378 ILLINOIS STATE ACADEMY OF SCIENCE, 



in the tops of anticlinal folds and domes. In areas where 

 the known dip or regional rise of the strata has brought 

 a shale capping to the surface where it has been largely 

 or entirely eroded, oil probably would, not be retained in 

 an immediately underlying porous rock. Any oil pre- 

 viously accumulated would have escaped long ago dur- 

 ing the preglacial erosion. This is applicable in Illinois 

 on the northward extension of the LaSalle anticline due 

 to the southward pitch of the axis, and also around the 

 rim of the central structural basin in southern, western, 

 and northern Illinois where the formations have been 

 truncated by erosion. 



SAND AND RESERVOIR ROCKS 



Where the necessary capping shales in the rock sec- 

 tion are present, the question of reservoir rocks should 

 next be considered. The positions of most of these are 

 known in the Illinois rock section. Structure and area 

 of deposition or subsequent erosion will affect the num- 

 ber of any such horizons that are present and can be 

 penetrated by the drill at any given locality. The prob- 

 able productivity of the sand, if it contains oil, com- 

 pared with the cost of drilling necessary is also an im- 

 portant consideration. The records of past drilling and 

 statistical information of oil production of the old fields 

 in Illinois are particularly beneficial in this connection, 

 although many operators have been very slow to realize 

 the great importance of keeping and preserving good 

 records. 



There are some lenticular porous sands in Illinois and 

 other beds which grade from impervious to porous, both 

 laterally and vertically. It is quite possible to find oil 

 accumulation in areas of regional folding without the 

 presence of complete local reversals, but for the same 

 reason that oil accumulates gravitationally in the arch of 

 an anticline, so oil in porous lenses with water sat- 

 uration would be expected in the higher parts of 

 the lenses. There have been cases of this kind in 

 Illinois, notably in the Colmar-Plymouth field, and 

 probably there are others awaiting discovery. It is 

 impossible, however, to obtain any indication of the 



