Oil and gas exist in underground traps under tremendous pressure.  When the sandstone or limestone containing the oil or gas is penetrated by the drill bit, , the oil and gas can blast out of the well with great force.

In the early days of oil drilling, there was no way to drill holes and keep this great force under control.  In the picture at the right, a well is shown "blowing wild".  This means that the tremendous gas pressure in the reservoir thousands of feet below the ground is forcing huge amounts of oil up the hole, blowing it into the air.

The circled well is the Mary Sudik #1, drilled by the Indian Territory Illuminating Oil Company in Oklahoma.  On March 26, 1930, this well blew out at a depth of 6,470 feet.  The force blew 20 pieces of 30-foot drill pipe out of the hole!

The  "Wild Mary Sudik" was finally plugged off 11 days after blowing out, through the heroic efforts of dozens of people.  Six years later, the Sudik lease had produced five million barrels of oil.

"Blowouts" have been extremely rare for over 50 years.   Equipment was invented later that allowed the drilling crew to keep the well and the pressure under total control at all times.  

Blowouts are now extremely rare.  A blowout is considered bad for the environment, bad for business, and is a very dangerous situation.  Great effort is made to avoid spilling even a single drop of oil on the ground.

At the right is a modern wellhead capping a well on a clean, well-maintained, and earth-friendly oil property.

Below is a modern, portable drilling rig.

Information provided by: http://www.abix.com