﻿PETROLEUM 
  GEOLOGY 
  — 
  HEROY 
  183 
  

  

  immense 
  accumulation 
  of 
  petroleum 
  as 
  is 
  present 
  in 
  the 
  East 
  Texas 
  

   field 
  has 
  been 
  derived 
  from 
  a 
  highly 
  restricted 
  and 
  local 
  source 
  ; 
  it 
  is 
  

   more 
  reasonable 
  to 
  consider 
  that 
  the 
  Woodbine 
  has 
  been 
  an 
  effective 
  

   carrier 
  bed 
  across 
  large 
  areas. 
  

  

  Between 
  such 
  extremes 
  the 
  geologist 
  meets 
  an 
  almost 
  complete 
  grada- 
  

   tion. 
  The 
  writer 
  considers 
  that 
  each 
  field 
  presents 
  a 
  special 
  problem 
  

   in 
  migration 
  and 
  that 
  a 
  simple 
  generalization 
  which 
  will 
  explain 
  all 
  

   cases 
  is 
  not 
  to 
  be 
  expected 
  or 
  even 
  sought. 
  

  

  ACCUMULATION 
  OF 
  PETROLEUM 
  

   METHOD 
  OF 
  ACCUMULATION 
  

  

  The 
  fundamental 
  force 
  which 
  determines 
  in 
  what 
  part 
  of 
  a 
  carrier 
  

   bed 
  accumulation 
  of 
  oil 
  and 
  gas 
  will 
  take 
  place 
  is 
  gravity. 
  Having 
  

   ordinarily 
  a 
  lower 
  specific 
  gravity 
  than 
  water, 
  they 
  have, 
  when 
  

   present 
  in 
  the 
  same 
  reservoir, 
  greater 
  buoyancy, 
  and 
  gravitational 
  

   separation 
  results. 
  The 
  mechanism 
  of 
  accumulation 
  has 
  been 
  well 
  

   stated 
  by 
  Kich 
  (1923) 
  : 
  

  

  Accumulation 
  results 
  from 
  the 
  selective 
  segregation 
  of 
  oil 
  and 
  gas, 
  which, 
  on 
  

   account 
  of 
  their 
  buoyancy, 
  always 
  tend 
  to 
  work 
  their 
  way 
  upward 
  toward 
  the 
  

   roof 
  of 
  the 
  reservoir 
  as 
  they 
  are 
  carried 
  along 
  by 
  the 
  water, 
  and 
  so 
  are 
  caught 
  in 
  

   anticlinal 
  or 
  similar 
  structural 
  [and 
  stratigraphic] 
  traps, 
  or 
  in 
  places 
  where 
  

   differences 
  of 
  porosity 
  cause 
  a 
  "screening" 
  section 
  which 
  permits 
  the 
  passage 
  

   of 
  water, 
  but 
  holds 
  back 
  oil 
  and 
  gas. 
  

  

  PLACE 
  OF 
  ACCUMULATION 
  

  

  General 
  statement. 
  — 
  A 
  trap 
  is 
  a 
  geologic 
  feature 
  causing 
  the 
  ac- 
  

   cumulation 
  of 
  fluids 
  in 
  porous 
  rocks; 
  in 
  petroleum 
  geology 
  the 
  use 
  

   of 
  the 
  term 
  is 
  restricted 
  to 
  an 
  accumulation 
  of 
  oil 
  and 
  gas 
  — 
  the 
  

   "structure" 
  of 
  the 
  practical 
  oil 
  man. 
  Various 
  classifications 
  which 
  

   make 
  it 
  possible 
  to 
  visualize 
  more 
  readily 
  the 
  wide 
  range 
  of 
  circum- 
  

   stances 
  under 
  which 
  such 
  accumulation 
  has 
  occurred 
  have 
  been 
  

   proposed 
  by 
  geologists 
  (Clapp, 
  1929; 
  Wilson, 
  1934). 
  The 
  porous 
  

   zone 
  in 
  which 
  the 
  oil 
  and 
  associated 
  fluids 
  have 
  accumulated 
  within 
  

   the 
  trap 
  is 
  called 
  the 
  reservoir. 
  Accumulation 
  occurs, 
  in 
  principle, 
  

   because 
  the 
  upward 
  and 
  lateral 
  movement 
  of 
  the 
  oil 
  and 
  gas 
  is 
  

   arrested 
  by 
  the 
  presence 
  of 
  a 
  barrier 
  — 
  in 
  other 
  words, 
  a 
  trap 
  or 
  

   closure. 
  

  

  Traps 
  are 
  formed 
  by 
  conditions 
  which 
  were 
  established 
  at 
  the 
  time 
  

   the 
  sediments 
  were 
  deposited, 
  by 
  the 
  diagenesis 
  and 
  lithification 
  to 
  

   which 
  the 
  sediments 
  have 
  later 
  been 
  subjected, 
  and 
  by 
  deformation. 
  

   Most 
  of 
  those 
  in 
  the 
  first 
  two 
  groups 
  (depositional 
  and 
  diagenetic), 
  

   described 
  in 
  the 
  following 
  paragraphs, 
  may 
  also 
  be 
  classified 
  as 
  

   stratigraphic 
  traps, 
  while 
  those 
  in 
  the 
  third 
  (deformational) 
  are 
  

   structural 
  traps 
  (Levorsen, 
  1936, 
  p. 
  524). 
  While 
  some 
  traps 
  are 
  of 
  

  

  