356 ILLINOIS STATE ACADEMY OF SCIENCE 



different from those which are concealed that what 

 is true of them at one place is not true at the other. 

 It is obvious that only a detailed, careful, and extensive 

 study of the cuttings from wells drilled in such covered 

 areas will enable the geologist to know the formations 

 underground. The geologist desires to know, however, 

 not only what the formations underground are like, but 

 also what their structure is, whether they are flat-lying 

 or folded. Where it is possible, the structure may be 

 determined from a key bed of some particular sort which 

 is encountered in the wells. In case there are no such 

 beds, however, it is necessary to rely on some other means 

 of identifying beds other than by gross lithological simi- 

 larity. This is where the microscope finds its usefulness, 

 and with its aid an attempt is made to determine what 

 characteristics, either lithological or paleontological, can 

 be used to identify certain horizons from which the struc- 

 ture of the beds may be interpreted. 



Aside from the purely scientific value of the infor- 

 mation obtained bearing on the subsurface structure of 

 beds, a practical worth is apparent. Oil and gas are 

 found associated with certain rock structures, and in the 

 scientific prospecting for oil and gas and the successful 

 exploitation of known deposits, it is desirable to know 

 the structure of the rock from which the oil or gas is 

 obtained. If the microscopic study of sediments, either 

 lithological or paleontological, aids in securing such data, 

 it renders a valuable service. 



The study of cuttings, however, particularly the litho- 

 logic study, is not confined to the identification of beds 

 of like stratigraphic position. The increase in the 

 amount of sand in a shale formation from well to well 

 may indicate the approach to an old shore line, where 

 sand lenses, possible oil reservoirs, may be expected. Of 

 a somewhat different nature are the determinations of 

 porosity and the state of aggregation of oil- and water- 

 bearing sands, which have, in cases at least, had a 

 definite bearing on successful and maximum oil pro- 

 duction. 



In the oil fields of the southwestern part of the United 

 States the microscopic study of cuttings has been found 



