The deepest holes of all are made for oils, and they go down to as much as 25000 feet. But we do not need to send men down to get the oil out, as we must with out mineral deposits. The holes are only borings, less then a foot in diameter. My particular experience is largely in oil, and the search for oil has done more to improve deep drilling than any other minning activity. When it has been decided where we are going to drill, we put up at the surface an oil derrick. It has to be tall because it is like a giant block and tackle, and we have to lower into the ground and haul out of the ground great lengths of drill pipe which is rotated by an engine at the top and fitted with a cutting bit at the bottom.
The geologist needs to know what rocks the drill has reached, so every so often a sample is obtained with a coring bit. It cuts a clean cylinder of rock, from with can be seen the strata the drill has been cutting through. Once we get down to the oil, it usually flow to the ground because the great pressure, either from gas or water, is pushing it. This pressure is must be under control, and we control it by means of the mud which we circulate down the drill pipe. We endeavour to avoid the old, romantic idea of a gusher, which wastes gas or oil. We want it stay down the hole until we can lead it off in a controled manner.
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