PAPERS ON GEOGRAPHY AND GEOLOGY 



383 



The new production being obtained from time to time 

 will continue to offset partially the natural decline of the 

 older wells, and serve to maintain the annual production 

 of Illinois with a very low decline rate. 



Legend 

 Oil well 



Detailed eologie 

 mapping 

 completed 



Reconnaissance 

 oil survey 

 completed 

 Cooperative 

 coal reports 

 completed 

 (reconnaissance 

 survey) 

 / Anticlinal axes 



^ Oil and gas 

 ^ fields 



Fig. 2. Map of Illinois showing location of oil and gas fields, anticlinal 

 axes and results of testing the anticlinal structures. Outlined areas 

 indicate whether detailed or reconnaissance geologic surveys have 

 been completed. 



