GEOLOGICAL P.\PERS 8l 



THE STRUCTURAL RELATIONS OF THE OIL FIELDS 



OF CRAWFORD AXD LAWRENXE COUNTIES, 



ILLINOIS."^ 



Raymond S. Blatchley. 



The State Geological Survey began a detailed investigation of 

 the geological conditions in the southern half of the eastern Illi- 

 nois oil fields in 1908. This work was carried on intermittently 

 and was finally completed Januar}- i, 1912. The specific area of 

 investigation lies in the southern half of Crawford and the cen- 

 tral portion of Lawrence counties. The priman.- object of the 

 work was to determine the cause of the accumulation of oil and 

 gas and to secure facts which would have a bearing on the origin 

 of oil. and also to determine, if possible, the relation of the 

 quantities of oil, gas, salt water, porosity of the sand, etc., to the 

 structural features of the sand. 



The work is based upon the elevations and records of 5,200 

 wells. The method of study is to map, by means of contour 

 lines, or lines through points of equal altitude, the geologic struc- 

 ture of the producing sands. The contours are made upon the 

 positive altitudes of the sands above a datum plane 1,500 feet 

 below mean sea level. These maps show the oil sands as if every- 

 thing above them had been removed. From the undulations on 

 the surface of the sands and from the initial productions of the 

 wells, the oil, gas and water relations to the structure are 

 observed. The La Salle anticline is the controlling feature of 

 the field. 



CRAWFORD COUNTY. 



The investigated area in Crawford county lies south of the 

 Illinois Central Railroad, between Oblong and Robinson. There 

 are 2,970 wells mapped in this area, of which 206, or 8.7 per 

 cent, are barren. The productive wells range in initial yield from 

 I to 1,600 barrels. The Robinson pool, or that of Crawford 

 county, is about seven miles wide between Robinson and Oblong, 

 but narrows to about three and one-half miles at the southern 

 limit of the county. The western boundary of the field trends 

 northwest and southeast and is sharply defined. The eastern 

 boundary-, on the other hand, is very irregular. 



^ This paper with illustrations was published by permission from the Secretary, in 

 Economic Geology", \ ol. \'II. p. 574, 1912. Also a more complete report with 

 abundant illustrations will appear as Bulletin 22, Illinois State Geological Survey. 



