﻿PETROLEUM 
  GEOLOGY 
  — 
  HEROY 
  185 
  

  

  weathering 
  just 
  mentioned. 
  Certainly 
  some 
  of 
  the 
  most 
  prolific 
  lime- 
  

   stone 
  reservoirs 
  are 
  in 
  dolomites 
  which 
  seem 
  to 
  be 
  of 
  secondary 
  origin. 
  

  

  In 
  the 
  Lima-Indiana 
  field 
  oil 
  has 
  accumulated 
  in 
  the 
  upper 
  zone 
  of 
  

   the 
  Trenton 
  limestone, 
  the 
  porosity 
  of 
  which 
  has 
  probably 
  resulted 
  

   from 
  dolomitization 
  (Carman 
  and 
  Stout, 
  1934). 
  In 
  the 
  fields 
  of 
  

   Michigan, 
  where 
  the 
  Dundee 
  formation 
  is 
  the 
  most 
  important 
  reser- 
  

   voir 
  rock, 
  dolomitization 
  is 
  the 
  principal 
  cause 
  of 
  porosity 
  (Hake, 
  

   1938) 
  . 
  In 
  the 
  Oklahoma 
  City 
  field 
  the 
  Arbuckle 
  limestone, 
  the 
  lowest 
  

   producing 
  horizon, 
  of 
  lower 
  Ordovician 
  age, 
  has 
  been 
  partly 
  eroded, 
  

   and 
  the 
  porosity 
  of 
  the 
  reservoir 
  has 
  evidently 
  resulted 
  from 
  weather- 
  

   ing 
  (McGee 
  and 
  Clawson, 
  1932). 
  

  

  Deformational 
  traps. 
  — 
  Tilting, 
  folding, 
  faulting, 
  and 
  intrusion 
  

   have 
  for 
  the 
  most 
  part, 
  resulted 
  from 
  movement 
  which 
  has 
  occurred 
  

   in 
  sedimentary 
  deposits 
  since 
  their 
  deposition, 
  and 
  each 
  has 
  been 
  

   responsible 
  for 
  the 
  formation 
  of 
  oil 
  and 
  gas 
  traps. 
  

  

  The 
  tilt 
  which 
  may 
  be 
  imparted 
  to 
  sediments 
  largely 
  controls 
  the 
  

   direction 
  in 
  which 
  oil 
  and 
  gas 
  move. 
  In 
  some 
  fields 
  the 
  accumulation 
  

   is 
  due 
  primarily 
  to 
  tilting 
  of 
  the 
  porous 
  bed 
  into 
  a 
  monoclinal 
  position. 
  

   If, 
  at 
  the 
  same 
  time, 
  erosion 
  has 
  exposed 
  its 
  margin 
  at 
  the 
  surface 
  the 
  

   oil 
  which 
  moves 
  upward 
  through 
  it 
  will 
  escape. 
  Lighter 
  oils 
  are 
  thus 
  

   drained 
  from 
  the 
  reservoir, 
  but 
  more 
  viscous 
  oils 
  may, 
  through 
  loss 
  of 
  

   lighter 
  fractions, 
  form 
  a 
  brea 
  which 
  gradually 
  seals 
  the 
  reservoir 
  near 
  

   the 
  surface 
  and 
  becomes 
  a 
  barrier 
  to 
  further 
  movement. 
  Examples 
  of 
  

   this 
  type 
  of 
  trap 
  are 
  found 
  in 
  the 
  Sunset-Midway 
  field, 
  California, 
  in 
  

   which 
  some 
  of 
  the 
  sands 
  are 
  filled 
  by 
  tar 
  near 
  the 
  surface 
  (Pack, 
  1920, 
  

   p. 
  87), 
  and 
  in 
  the 
  Lagunillas 
  field, 
  Venezuela, 
  where 
  the 
  oil 
  gradually 
  

   decreases 
  in 
  Baume 
  gravity 
  eastward 
  as 
  the 
  sands 
  rise 
  toward 
  the 
  out- 
  

   crop. 
  More 
  commonly, 
  however, 
  tilting 
  has 
  produced 
  traps 
  through 
  

   combination 
  with 
  other 
  factors, 
  such 
  as 
  stratigraphic 
  variation. 
  

  

  Folding, 
  causing 
  anticlines 
  and 
  synclines, 
  domes 
  and 
  basins, 
  is 
  in 
  all 
  

   probability 
  the 
  most 
  important 
  factor 
  in 
  producing 
  traps 
  for 
  oil 
  and 
  

   gas. 
  The 
  first 
  structural 
  form 
  recognized 
  as 
  controlling 
  accumulation 
  

   was 
  the 
  anticline, 
  and 
  for 
  many 
  years 
  the 
  petroleum 
  geologist 
  was 
  

   chiefly 
  concerned 
  with 
  the 
  finding 
  of 
  domes 
  in 
  which 
  the 
  trapping 
  of 
  

   oil 
  was 
  primarily 
  dependent 
  on 
  structural 
  closure. 
  Fields 
  in 
  which 
  

   folding 
  of 
  this 
  type 
  is 
  the 
  primary 
  cause 
  of 
  accumulation 
  are 
  numer- 
  

   ous, 
  and 
  only 
  a 
  few 
  of 
  the 
  most 
  prominent 
  need 
  be 
  mentioned. 
  Long 
  

   Beach, 
  Calif., 
  is 
  the 
  most 
  productive 
  dome 
  in 
  the 
  United 
  States, 
  with 
  

   a 
  total 
  yield 
  of 
  over 
  630,000,000 
  barrels. 
  Santa 
  Fe 
  Springs, 
  Elk 
  Hills, 
  

   and 
  Kettleman 
  Hills 
  are 
  other 
  California 
  fields 
  of 
  this 
  type. 
  Salt 
  

   Creek, 
  Wyo., 
  the 
  most 
  productive 
  field 
  of 
  the 
  Rocky 
  Mountain 
  area, 
  is 
  

   a 
  domal 
  structure. 
  Accumulation 
  in 
  the 
  Seminole 
  fields 
  of 
  Oklahoma 
  

   is 
  primarily 
  on 
  domal 
  folds 
  (Levorsen, 
  1929). 
  Domal 
  folding 
  has 
  

   been 
  the 
  controlling 
  factor 
  in 
  forming 
  traps 
  in 
  the 
  Hobbs 
  field, 
  New 
  

   Mexico, 
  and 
  in 
  the 
  Yates 
  and 
  Big 
  Lake 
  fields 
  in 
  west 
  Texas. 
  

  

  