﻿PETROLEUM 
  GEOLOGY 
  — 
  HEROY 
  191 
  

  

  balance 
  between 
  the 
  portion 
  submerged 
  under 
  salt 
  water 
  in 
  the 
  conti- 
  

   nental 
  shelf 
  and 
  the 
  portion 
  lying 
  under 
  the 
  land. 
  His 
  reasoning 
  may 
  

   be 
  extended 
  to 
  coastal 
  plain 
  beds 
  which 
  have 
  no 
  surface 
  outcrops 
  be- 
  

   cause 
  of 
  overlap 
  or 
  up-dip 
  pinching, 
  but 
  which 
  may 
  have 
  a 
  submarine 
  

   outcrop 
  at 
  the 
  edge 
  of 
  the 
  shelf. 
  An 
  explanation 
  is 
  thus 
  afforded 
  of 
  

   the 
  normal 
  reservoir 
  pressures 
  noted 
  in 
  coastal 
  plain 
  oil 
  fields 
  (Cannon 
  

   and 
  Craze, 
  1938) 
  . 
  Fluid 
  relations 
  in 
  a 
  large 
  coastal 
  plain 
  field, 
  Conroe, 
  

   Tex., 
  have 
  been 
  well 
  described 
  by 
  Michaux 
  and 
  Buck 
  (1936). 
  

  

  Keservoirs 
  which 
  are 
  essentially 
  sealed 
  by 
  surrounding 
  rocks 
  of 
  low 
  

   permeability 
  may 
  contain 
  oil 
  and 
  gas 
  under 
  pressures 
  which 
  have 
  no 
  

   direct 
  relation 
  to 
  the 
  hydrostatic 
  pressures 
  corresponding 
  to 
  their 
  

   depth. 
  The 
  oil 
  and 
  gas 
  in 
  such 
  reservoirs 
  may 
  have 
  pressures 
  sub- 
  

   stantially 
  in 
  excess 
  of 
  the 
  equivalent 
  hydrostatic 
  pressure 
  and 
  tending 
  

   to 
  approach 
  the 
  pressure 
  corresponding 
  to 
  the 
  weight 
  of 
  the 
  over- 
  

   burden 
  (Cannon 
  and 
  Craze, 
  1938). 
  It 
  has 
  been 
  noted 
  that 
  in 
  some 
  

   cases 
  shallower 
  reservoirs 
  may 
  have 
  higher 
  reservoir 
  pressures 
  than 
  

   those 
  in 
  the 
  same 
  area 
  at 
  greater 
  depths 
  (Millikan, 
  1932). 
  Reservoirs 
  

   which 
  are 
  essentially 
  lenticular 
  in 
  form 
  and 
  isolated 
  from 
  artesian 
  

   conditions 
  may 
  be 
  expected 
  to 
  contain 
  oil 
  and 
  gas 
  under 
  a 
  pressure 
  

   determined 
  primarily 
  by 
  the 
  amount 
  of 
  gas 
  which 
  has 
  migrated 
  into 
  

   the 
  reservoir 
  and, 
  if 
  the 
  surrounding 
  section 
  were 
  highly 
  impervious, 
  

   pressures 
  approaching 
  the 
  weight 
  of 
  the 
  overburden 
  may 
  be 
  built 
  up. 
  

   Excess 
  pressures 
  in 
  shallower 
  strata 
  may, 
  however, 
  in 
  some 
  cases 
  be 
  

   caused 
  by 
  supercharging 
  from 
  deeper 
  horizons 
  as 
  a 
  result 
  of 
  upward 
  

   migration 
  of 
  oil 
  and 
  gas 
  along 
  zones 
  of 
  fracture 
  or 
  faulting. 
  Unfortu- 
  

   nately 
  geologists, 
  on 
  the 
  whole, 
  have 
  given 
  scant 
  attention 
  to 
  this 
  

   phase 
  of 
  petroleum 
  geology, 
  leaving 
  this 
  interesting 
  field 
  mostly 
  to 
  

   petroleum 
  engineers. 
  

  

  CONCLUSION 
  

  

  The 
  natural 
  history 
  of 
  petroleum, 
  from 
  genesis 
  to 
  dispersion, 
  is 
  

   cyclical. 
  In 
  some 
  regions 
  where 
  petroleum 
  deposits 
  occur, 
  only 
  a 
  

   single 
  cycle 
  is 
  represented, 
  while 
  in 
  others 
  several 
  cycles, 
  either 
  partial 
  

   or 
  complete, 
  may 
  have 
  occurred. 
  In 
  the 
  preceding 
  pages 
  the 
  writer 
  has 
  

   described 
  in 
  sequence 
  the 
  phases 
  which 
  form 
  a 
  complete 
  cycle 
  and 
  the 
  

   character 
  of 
  each. 
  While 
  each 
  phase 
  has 
  been 
  the 
  objective 
  of 
  much 
  

   scientific 
  investigation, 
  there 
  is 
  great 
  variation 
  in 
  the 
  quality 
  and 
  com- 
  

   pleteness 
  of 
  the 
  results 
  attained. 
  This 
  is 
  partly 
  inherent 
  in 
  the 
  nature 
  of 
  

   the 
  problems 
  themselves. 
  Science, 
  however 
  advanced, 
  is 
  as 
  yet 
  not 
  

   adequately 
  implemented 
  to 
  investigate 
  some 
  of 
  them, 
  and 
  the 
  progress 
  

   in 
  the 
  solution 
  of 
  others 
  may 
  be 
  ascribed 
  to 
  the 
  immediate 
  importance 
  

   of 
  the 
  results; 
  economic 
  considerations 
  frequently 
  determine 
  the 
  

   amount 
  and 
  thoroughness 
  of 
  research. 
  

  

  Although 
  petroleum 
  geologists 
  are 
  conscious 
  of 
  the 
  inadequacy 
  of 
  

   their 
  present 
  information 
  as 
  to 
  many 
  details 
  of 
  the 
  petroleum 
  cycle, 
  

  

  