PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 351 



places whore there are marks of disturbance and displacement of rocks. 

 The cavities have probably been caused sometimes by uplifts, and some- 

 times by erosion and the dissolving' action of water; but, wliatevcr maybe 

 their origin, tliey are not usually of g'reat horizontal extent. 



The Wells Vary in Depth. 



It is seldom that the neig'hboring wells strike oil at the same depth, 

 whether the strata be horizontal or dipping. It is one chance out of many 

 to strike oil at all, even in the neighborhoods where it exists in abundance. 

 The drill, as it enters the cavity, sinks variously, from four or five inches 

 to as many feet, sometimes sticking fast, as if between the oblique sides of 

 a narrow fissure. But there are facts connected with the history of oil 

 wells, particularl}^ in their intermittent action, and their interference with 

 one another which serve to show the existence, in many cases, of systems 

 of cavities connected together by channels cf communication more or 

 less free, running' sometimes along- the strata and sometimes across them. 

 The productiveness of a well depends on its entering either one of the main 

 reservoirs, or some of its important connections. 



Isolated Oil Cavities. 



Let us begin with the most simple case, that of single or isolated oil 

 cavity running obliquely upward. Every collection of oil is accompanied 

 with varying quantities of gas and water, the gas occupying of course the 

 top of the cavity, and the water the bottom, according to the order of spe- 

 cific gravities. 



1. Suppose a well is bored to enter the top of the cavity. The gas being 

 in a high state of tension escapes, sometimes with explosive violence, car- 

 rying out with it whatever water may be collected in the boring. If the 

 water flows in rapidly, as is usually the case, the oil on its surface is driven 

 into the lower end of the tube, and may be pumped out until the water 

 reaches the tube, when mixed oil and water will be drawn. It often hap- 

 pens that the water rises faster than it can be thus exhausted, and the oil, 

 dri%'en into tlie very top of the cavity, is lost until the water can be reduced 

 by machinery of great working power. But as it cannot be driven below 

 the mouth of the tube, unmixed oil cannot be obtained from the well. In 

 all wells from whicli the gas has escaped there is, ultimately, a saving of 

 oil, if the oil is pumped out as rapidly as possible before the intrusion of 

 water. 



2. Suppose the boring enters the cavity at a lower point than where the 

 gas is held, and enters the oil. In this case the oil enters first in the tube 

 to a height depending on the tension of the gas above it, a mode of action 

 which is illustrated by the familiar apparatus called the fountain with con- 

 densed air. Sometimes it is thrown into the air to the distance of thirty or 

 forty feet, and large quantities are wasted. If the oil continues to be 

 ejected till its surface in the cavity descends to the mouth of the tube, the 

 fact first becomes known by a gurgling sound and a spurting action, and 

 the gas, or a greater portion of it, escapes, after which the pump becomes 

 necessary, and the same series of actions take place as in the first case. 



