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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1943 
  

  

  simple 
  types, 
  most 
  of 
  them 
  are 
  formed 
  by 
  a 
  combination 
  of 
  factors. 
  

   Oil 
  fields 
  may 
  be 
  due 
  to 
  only 
  a 
  single 
  trap, 
  but 
  more 
  frequently 
  

   several 
  traps 
  are 
  present 
  and 
  these 
  may 
  be 
  of 
  more 
  than 
  one 
  type. 
  In 
  

   the 
  following 
  summary 
  the 
  more 
  important 
  types 
  of 
  each 
  group, 
  and 
  

   some 
  of 
  the 
  oil 
  fields 
  which 
  are 
  illustrative 
  of 
  each, 
  are 
  mentioned. 
  

  

  Depositional 
  traps. 
  — 
  Traps 
  which 
  are 
  the 
  result 
  of 
  conditions 
  estab- 
  

   lished 
  when 
  the 
  sediments 
  were 
  deposited 
  are 
  of 
  two 
  principal 
  types 
  : 
  

   those 
  in 
  which 
  the 
  reservoir 
  bed 
  wedges 
  out 
  laterally 
  between 
  less 
  

   permeable 
  strata 
  or 
  in 
  which 
  the 
  character 
  of 
  the 
  reservoir 
  bed 
  

   changes 
  lithologically 
  ; 
  and 
  those 
  in 
  which 
  the 
  porous 
  bed 
  has 
  been 
  

   truncated 
  and 
  the 
  beveled 
  edge 
  has 
  been 
  overlapped 
  by 
  a 
  less 
  

   permeable 
  bed. 
  

  

  In 
  order 
  for 
  the 
  wedge 
  to 
  be 
  an 
  effective 
  trap 
  it 
  must 
  point 
  up-dip, 
  

   and 
  this 
  inclination 
  may 
  be 
  either 
  initial 
  dip 
  or 
  the 
  result 
  of 
  sub- 
  

   sequent 
  tilting. 
  Important 
  oil 
  fields 
  of 
  this 
  type 
  are 
  Burbank 
  (Sands, 
  

   1927) 
  and 
  Glenn 
  (Wilson, 
  1927), 
  Oklahoma, 
  and 
  the 
  East 
  Coalinga 
  

   field, 
  California 
  (Atwill, 
  1940). 
  These 
  fields 
  occur 
  on 
  the 
  margin 
  of 
  

   sands 
  which 
  have 
  large 
  regional 
  extent. 
  A 
  similar 
  type 
  of 
  accumula- 
  

   tion 
  occurs 
  in 
  lenticular 
  and 
  "shoestring" 
  sands 
  where 
  the 
  areal 
  

   extent 
  of 
  the 
  trap 
  may 
  be 
  quite 
  small, 
  as 
  in 
  the 
  fields 
  of 
  eastern 
  

   Kansas 
  and 
  western 
  Pennsylvania 
  (Rich, 
  1938) 
  . 
  

  

  The 
  truncation 
  of 
  a 
  porous 
  bed 
  and 
  the 
  unconformable 
  deposition 
  

   across 
  its 
  edge 
  of 
  a 
  cover 
  rock 
  forms 
  an 
  excellent 
  trap. 
  An 
  out- 
  

   standing 
  example 
  is 
  the 
  East 
  Texas 
  field 
  (Minor 
  and 
  Hanna, 
  1933), 
  

   in 
  which 
  the 
  oil 
  has 
  accumulated 
  in 
  the 
  truncated 
  edge 
  of 
  the 
  Wood- 
  

   bine 
  sand. 
  The 
  sands 
  of 
  the 
  Simpson 
  group 
  in 
  the 
  Oklahoma 
  City 
  

   field 
  are 
  overlapped 
  by 
  Pennsylvania 
  strata 
  to 
  form 
  traps 
  of 
  this 
  

   character 
  (McGee 
  and 
  Clawson, 
  1932). 
  

  

  Diagenetic 
  traps. 
  — 
  In 
  this 
  group 
  are 
  included 
  those 
  traps 
  which 
  

   have 
  resulted 
  from 
  changes 
  in 
  the 
  petrology 
  of 
  the 
  reservoir 
  rocks 
  

   subsequent 
  to 
  their 
  deposition 
  — 
  that 
  is, 
  from 
  diagenesis. 
  Accumu- 
  

   lation 
  in 
  sandstones 
  may 
  be 
  controlled 
  by 
  cementation, 
  as 
  in 
  some 
  

   of 
  the 
  Venango 
  sand 
  fields 
  of 
  Pennsylvania 
  (Torrey, 
  1934, 
  p. 
  472) 
  

   and 
  the 
  Clinton 
  sand 
  fields 
  of 
  Ohio 
  (Billingsley, 
  1934, 
  p. 
  505). 
  

   More 
  frequently, 
  however, 
  traps 
  of 
  this 
  group 
  are 
  formed 
  by 
  the 
  

   solution 
  and 
  recrystallization 
  of 
  limestones 
  (Howard, 
  1928; 
  Adams, 
  

   1934). 
  Long 
  after 
  they 
  were 
  deposited 
  diastrophic 
  movements 
  have 
  

   frequently 
  exposed 
  limestones 
  to 
  weathering. 
  Subaerial 
  exposure 
  

   and 
  solution 
  by 
  ground 
  water 
  have 
  then 
  produced 
  secondary 
  porosity 
  

   in 
  the 
  limestone, 
  and, 
  when 
  the 
  weathered 
  surface 
  was 
  covered 
  by 
  

   later 
  sediments, 
  a 
  trap 
  suitable 
  for 
  oil 
  accumulation 
  has 
  resulted. 
  

   The 
  formation 
  of 
  dolomite 
  by 
  the 
  recrystallization 
  of 
  calcite 
  and 
  

   aragonite 
  is 
  probably 
  responsible 
  for 
  increasing 
  the 
  porosity 
  of 
  lime- 
  

   stones, 
  and 
  this 
  alteration 
  seems 
  frequently 
  to 
  be 
  a 
  part 
  of 
  the 
  paleo- 
  

  

  