Crude oil and natural gas are created when plankton die, fall to the sea floor, are trapped in sediment, and then undergo immense pressure and heat through millennia of accumulating debris. The oil and gas become trapped in porous rock (known as reservoir rock) surrounded by impervious rock (known as cap rock).
Geologists look for signs of these conditions in order to discover new reservoirs. They may look at satellite images and collect surface samples to start. When oil flows it creates slight disruptions in the Earth’s gravitational and magnetic fields, which sensitive magnetometers and gravity meters can pick up on. Petroleum also produces a distinct smell, which electronic noses can pick up on if they’re sensitive enough. Lastly seismologists create vibrations in the earth to locate potential sites.
Once a site is located, the oil company gets any permissions required, checks the environmental impact, clears the surrounding arlliea according to regulation guidelines, and gets started. A blowout preventer is used to close off the hole in an emergency.
Drilling is done in stages, with each stage using a progressively smaller drill. The first stage of drilling is usually done before the rig is set up, using a truck equipped for the purpose. As they drill, drill mud is pumped into the hole to expel the cut bits of rock from the hole, allowing the drill to continue unimpeded. The mud also serves to cool the drill and to keep the hole from caving in on itself. As the drill goes deeper, new segments of pipe are added back at the rig. When the desired depth is reached, the drill then pulls out and is replaced by casing. Cement is then pumped down the hole, expelling the mud and holding the casing in place.
Once the reservoir is reached and the well is deep enough, a perforating gun pokes holes in the casing and a tube is snaked through. A packer is used to seal the outside of the tubing and a Christmas tree (another device) is placed on top to control the well’s output. Special fluid containing acid or proppants is then used to dissolve the reservoir rock, allowing the trapped petroleum to flow. And with that, the job is done.