6 RESOURCES OF TENNESSEE. 



delivered to the surface, that drillers in quest of these products 

 are on the constant lookout for "oil sands" ; but they call any 

 rock that yields oil a "sand". 



The fine-grained, or rather the close textured rocks that 

 contain oil and gas, are limestones and shales. Water, like oil 

 and gas, will flow through the coarse-grained sandstones and 

 conglomerates, and many of these are the source of water 

 supply. But if water in quantity is found in close textured 

 limestone or in shale, it must come from joints. The same is 

 true of oil and gas. Such rocks are not considered so desirable 

 as sandstones, for reservoirs. 



Oil and gas nearly always occur in stratified rocks, by which 

 is meant those that are in layers or beds. Igneous rocks are 

 not in well defined layers and but few are known to contain oil 

 and gas. The best conditions in rocks saturated with water, 

 occur where the rock beds are bowed upward, forming 

 anticlines. Where the rocks are dry, the oil tends to move 

 down any possible slope and accumulate in the troughs or 

 synclines. Hence in regions of saturated rocks, the anticlines 

 are sought by experienced oil men, while in regions of dry 

 rocks, synclines are sought. Since it is more common to find 

 the rocks saturated than dry, attention is mainly directed to the 

 anticlines. 



Another essential for a reservoir is that the open rock be 

 covered with a "tight" or close textured one, so as to seal the oil 

 and gas in. Without such a covering, these elusive products 

 will escape upward and be scattered through the overlying 

 rocks in inappreciable amounts, or what is more probable in 

 the case of gas, will be given off into the air above. 



It seems not improbable that nearly all stratified rocks 

 originally contained organic matter that has produced more or 

 less gas or oil, but if so, most of these have given up their 

 products by dispersion to other rocks and to the atmosphere, 

 from want of adequate sealing-in conditions. In the case of 

 oil under such conditions, much of the gaseous matter has 

 escaped from it, leaving asphalt and its kindred products, such 

 as albertite, ozokerite, grahamite, and gilsonite. 



Yet another condition usually given as an essential for a 

 reservoir, is that the open rock be underlain by a tight layer, 



