﻿PETROLEUM 
  GEOLOGY 
  — 
  HEROY 
  171 
  

  

  most 
  common 
  original 
  source 
  and 
  that 
  the 
  oil 
  found 
  in 
  arenaceous 
  sand- 
  

   stones 
  was 
  supposed 
  to 
  have 
  been 
  derived 
  from 
  the 
  shales 
  above 
  or 
  

   below. 
  Black 
  shales 
  were 
  thought 
  to 
  be 
  rich 
  in 
  oil, 
  probably 
  because 
  

   "coal 
  oil" 
  had 
  been 
  distilled 
  from 
  them. 
  A 
  relationship 
  was 
  thus 
  early 
  

   assumed 
  to 
  exist 
  between 
  the 
  amount 
  of 
  organic 
  matter 
  present 
  in 
  sedi- 
  

   ments 
  and 
  their 
  capacity 
  to 
  originate 
  petroleum. 
  The 
  term 
  "source 
  

   beds" 
  gradually 
  came 
  into 
  use 
  to 
  distinguish 
  those 
  rocks 
  from 
  whose 
  

   organic 
  matter 
  petroleum 
  has 
  originated. 
  As 
  the 
  science 
  advanced, 
  

   however, 
  it 
  became 
  apparent 
  that 
  such 
  generalizations 
  were 
  not 
  com- 
  

   pletely 
  true, 
  and 
  about 
  20 
  years 
  ago 
  the 
  need 
  for 
  experimental 
  investi- 
  

   gation 
  of 
  source 
  beds 
  began 
  to 
  be 
  recognized. 
  This 
  took 
  definite 
  form 
  

   in 
  1926 
  when 
  the 
  American 
  Petroleum 
  Institute 
  sponsored 
  a 
  research 
  

   program 
  which 
  is 
  still 
  continuing. 
  

  

  Investigation 
  of 
  so 
  complex 
  a 
  subject 
  was 
  initially 
  faced 
  with 
  in- 
  

   herent 
  difficulties. 
  The 
  assumption 
  that 
  the 
  source 
  beds 
  were 
  strati- 
  

   graphically 
  closely 
  associated 
  with 
  the 
  reservoir 
  beds 
  depends 
  for 
  its 
  

   validity 
  on 
  the 
  premise 
  that 
  oil 
  has 
  accumulated 
  near 
  the 
  zone 
  and 
  

   area 
  of 
  origin 
  and 
  has 
  not 
  migrated 
  horizontally 
  or 
  vertically 
  for 
  long 
  

   distances, 
  a 
  premise 
  on 
  which 
  petroleum 
  geologists 
  are 
  by 
  no 
  means 
  

   in 
  agreement. 
  If 
  organic 
  matter 
  in 
  the 
  form 
  of 
  petroleum 
  has 
  origi- 
  

   nated 
  in 
  particular 
  strata, 
  then 
  the 
  movement 
  of 
  the 
  petroleum 
  out 
  

   of 
  these 
  strata 
  will 
  leave 
  them 
  poorer 
  in 
  organic 
  matter 
  than 
  they 
  

   were 
  originally 
  ; 
  hence 
  present 
  organic 
  content 
  may 
  not 
  be 
  conclusive 
  

   as 
  to 
  whether 
  or 
  not 
  a 
  particular 
  bed 
  has 
  acted 
  as 
  a 
  source 
  of 
  petroleum. 
  

   On 
  the 
  other 
  hand, 
  if 
  certain 
  strata 
  were 
  originally 
  sufficiently 
  rich 
  in 
  

   organic 
  matter 
  to 
  originate 
  petroleum 
  they 
  may 
  still, 
  even 
  after 
  giving 
  

   up 
  some 
  of 
  their 
  organic 
  content, 
  be 
  richer 
  in 
  organic 
  matter 
  than 
  other 
  

   sedimentary 
  deposits. 
  If 
  the 
  geologic 
  forces 
  to 
  which 
  an 
  area 
  has 
  

   been 
  subjected 
  subsequent 
  to 
  the 
  deposition 
  of 
  the 
  source 
  beds 
  are 
  an 
  

   important 
  factor 
  in 
  the 
  genesis 
  of 
  petroleum, 
  then 
  the 
  amount 
  and 
  

   character 
  of 
  the 
  organic 
  matter 
  originally 
  present 
  in 
  the 
  sediments 
  

   may 
  not 
  be 
  the 
  major 
  factor; 
  the 
  dynamic 
  history 
  may 
  be 
  controlling. 
  

   These 
  and 
  other 
  questions 
  complicate 
  the 
  problem 
  of 
  recognizing 
  source 
  

   beds. 
  Work 
  on 
  details 
  of 
  the 
  problem 
  has 
  led 
  to 
  some 
  specific 
  con- 
  

   clusions, 
  and 
  in 
  the 
  following 
  paragraph 
  the 
  writer 
  has 
  attempted 
  to 
  

   summarize 
  present 
  prevailing 
  opinion 
  on 
  this 
  subject. 
  

  

  Recent 
  marine 
  sediments 
  contain 
  as 
  high 
  as 
  7 
  percent 
  organic 
  matter 
  

   with 
  the 
  average 
  around 
  2.5 
  percent. 
  Some 
  older 
  rocks, 
  such 
  as 
  Mon- 
  

   terey 
  shale, 
  may 
  have 
  had 
  a 
  higher 
  organic 
  content 
  than 
  recent 
  sedi- 
  

   ments 
  at 
  the 
  time 
  of 
  deposition. 
  All 
  ancient 
  sediments 
  have 
  probably 
  

   lost 
  some 
  of 
  their 
  original 
  organic 
  content, 
  and 
  the 
  loss 
  through 
  aging 
  

   may 
  be 
  as 
  much 
  as 
  40 
  percent. 
  The 
  proportion 
  of 
  the 
  original 
  organic 
  

   content 
  that 
  may 
  have 
  been 
  converted 
  into 
  petroleum 
  is 
  unknown 
  but 
  

   has 
  been 
  estimated 
  at 
  from 
  5 
  to 
  10 
  percent. 
  In 
  recent 
  sediments 
  the 
  

  

  