82 ILLINOIS ACADEMY OF SCIENCE. 



All the penetrated rocks in the producing areas of Crawford 

 county belong to the Pennsylvanian series. This series is repre- 

 sented by about 480 feet of the McLeansboro, 300 feet of the 

 Carbondale, and about 100 feet of the Pottsville formations. The 

 rocks are all of sedimentary origin, being principally shales with 

 variable intergradations of sandstones, limestones and coal. The 

 oil sands lie at the top of the Pottsville rocks, which are the lowest 

 members of the Pennsylvanian, and are essentially coarse sand- 

 stones, merging into sandy shales at the top, and often split with 

 lenses of shale and pockets of coal. 



The Crawford county pools have one general oil-producing 

 zone, known as the Robinson sand, lying between 800 and 975 

 feet deep. This sand is so broken and lenticular that it ofifers 

 little opportunity for structural study. In some portions of the 

 field it assumes regularity in its distribution. It is split into 

 two or three persistent lenses that show an average interval of 

 about fifty feet and a thickness of from two to fifty feet. The 

 lenses often merge into each other and are even united in some 

 wells with a maximum thickness of 122 feet. Again, the lenses 

 pinch out, and in several wells are entirely absent. 



Owing to the irregular deposition of sands and shales, it is 

 impossible to correlate and contour any sand beds definitely, 

 except the top lens of the Robinson sand. Even this work loses 

 much of its scientific value in places where the sand wedges out 

 or is overlapped. 



The altitudes of the top lens are assembled and contoured with 

 intervals of twenty feet. The general structure of the Robinson 

 pool reveals a broad and gentle arch, which is divided into two 

 parts by a transverse basin. The north part of the arch is six 

 miles wide, with its crest ninety-five feet above the lowest 

 explored portions of the limbs. The south part of the arch 

 is about four miles wide and no feet high. The 1,100-foot con- 

 tour follows the limits of the pool in a general way and seems 

 to include most of the productive zone. The small irregularities 

 of the map probably do not represent minor folds, but irregular 

 porosity of the general sand zone, as determined by the driller. 



In studying the distribution of oil over Crawford county, the 

 lower lenses are found slightly more productive than the top 

 lens. There is considerable unevenness of distribution due to the 

 following factors : 



I. The sands vary in porosity and in many places are prac- 

 tically impervious to oil. 



