376 ILLINOIS STATE ACADEMY OF SCIENCE 



Correct judgment of the oil geologist must involve the 

 following considerations, and their application in Illi- 

 nois will be discussed briefly. 



REGIONAL STRUCTURE 



The bedded rock formations in Illinois, as far as the 

 drill has reached, are composed mainly of shales, sand- 

 stones, and limestones. These beds are so tilted and 

 folded that in central Illinois they form a spoon-shaped 

 basin, a gentle rise continuing out to the edges of the 

 State with the exception of the eastern edge of the basin 

 where the beds rise sharply forming an uplifted fold 

 called the LaSalle anticline. This is the most pronounced 

 uplift or folded structure in Illinois, and the main axis 

 of the folding runs in a direction slightly east of south 

 from the northwest of the State through LaSalle to the 

 vicinity of St. Francisville near the Indiana boundary in 

 the southeast with a general inclination or pitch to the 

 south. In contrast to the dip or inclination of the beds, 

 the rock surface in Illinois was more, or less base leveled 

 by erosion, and over this surface was deposited the gla- 

 cial drift consisting of clay, silts, sands, and gravels, 

 which at the present time cover the surface of most of 

 Illinois. These late deposits are cut through by the pres- 

 ent larger rivers and streams exposing the underlying 

 rock, the out-crops of which afford a source of consid- 

 erable information to the geologist. However, there are 

 large areas where no outcrops can be seen, and the geolo- 

 gist has to derive much of his knowledge of the subsur- 

 face conditions from a study of the logs of wells and 

 samples of the rock beds penetrated. 



The important relationships of oil to the structure 

 or folding of the bedded rocks was first recognized 

 by I. C. White and is known as the anticlinal theory 

 of oil accumulation. In a general way this has been 

 found to hold true for oil accumulation in Illinois, 

 although oil geologists are recognizing other relation- 

 ships which are applicable in certain circumstances, 

 and in others modify the application of the anti- 

 clinal theory. Thus we note (Figs. 2 and 3) the main 

 oil fields developed along the axis of the LaSalle an- 



