OIL AND GAS IN THE CENTRAL BASIN. 7 



like the overlying one, for the reason that oil or gas may be 

 dissipated downward as well as upward. 



Size of the reservoir. The size of the reservoirs in which oil 

 and gas are stored may not be larger than a pea, or it may 

 cover many square miles. Examples of the small ones may be 

 found in many limestones, in which occur cavities filled or 

 partly filled with petroleum. Such limestones are seductive, 

 for they excite prospecting, but seldom yield oil in paying 

 quantity. The cavities may be numerous and all may contain 

 some oil, but they are not connected, and consequently there is 

 no possibility of the oil flowing from one to another and finally 

 into the drill hole, from which it can be pumped to the surface. 



Most of the large reservoirs are in sandstone, the storage 

 space being composed of the infinite number of pores between 

 the sand grains composing the rock. In fact each one of these 

 small pores is a reservoir in itself, but they are all connected, 

 and because of this the oil or gas will flow through the rock 

 a limited distance to a drill hole. The actual distance through 

 which it will flow varies, being short for small pores and low 

 rock pressure, and comparatively great for large pores and 

 high pressure. 



Shape of the reservoir. Where folding is a factor in forming 

 the reservoir, its shape depends upon that of the structural 

 features. If in an anticline, and the anticline be dome shaped 

 the outline of the reservoir will be circular ; but if the anticline 

 should be of the common, elongated kind, the reservoir will be 

 more or less linear in outline. For this reason, geologists and 

 experienced well drillers work out the structure and follow it, 

 not only in trying out an undeveloped field, but if tha rocks 

 prove to be saturated with water, in extending it after a 

 successful well has been drilled. 



But here the layman must be warned against making the 

 common mistake of supposing that the bending of the rock 

 layers conforms to the surface irregularities. The rock layers 

 cannot be supposed to bow upward where there are hills and 

 downward where there are valleys. If they are not level, they 

 perhaps more commonly bow downward in the hills as syn- 

 clines and upward beneath the valleys as anticlines; or more 

 commonly yet, they dip in one direction as monoclines. In the 



