GEOLOGICAL PAPERS 8" 



The accumulation of oil and gas in their present position may 

 be looked upon as ideal, and is presumably due to the following 

 factors : 



1. There is an extensive anticline with a marked basin on at 

 least one side. 



2. The depressions on both sides of the fold, showing abun- 

 dant water, comprise extensive "feeding" areas for the arch. 



3. The sands are commonly porous, and hence form suitable 

 reser^'oirs. 



4. There are abundant shales and some limestones overlying 

 the sandstones, which probably serve a? impers^ous covers to the 

 reservoirs. 



5. The sands in both limbs of the anticline are abundantly satu- 

 rated with salt water, which is probably instrumental in holding 

 the oil and gas captive in its present position. 



6. Although the general structure of the oil fields is dominated 

 by a major fold, its crest is very irregular and is interrupted 

 by numerous minor domes and transverse depressions, which, 

 together with irregularities in porosit}\ have been instrumental 

 in segregating the pools. 



7. With one exception, the best collection of oil was found 

 over the broad flat areas. The domes over the entire field are 

 logical gas reservoirs, but. contrary to expectation, the largest 

 amounts of gas and oil do not lie at the apexes of the domes, but 

 a short distance below. 



GEOLOGICAL SEOL'E.\XE IX THE MCIXITY OF LA 

 SALLE AS REVE-ALED BY RECEXT DRILLIXG. 



Gilbert H. Cady. 



Within the last two months the State Geological Survey has 

 received the drilling samples from two new wells in the vicinity 

 of La Salle that penetrate strata to the deptli of from 1.200 to 

 1,500 feet. The presentation of the records or logs resulting 

 from the study of these two sets of samples, and their interpre- 

 tation, is the first purpose of this discussion. There were also 

 available for comparison the records of several wells in the 

 vicinit}' of La Salle, some quite recently drilled, and others dating 

 back before 1890. Of some of the latter, the drilling samples 

 have been studied bv Dr. T. A. L'dden. and the results of his 



