STRUCTURE 663 



"Thus on modern deltas are found in appropriate proportion all the features 

 described as characteristic of the Robinson sand, as determined from the well 

 records. Such an explanation harmonizes the barlike form of the larger sand 

 masses with the channel features and the irregularity of certain of the smaller 

 pools and with the even, thin sand-beds of some of the outliers. According to 

 this explanation, the Flat Rock and Parker pools are sand-bars probably of the 

 same age ; the Birds pool is a larger bar parallel to the first and built pre- 

 sumably at a later time farther offshore. The smaller and more irregular pools 

 maj^ be river-channel deposits, portions of a delta top or front ; smaller, more 

 irregular bars, or lagoons that the delta deposits formed opposite the mouths 

 of tidal inlets." 



The clear-cut boundary of the sand masses on the southeast side and the 

 outfingering lenses toward the northwest seem to bear out the belief that 

 the open sea lay to the southeast and the land to the northwest in Upper 

 Pottsville time. 



If this hypothesis is correct, the prediction of any extension eastward 

 of the Crawford County fields becomes extremely hazardous. There are 

 untested areas fully as large as any of the producing pools mentioned 

 above^ but in most of these untested areas there is absolutely no surface 

 indication upon which the geologist may base a prediction as to the exist- 

 ence of favorable structure, much less predict the presence or absence of 

 sands capable of holding oil. In order to oifset the disadvantage under 

 which the geologist works, the Illinois Geological Survey has published a 

 map which shows the location of all dry holes that have been drilled deep 

 enough to test the Eobinson sand. Thus attention is called to the areas 

 which 3^et remain to be tested. 



HEAVY OIL IN FLAT ROCK POOL 



In parts of the Flat Eock pool the presence of oil of 19° Baume gravity, 

 as compared with much lighter oils in other parts of the same sand in 

 Crawford County, has given rise to much speculation as to its cause. 

 Eecent detailed studies of the well records seem to show that in most 

 cases where the heavy oil exists there is present above the regular oil sand 

 a higher lenticular mass of sand which has a direct connection with the 

 underlying oil sand. In practically all cases the upper lens is filled with 

 gas, which suggests that the lighter materials have been given an oppor- 

 tunity to escape from the regular oil sand, and that the gravity of the oil 

 wliich remains is dependent on the extent to which this action has taken 

 place. Heavy and light oils occur in the same sands and in the same 

 pool, but in all cases they seem to bear a definite relation to the existence 

 of tlic hiii'lier lenses as mentioned above. 



