Locating Oil

Whether employed directly by an oil company or under contract from a private firm, geologists are the ones responsible for finding oil. Their task is to find the right conditions for an oil trap — the right source rock, reservoir rock and entrapment. Many years ago, geologists interpreted surface features, surface rock and soil types, and perhaps some small core samples obtained by shallow drilling. Modern oil geologists also examine surface rocks and terrain, with the additional help of satellite images. However, they also use a variety of other methods to find oil. They can use sensitive gravity meters to measure tiny changes in the Earth’s gravitational field that could indicate flowing oil, as well as sensitive magnetometers to measure tiny changes in the Earth’s magnetic field caused by flowing oil. They can detect the smell of hydrocarbons using sensitive electronic noses called sniffers. Finally, and most commonly, they use seismology, creating shock waves that pass through hidden rock layers and interpreting the waves that are reflected back to the surface.

In seismic surveys, a shock wave is created by the following:

  • Compressed-air gun– shoots pulses of air into the water (for exploration over water)
  • Thumper truck– slams heavy plates into the ground (for exploration over land)
  • Explosives– detonated after being drilled into the ground (for exploration over land) or thrown overboard (for exploration over water)

The shock waves travel beneath the surface of the Earth and are reflected back by the various rock layers. The reflections travel at different speeds depending upon the type or density of rock layers through which they must pass. Sensitive microphones or vibration detectors detect the reflections of the shock waves — hydrophones over water, seismometers over land. Seismologists interpret the readings for signs of oil and gas traps.

Once geologists find a prospective oil strike, they mark the location using GPS coordinates on land or by marker buoys on water.

Oil Drilling Preparation

Once the site has been selected, scientists survey the area to determine its boundaries, and conduct environmental impact studies if necessary. The oil company may need lease agreements, titles and right-of way accesses before drilling the land. For off-shore sites, legal jurisdiction must be determined.

After the legal issues are settled, the crew goes about preparing the land:

  • The land must be cleared and leveled, and access roads may be built.
  • Because water is used in drilling, there must be a source of water nearby. If there is no natural source, the crew drills a water well.
  • The crew digs a reserve pit, which is used to dispose of rock cuttings and drilling mud during the drilling process, and lines it with plastic to protect the environment. If the site is an ecologically sensitive area, such as a marsh or wilderness, then the cuttings and mud must be disposed of offsite — trucked away instead of placed in a pit.

Once the land has been prepared, the crew digs several holes to make way for the rig and the main hole. A rectangular pit called a cellar is dug around the location of the actual drilling hole. The cellar provides a work space around the hole for the workers and drilling accessories. The crew then begins drilling the main hole, often with a small drill truck rather than the main rig. The first part of the hole is larger and shallower than the main portion, and is lined with a large-diameter conductor pipe. The crew digs additional holes off to the side to temporarily store equipment — when these holes are finished, the rig equipment can be brought in and set up.

Depending upon the remoteness of the drill site and its access, it may be necessary to bring in equipment by truck, helicopter or barge. Some rigs are built on ships or barges for work on inland water where there is no foundation to support a rig (as in marshes or lakes).

In the next section, we’ll look at the major systems of an oil rig.