STRUCTURE  OF  THE  BOULDER  OIL  FIELD,  COLORADO,  WITH 
RECORDS  FOR  THE  YEAR  1903. 
By  N.  M.  Fenneman. 
The  field  here  referred  to  has  be  n  producing  petroleum  in  moderate 
quantities  for  the  past  two  years.  It  comprises  a  small  area  centering 
about  3  miles  northeast  of  Boulder,  Colo.,  east  of  and  within  3  miles  of 
the  foothills  of  the  Front  Range.  Above  3,000  feet  above  sea  level  the 
entire  zone  traversed  in  drilling  is  within  the  Fort  Pierre  shales  of  the 
Cretaceous.  Small  quantities  of  oil  have  been  found  at  various  depths, 
from  a  few  hundred  to  3,400  feet.  Sometimes  the  oil  (or  gas)  has 
been  in  the  sandier  beds,  which  may  occur  at  any  and  all  depths,  and 
sometimes  in  the  shale  itself.  Neighboring  wells  find  oil  at  very 
diverse  depths.  Further  details  of  the  occurrence  of  the  oil  and  the 
work  of  obtaining  it  have  been  given  in  a  former  publication. a 
From  all  observations  made  previous  to  the  report  just  noted,  there 
appeared  to  be  no  law  governing  the  distribution  of  oils  in  the  Fort 
Pierre  shales.  Oils  are  found  in  various  parts  of  Colorado  and 
neighboring  States,  but  their  structural  relations,  so  far  as  known, 
gave  vague  explanations  of  their  presence  and  left  the  prospector  with 
the  impression  that  oil  might  or  might  not  be  found  at  almost  any 
place  in  the  Fort  Pierre  shales. 
The  want  of  any  evidence  of  anticlinal  arches  suggested  the  infer- 
ence that  such  structures  were  unnecessary  to  form  receptacles  for 
oil  and  gas  in  so  dense  a  rock  as  the  Pierre  shales.  The  idea  that 
the  occurrence  of  the  oil  had  some  connection  with  folds  was,  however, 
not  given  up,  and  renewed  efforts  were  made  to  induce  companies  and 
drillers  to  preserve  samples  and  careful  records,  with  a  view  to  deter- 
mining the  exact  position  of  the  strata. 
During  the  last  summer  a  detailed  structural  study  has  been  made  of 
an  area  of  150  square  miles,  of  which  the  oil  field  occupies  the  center. 
The  careful  study  of  the  borders  of  this  area  was  made  necessary  by 
the  fact  that  its  broad  central  portion,  containing  the  wells,  is  under- 
lain by  the  Fort  Pierre  shales,  which  are  in  general  so  nonresistant 
and  homogeneous  that  exposures  of  solid  rock  are  extremely  rare. 
a  Bull.  U.  S.  Geol.  Survey  No.  213,  1903,  p.  312. 
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