﻿174 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1943 
  

  

  buried 
  sediments, 
  and 
  geologists, 
  contrasting 
  conditions 
  in 
  such 
  sedi- 
  

   ments 
  with 
  those 
  occurring 
  at 
  the 
  surface, 
  have 
  naturally 
  sought 
  in 
  

   some 
  way 
  to 
  relate 
  the 
  origin 
  of 
  petroleum 
  to 
  them. 
  

  

  As 
  a 
  section 
  of 
  sedimentary 
  rocks 
  is 
  deposited, 
  the 
  weight 
  of 
  the 
  

   overburden 
  progressively 
  increases, 
  and 
  the 
  deeper 
  beds 
  become 
  

   heavily 
  loaded. 
  The 
  pressure 
  thus 
  created 
  causes 
  compaction 
  of 
  

   the 
  sediments, 
  which 
  become 
  more 
  dense 
  with 
  increase 
  in 
  depth 
  

   (Athy, 
  1930a). 
  Compaction 
  is 
  accompanied 
  by 
  closer 
  spacing 
  of 
  the 
  

   grains 
  and 
  by 
  the 
  gradual 
  displacement 
  from 
  the 
  sediment 
  of 
  most 
  

   of 
  the 
  interstitial 
  water. 
  The 
  friction 
  resulting 
  from 
  movement 
  of 
  

   the 
  grains 
  may 
  produce 
  some 
  heat 
  (Pratt, 
  1934, 
  p. 
  242), 
  and 
  chemical 
  

   reactions, 
  such 
  as 
  the 
  oxidation 
  of 
  pyrite, 
  may 
  be 
  another 
  source, 
  

   but 
  it 
  is 
  probable 
  that 
  the 
  internal 
  heat 
  of 
  the 
  earth 
  is 
  the 
  chief 
  

   cause 
  of 
  increase 
  in 
  temperature 
  with 
  depth. 
  Whatever 
  the 
  cause, 
  

   recent 
  drilling 
  in 
  sedimentary 
  basins 
  to 
  depths 
  approaching 
  3 
  miles 
  

   has 
  revealed 
  the 
  existence 
  of 
  temperatures 
  of 
  nearly 
  300° 
  F. 
  Such 
  

   temperatures 
  are 
  greater 
  than 
  those 
  which 
  chemists 
  have 
  usually 
  

   considered 
  would 
  be 
  available 
  for 
  geochemical 
  reactions 
  leading 
  to 
  

   the 
  formation 
  of 
  petroleum. 
  The 
  opinion 
  (McCoy 
  and 
  Keyte, 
  1934, 
  

   p. 
  269) 
  that 
  most 
  of 
  the 
  known 
  oil 
  fields 
  were 
  formed 
  at 
  temperatures 
  

   lower 
  than 
  140° 
  F. 
  may, 
  therefore, 
  require 
  revision. 
  Even 
  tempera- 
  

   tures 
  of 
  around 
  300° 
  F. 
  are, 
  however, 
  lower 
  than 
  those 
  usually 
  

   considered 
  to 
  be 
  within 
  the 
  "cracking 
  range," 
  and 
  the 
  reactions 
  by 
  

   which 
  petroleum 
  has 
  been 
  formed 
  may 
  still, 
  from 
  the 
  chemist's 
  

   standpoint, 
  be 
  considered 
  low-temperature 
  changes. 
  The 
  reactions 
  

   by 
  which 
  organic 
  matter 
  is 
  converted 
  to 
  petroleum 
  are 
  essentially 
  

   endothermic, 
  and 
  the 
  energy 
  present 
  in 
  the 
  form 
  of 
  heat 
  in 
  the 
  sedi- 
  

   ments 
  where 
  petroleum 
  is 
  being 
  formed 
  accordingly 
  facilitates 
  them. 
  

  

  Pressure 
  may 
  have 
  had 
  an 
  important 
  effect 
  in 
  the 
  formation 
  of 
  

   petroleum 
  by 
  favoring 
  polymerization 
  (Brooks, 
  1938, 
  p. 
  51). 
  Hy- 
  

   drogenation 
  and 
  other 
  types 
  of 
  chemical 
  transformation 
  of 
  hydro- 
  

   carbons 
  are 
  also 
  facilitated 
  by 
  pressure 
  (Pratt, 
  1934, 
  p. 
  241). 
  In 
  

   addition 
  to 
  the 
  weight 
  of 
  the 
  overburden, 
  pressure 
  in 
  sediments 
  may 
  

   be 
  due 
  to 
  hydrostatic 
  head, 
  the 
  presence 
  of 
  petroleum 
  gases 
  in 
  porous 
  

   reservoirs, 
  and 
  possibly 
  other 
  factors 
  such 
  as 
  cementation 
  and 
  

   chemical 
  metamorphism. 
  As 
  water-free 
  sedimentary 
  rocks 
  have 
  an 
  

   average 
  specific 
  gravity 
  above 
  2.5, 
  the 
  weight 
  of 
  the 
  overburden 
  

   might 
  be 
  expected 
  to 
  result 
  in 
  pressures 
  at 
  depths 
  greatly 
  in 
  excess 
  

   of 
  the 
  weight 
  of 
  a 
  column 
  of 
  water 
  of 
  equivalent 
  height. 
  Measure- 
  

   ments 
  indicate, 
  however, 
  that 
  the 
  hydrostatic 
  pressures 
  existing 
  in 
  

   underground 
  reservoirs 
  normally 
  correspond 
  to 
  the 
  weight 
  of 
  such 
  

   a 
  column 
  of 
  water 
  (Versluys, 
  1932) 
  rather 
  than 
  to 
  the 
  weight 
  of 
  

   the 
  overlying 
  sediments. 
  This 
  has 
  been 
  found 
  true 
  in 
  wells 
  drilled 
  to 
  

   depths 
  of 
  over 
  13,000 
  feet 
  in 
  which 
  the 
  reservoir 
  pressures 
  exceed 
  

  

  