350  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
that  this  oil  has  been  formed  from  the  shale  in  which  it  is  found,  and  that  the  oil-bearing 
sands  represent  local  sandy  layers  more  or  less  perfectly  surrounded  by  shales  in  which 
the  oil  has  accumulated.  This  is  the  case  also  in  the  Boulder  and  Florence  fields,  although 
at  those  localities  the  shales  are  geologically  younger.  Where  water  is  absent  from  the 
oil-bearing  beds  oil  tends  to  move  down  the  dip,  and  so  far  as  the  continuity  of  the  porous 
beds  allows  will  collect  in  the  troughs  of  the  synclines.  This  is  apparently  the  case  in 
this  field,  and  the  position  of  the  syncline  and  the  depth  of  the  oil-bearing  shale  at  its 
lowest  point  thus  becomes  one  of  considerable  economic  importance.  At  Hilliard  the 
lowest  point  in  the  shale  bed  is  over  11,000  feet  from  the  surface  and  the  dip  of  the  hods 
is  such  that  a  deep  well  would  be  extremely  difficult  to  sink.  Between  Hilliard  and  the 
Aspen  tunnel  the  syncline  rises  and  these  beds  may  be  thoroughly  prospected  with  a  hole 
2,500  to  3,000  feet  deep.  This  is  a  good  location,  although  the  oil-spring  fault  to  the 
west  introduces  a  point  of  leakage  and  the  deepening  of  the  syncline  to  the  south  a  I  fords 
a  lower  point  of  accumulation.  Because  of  the  normal  character  of  the  syncline  at  ths 
point,  a  well  properly  placed  may  be  drilled  entirely  in  nearly  horizontal  strata. 
Northward  the  synclinal  trough  deepens  rapidly,  and  at  the  Lazeart  mine  the  oil  shale 
is  perhaps  10,000  feet  from  the  surface.  At  Round  Mountain  the  strata  are  overturned 
and  faulted  and  the  site  is  not  favorable  for  an  oil  well.  Between  Round  Mountain  an| 
the  top  of  the  Adaville  beds,  north  of  Little  Muddy  Creek,  the  synclinal  trough  rises.  The 
rise  is  gradual,  the  syncline  is  normal,  and  the  locality  in  these  respects  is  the  best  in  the 
region.  1  low  ever,  the  great  depth  of  the  oil-bearing  shales — 5,000  to  7,000  feet — is  practi- 
cally prohibitive.  To  the  north  the  syncline  sinks,  and  at  the  Oregon  Short  Line  the  lowes| 
point  of  the  oil-bearing  beds  is  about  15,000  feet  from  the  surface.  Farther  north  the  syn- 
cline rises,  and  in  the  long  trough  north  of  a  point  10  to  15  miles  north  of  Kcmmerer  the 
conditions  are  very  favorable.  The  depth  of  the  oil-bearing  shale  in  the  center  of  the  syiij 
cliue  is  such  that  wells  could  be  readily  sunk,  and  test  holes  here  are  likely  to  yield  return! 
This  region  is  beyond  that  examined  this  year,  but  enough  was  learned  regarding  it  to  waa 
rant  the  above  suggestion.  Moreover,  oil  springs  are  reported  in  this  region,  and  it  is  hoped 
that  they  may  be  critically  examined  next  year. 
In  general,  in  the  region  covered  by  this  report  the  depth  of  the  oil-bearing  shales  at  the 
axis  of  the  syncline  is  practically  prohibitive,  but  the  soft  character  of  the  strata  suggests 
that  the  diminut  ion  of  pore  space,  due  to  the  pressure  of  the  superincumbent  beds,  may  be 
SO  great  that  the  maximum  accumulation  of  oil  will  be  at  some  point  on  the  limb  of  the 
syncline.  between  t  he  axis  and  the  outcrop.  Indeed,  though  the  oil-bearing  shales  underlie 
a  much  larger  area,  prospecting  should  he  restricted  to  the  region  between  the  axis  of  the 
Lazeart  syncline  and  the  eroded  edge  of  the  bed  on  the  west  Hank  of  the  Meridian  anti- 
cline (PI.  XI). 
The  well  of  the  Pittsburg  and  Salt  Lake  Oil  Company,  in  sec.  10,  T.  U  N.,  It.  118  W., 
develops  an  entirely  differenl  horizon.  It  is  so  situated  that  after  passing  through  the 
Wasatch  beds  it  strikes  the  older  rock  below  the  oil-bearing  Benton  shale  (PI.  XII,  A).  ■ 
develops  two  oil-bearing  sand  beds  in  a  light-blue  shale,  which  is  probably  Jurassic  and  of  the 
same  age  as  the  shale  exposed  on  Twin  Creek  wesl  of  Nugget.  The  oil  is  black  and  more 
in  the  nature  of  a  lubricating  oil  than  that  from  the  upper  horizon.  Prospecting  for  these 
lower  horizons  may  well  be  carried  on  near  and  just  west  of  the  Meridian  anticline.  If 
in  these  lower  strata  water  is  also  absent,  the  greatest  yield  will  be  found  down  the  dip. 
In  attempting  to  develop  the  shales  of  the  same  age  on  Twin  Creek  test  wells  should  be 
located  west  of  the  outcrop  and  in  such  a  position  that  500  or  1,000  feet  of  red  beds  may 
be  drilled  through  before  reaching  the  shales. 
