PAPERS OX GEOGRAPHY AND GEOLOGY 379 



presence or actual location of such local petroliferous 

 lenses from surface observations on outcrops, or from 

 the data supplied by former neighboring wells. But hori- 

 zons containing such lenticular " sands" likely to occur 

 in Illinois are fairly well known and in several localities 

 supply an extra sand chance in addition to the regular 

 sands or more prevalent porous horizons that have pro- 

 duced oil. 



PRESENCE OF LOCAL FOLDS AND UNCONFORMITIES 



The stratigraphic position and the nature of the shale 

 and reservoir rocks may be ideal, but in Illinois at least 

 the oil accumulations are associated with anticlinal fold- 

 ing or doming of the beds. This, then, is of vital import- 

 ance in recommending the most likely places for oil tests. 

 In addition to the extension of the old fields on the La- 

 Salle-St. Francisville uplift there are many local minor 

 structural anticlines and domes in the south and west 

 parts of the State. The knowledge of the presence and 

 location of these structures is one of the keys to the 

 wise location of exploratory test wells. 



In Illinois, as elsewhere, there have been time intervals 

 of erosion accompanied in some instances by some tilting 

 or warping of the strata between successive depositions 

 of sedimentary strata. The bedding of the overlying 

 strata may not, then, conform exactly with the bedding 

 or the eroded surface of the underlying formations. This 

 sometimes results in favorable traps in reservoir rocks 

 suitably capped or pinched in by impervious bods, but 

 they can not be predicted directly from knowledge of the 

 dip of the strata outcropping at the surface. The pres- 

 ence of such unconformities at certain horizons is known, 

 however, from other geological evidence, and their influ- 

 ence on oil accumulation can be more or less evaluated 

 by the oil geologist. 



The geological information that the oil geologist can 

 bring to bear on the oil problems of Illinois can be ob- 

 tained in part by studies and surveys of rock outcrops, 

 but in very large measure it must be obtained through the 

 study of subsurface records and samples of cuttings 

 taken from drilling wells. The State Geological Survey 



