﻿PETROLEUM 
  GEOLOGY 
  — 
  HEROY 
  181 
  

  

  ability 
  of 
  rocks 
  are 
  congenital; 
  others 
  are 
  the 
  result 
  of 
  subsequent 
  

   geological 
  processes. 
  

  

  Congenital 
  factors 
  are 
  lithology 
  and 
  areal 
  extent. 
  Sands 
  are 
  ini- 
  

   tially 
  more 
  permeable 
  than 
  marls 
  and 
  clays; 
  coarse 
  sands 
  more 
  per- 
  

   meable 
  than 
  finer 
  ones. 
  Certain 
  fragmental 
  or 
  oolitic 
  limestones, 
  

   however, 
  originally 
  had 
  permeability 
  comparable 
  to 
  that 
  of 
  sands. 
  

   Porous 
  rocks 
  that 
  were 
  deposited 
  over 
  limited 
  areas 
  in 
  the 
  form 
  of 
  

   lenses 
  or 
  that 
  gradually 
  became 
  less 
  porous 
  laterally 
  as 
  a 
  result 
  of 
  

   change 
  in 
  composition 
  are 
  obviously 
  less 
  effective 
  as 
  carriers 
  than 
  

   those 
  which 
  were 
  laid 
  down 
  over 
  wide 
  areas. 
  Even 
  "blanket" 
  sand- 
  

   stones 
  such 
  as 
  the 
  St. 
  Peter, 
  Berea, 
  Dakota, 
  and 
  Woodbine, 
  which 
  are 
  

   regionally 
  continuous, 
  are, 
  however, 
  by 
  no 
  means 
  uniform 
  in 
  per- 
  

   meability 
  and 
  frequently 
  contain 
  "tight" 
  areas. 
  Limestones 
  show 
  an 
  

   even 
  greater 
  variation, 
  such 
  as 
  that 
  which 
  accompanies 
  a 
  change 
  from 
  

   reef 
  to 
  offshore 
  f 
  acies. 
  

  

  Factors 
  which 
  have 
  reduced 
  the 
  permeability 
  of 
  rocks 
  subsequent 
  

   to 
  deposition 
  include 
  compaction, 
  lithification, 
  cementation, 
  and 
  re- 
  

   crystallization. 
  The 
  effect 
  of 
  compaction 
  in 
  reducing 
  the 
  size 
  of 
  pore 
  

   spaces 
  has 
  already 
  been 
  mentioned. 
  Changes 
  in 
  density, 
  such 
  as 
  from 
  

   sands 
  to 
  sandstones 
  and 
  from 
  clays 
  to 
  shales, 
  however 
  they 
  may 
  have 
  

   been 
  accomplished, 
  usually 
  reduce 
  the 
  effectiveness 
  of 
  rocks 
  to 
  conduct 
  

   fluids 
  by 
  forcing 
  the 
  individual 
  grains 
  into 
  closer 
  contact. 
  The 
  factors 
  

   just 
  mentioned 
  are 
  usually 
  most 
  effective 
  soon 
  after 
  the 
  sediments 
  are 
  

   deposited. 
  Cementation, 
  which 
  may 
  occur 
  either 
  during 
  the 
  early 
  

   or 
  the 
  later 
  history 
  of 
  the 
  sediments, 
  may 
  change 
  a 
  rock 
  which 
  was 
  

   originally 
  a 
  competent 
  carrier 
  to 
  one 
  which 
  may 
  be 
  highly 
  impervious, 
  

   as 
  illustrated 
  by 
  the 
  change 
  from 
  sandstone 
  to 
  quartzite 
  (Twenhofel, 
  

   1932, 
  p. 
  229). 
  Eecrystallization 
  may 
  in 
  some 
  cases 
  increase 
  porosity 
  

   but 
  it 
  is 
  quite 
  as 
  likely 
  to 
  decrease 
  permeability 
  by 
  closing 
  capillary 
  

   passages. 
  Dynamic 
  metamorphism 
  has 
  an 
  influence 
  of 
  the 
  second 
  

   order 
  by 
  bringing 
  into 
  play 
  some 
  of 
  the 
  factors 
  which 
  have 
  been 
  

   specified. 
  

  

  Permeability 
  has, 
  on 
  the 
  other 
  hand, 
  been 
  increased 
  by 
  other 
  factors 
  

   such 
  as 
  solution 
  and 
  fracturing. 
  Solution 
  has 
  functioned 
  chiefly 
  in 
  

   calcareous 
  and 
  dolomitic 
  rocks. 
  It 
  has 
  resulted 
  from 
  subaerial 
  ex- 
  

   posure 
  and 
  weathering 
  and 
  in 
  many 
  limestone 
  fields 
  has 
  unquestion- 
  

   ably 
  increased 
  the 
  porosity 
  of 
  the 
  reservoir 
  (Adams, 
  1934). 
  Such 
  

   paleogeographic 
  conditions 
  would 
  presumably 
  be 
  local 
  rather 
  than 
  

   regional 
  and 
  may 
  therefore 
  have 
  influenced 
  migration 
  of 
  oil 
  over 
  

   limited 
  areas. 
  Solution 
  has 
  in 
  many 
  cases 
  acted 
  to 
  increase 
  the 
  ini- 
  

   tial 
  porosity, 
  and 
  it 
  then 
  becomes 
  difficult 
  to 
  determine 
  how 
  much 
  

   of 
  the 
  permeability 
  is 
  primary 
  and 
  how 
  much 
  secondary. 
  The 
  impor- 
  

   tance 
  of 
  fracturing 
  in 
  increasing 
  permeability 
  also 
  varies 
  greatly 
  

   among 
  different 
  areas. 
  Such 
  highly 
  permeable 
  limestones 
  as 
  the 
  

  

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