PAPERS OX GEOGRAPHY AND GEOLOGY 377 



ticlinal uplift in southeast Illinois. The producing area 

 is subject to certain limitations toward the. north, due 

 partly to the pitch of the axis. Most of the oil producing 

 horizons come up to the surface towards the north and 

 have either been eroded or were never deposited. 



Other small anticlines or local folds or domes are 

 known in the gently rising beds to the southwest and 

 western parts of the State, some of which have proved 

 productive. Xo production has been found in Illinois as 

 yet that is not in some way connected with anticlinal 

 structure or doming of the strata. 



SHALE COXDITIoX> AND POSITION IX THE BOCK SECTIi "'X 

 SHALE AS A SOURCE 



Oil is generally believed to have had its origin in shales 

 containing certain kinds of organic material which have 

 been converted by pressure and some heat to the hydro- 

 carbon constituents of oil particles. These were collect- 

 ed by the aid of circulating fluids and rock movements 

 and accumulated in favorable traps in reservoir rocks. 



The presence of the necessary shales as a source 

 either in juxtaposition to the reservoir rock or connected 

 with it at some time by porous channels is one of the 

 requisites to bear in mind when considering an area for 

 possible oil accumulation. The exact nature of the shale 

 necessary as a source, its history of sedimentation and 

 subsequent alteration are subjects about which the oil 

 geologist should know to enable him to apply more fully 

 the science of geology to the economic problem of find- 

 ing oil accumulations. It is hoped that the results of 

 present studies of sedimentation, now being undertaken 

 by some of the leading geologists, will Vie especially help- 

 ful to the oil geologist. 



At present, however, ahead of actual drilling, certain 

 areas can be considered as having very slight chance for 

 oil when we know that the underlying or associated rock 

 section does not contain adequate amounts of shale. 



IMPERVIOUS CAPPIX'G OF RESERVOIR ROCKS 



Shale as an impervious medium overlying reservoir 

 rocks has an important role in the accumulation of oil 



