Sl^RAl^IGRAPEtY 657 



Stratigkaphy 



The producing sands in Illinois range in age from the top of the Car- 

 bondale formation of the Pennsylvanian series down to the npper part of 

 the Trenton limestone. The latter formation produces in only one lo- 

 cality^ at the north end of the Clark County field, at a depth of about 

 2,200 feet. Elsewhere in the State it has not yet proved to be commer- 

 cially important. By far the greatest output is derived from the sand- 

 stones of the Carbondale and Pottsville formations of the Pennsylvanian, 

 tcgether with the Chester group and the Sainte Genevieve formation of 

 the Mississippian. A sandstone at the base of the Niagaran produces in 

 one small locality at Colmar, McDonough County, in western Illinois. 



All of the producing beds are sands of variable thickness and character 

 with the exception of the so-called McClosky sand, which is in reality the 

 oolitic Sainte Genevieve limestone lying immediately beneath the Chester 

 group. Of all the producing beds in the State, those of the Chester are 

 the most regular in their development. The sands of the Pennsylvanian 

 are extremely irregular in thickness and character, and it is often impos- 

 sible to correlate them from one well to another with any degree of suc- 

 cess. The Hoing sand, lying at the base of the Niagaran in the Colmar 

 field, is the most restricted in its development of any producing horizon 

 in the State. It appears to have been deposited in depressions on the 

 Maquoketa surface during the encroachment of the Niagaran sea, but 

 outside of the small area at Colmar numerous drill-holes in the western 

 part of Illinois have failed to disclose its presence, and it does not seem 

 to exist in the main fields of Clark, Crawford, and Lawrence counties. 



The producing sands pinch out to the north along the La Salle anti- 

 cline, due in all probability to the existence of the anticline previous to 

 the deposition of the oil-bearing beds. The important Chester group is 

 present only in the southern one-third of the State. Consequently pro- 

 duction outside the area shown must come from the Pottsville, the Niag- 

 aran, or the Trenton. 



Steucture , 



general structure of state 



Structurally the State is a spoon-shaped basin, the tip lying in the 

 northwest corner and the deepest part of the bowl in Wayne, Edwards, 

 Hamilton, and White counties, in the southeast corner of the State. The 

 long axis of the spoon extends northwest-southeast, parallel to the main 

 oil fields. In the west and central parts of the State the dip toward the 

 axis of the basin is commonly as low as 10 feet per mile. The westward 



