            factors  in  the  evolution  of  pore  pressures  in  such  environments.  Subnormal  pressures
            can be created by uplift and erosion  through  the reduction  of temperature,  which causes
            shrinkage  of the  fluids  and  porosity  rebound.  This  can  create  a  differential  pressure  in
            the overall hydrodynamics  of the uplifted  sediments.  Dobrynin  and Serebryakov  (1989)
            attributed  the  occurrence  of abnormally  low  pressures  located  in  the  Nepsko-Botuobin
            anticline  in  eastern  Siberia  to  changes  in  the  surface  temperatures  of  the  Earth  during
            geologic times.
               There  are three  commonly  accepted practical  classifications  of pore  waters based  on
            their chemistry and the geochemical  interpretation  of the results:
              9 Palmer (1911) m  American
              9 Sulin (1946) m  Russian
              9 Schoeller (1955) m  French
               Collins  (1975)  gave  an  excellent  review  of  the  water  analysis  procedures  and
            resulting  interpretations  from  the  above  classification  schemes  for  the  Gulf  Coast  and
            mid-continent  reservoir pore waters.
               The present-day  understanding  on how these pore  waters originate,  what affects their
            chemical compositions,  and how fluids migrate are presented and debated in this chapter.
            Arguments  and perspectives,  both pro and con,  are presented  to facilitate examining the
            role of pore-water chemistry in excessive-pressured  zones.



            OVERVIEW  AND  CONSTRAINTS
               The  chemical  composition  of  pore  waters  in  abnormally  high  pressure  zones  in
            sedimentary  basins  often  differs  from  the  composition  of the  pore  waters  in  associated
            normally  pressured  formations.  This  is  especially  true  for  subsiding  immature  Tertiary
            and  young  Quaternary  sedimentary  basins  with  high  sedimentation  rates  and  thick
            shale  components.  Chilingarian  et  al.  (1994)  pointed  out  that  overburden  pressures
            on  sediments  in  these  basins  may  reach  as  high  as  45,000  psi  (about  300  MPa).
            This  results  in  a  strong  driving  force  for  the  migration  of  excess  pore  waters  during
            gravitational  compaction  when  large  amounts  of  interstitial  water  are  squeezed  out  of
            argillaceous  sediments  and  are  expelled  into  the  associated  permeable  beds.  Since  the
            mid-1950s,  there  has  been  a  steady  progress  toward  comprehending  how  subsurface
            pore  fluids  have evolved under gravitational  compaction  conditions,  and  the differences
            in pore-water  chemistry between  normally compacted  and  under-compacted  sediments.
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