660 



F. H. KAY OIL FIELDS OF ILLINOIS 



condition as to be oil bearing. For each sand, tben, the general result is 

 a develoj^ment here and there at the same horizon of lenticular masses of 

 sand ca})able of acting as reservoirs for oil and gas, surrounded b}^ beds 

 which are now impervious to the mov-ement of these materials. In a 

 broad sense, accumulation is controlled by the anticlinal structure, but 

 has taken place independently in the different lenses of each sand. Other- 

 wise it would be difficult to understand the existence of salt water along 

 the axis of the anticline in positions considerably higher than oil in the 

 same sand in other parts of the anticline or further down the axis in the 

 same sand. 



The asymmetrical nature of the La Salle anticline has caused the main 

 oil fields to be limited much more closely on the west side than on the 

 east. On the west side all of the producing sands are carried beneath the 

 level of permanent salt-water saturation along a rather definite line. The 

 sands of Lawrence County show abundant water along the flanks of the 

 anticline and but little through the center of the field, except in the lower 

 Bridgeport and Buchanan sands of the Pottsville. These rocks contain 

 large quantities of water over the entire Lawrence County area, whereas 

 farther north, along the axis of the anticline, in Crawford County, water 

 is not a disturbing feature except at the edges of the field. For this rea- 

 son it appears that there is probably a better lateral connection between 

 the different lenses of the Pottsville rocks than exists between the sepa- 

 rated porous portions of the lower sands. 



ISOLATED POOLS ON EAST-SIDE ANTICLINE 



On the east side of the La Salle anticline are a number of small isolated 

 pools which are of more than ordinary interest to the geologist because of 

 their possible mode of origin. Two of these pools — the Flat Eock and 

 Birds — have their longer axes parallel to each other in a northeast-south- 

 west direction almost at right angles to the axis of the La Salle anticline. 

 For this reason, before detailed studies were made in the field, it was sup- 

 posed that the accumulation in the smaller pools to the east was con- 

 trolled by small cross-anticlines, and that the existence of other similar 

 structural folds might be determined by geological work on surface out- 

 crops. Eecently Dr. John L. Rich has made careful studies of the prob- 

 lem in connection with field-work on the Birds and Vincennes quad- 

 rangles, in cooperation with the United States Geological Survey, and the 

 writer is indebted to Doctor Rich for much material. The four principal 

 productive pools on the east side of the anticline are the New Hebron, 

 Flat Rock, Parker, and Birds, and they are almost, if not entirely, sepa- 

 rated from each other. The principal characteristic of each pool is the 



