GEOLOGICAL PAPERS 83 



2. The sands thin and thicken rapidly, and in some localities 

 pinch out altogether. 



3. The sands are so irregularly interbedded with the shales 

 along the productive zone in some areas as to prohibit extensive 

 collection of oil, gas and water. 



4. The best productive areas have twenty to forty feet of sand 

 and are usually free from large amounts of salt water. 



5. Local dry spots in the midst of very productive territory 

 cannot be attributed to small depressions or knolls in the sand 

 bodies, but rather to the non-porosity of the bed. 



Salt water does not uniformly fill the rocks of the region, 

 although there are many dry strata which are porous. Great 

 quantities of salt water occur upon the limbs of the anticline 

 beyond the productive area and in the Illinois basin to the west. 

 All the lenses of the Robinson sand are well saturated along the 

 definite boundary line on the western side of the pool. The 

 upper lenses are generally barren of water within the pool, while 

 the lower lenses reveal water across the fold and, in some locali- 

 ties, under the oil. Since the oil lies near the top of the lower 

 sand lens, but few wells pass through the oil stratum into the 

 water, for fear of drowning out the oil. 



It is obvious from the position of the water and oil along 

 the La Salle anticline that the water has controlled the accumu- 

 lation of oil in the arch. The water probably has originally per- 

 mitted the oil to migrate long distances up the slope of the 

 Illinois basin into the arch. This was effective for all lenses of 

 the Robinson sand, although the degree of saturation is variable 

 over the crest of the arch. 



LAWRENCE COUNTY. 



The oil field of Lawrence county offered the best opportunity 

 for geological study because of the depth and number of oil 

 horizons and the abundance of records. The field is seventeen 

 miles long and three miles wide. There were 2,180 wells studied, 

 of which 156 or 5^0 per cent were dry. The range of initial pro- 

 duction lies between i and 2,400 barrels. The field trends north- 

 west and southeast, with the northern limit on the Crawford-Law- 

 rence county line. The field changes its course about twenty 

 degrees near Bridgeport. The western edge of the field is well 

 defined like that of Crawford county, and similarly. The eastern 

 edge is irregular. The Lawrence county field is the richest in 



