| |
|
|
How Oil Forms:
Burial of Sediments:
Petroleum, or oil, forms from the remains of plants and animals that lived in the ocean between 10 to 160 million years ago. When organisms died and sank to the bottom, they were covered in mud, sand, and other mineral deposits. This rapid burial prevented immediate decay, which would normally occur if organisms remained exposed on the sea floor.
The lack of oxygen in the sedimentary layers caused organisms to slowly decay into carbon-rich compounds. These compounds mixed with surrounding sediments and formed source rock, which is a type of fine-grained shale. As more layers were deposited on top of one another, pressure and heat acting on the source rock compressed the organic material into crude oil.
Traped in the Earth:
As time passes, oil travels into rocks that have larger spaces, or pores, to hold it. Limestone and sandstone are two types of rocks with large pores, and they are called porous rocks. The rocks that oil accumulates within are reservoir rock and oil remains within these reservoirs due to rock structures called caps or traps. Cap or trap rock can hold oil within the ground because these rocks are impermeable, which means they lack pores through which oil can travel. Extracting oil from the ground therefore involves drilling through the cap rock in order to access the underlying oil.
Finding Oil:
Discovering the location of oil within the earth is difficult because of the presence of cap rock, which can be miles thick in some locations. Oil geologists study surface rocks and terrain to determine if oil is present underground, but the best evidence comes from various satellite imaging techniques. Oil flows may disrupt earth's gravitational or magnetic field, so gravity meters and magnetometers can detect some oil sources. The most reliable method for finding oil is through the use of shock waves in a process called seismology.
In a seismological survey a shock wave is aimed at the surface of the water or land and the length of time it takes for the waves to reflect back to the sensor is recorded. The speed of the shock wave depends on the type of rock it travels through, and by comparing the travel times to known densities of rock, seismologists can determine what rocks are underground and predict if they might contain oil. Although this technology is superior to other oil detection methods based solely on examining surface rock features, it has only a 10% success rate of finding new oil sites.
Sources:
Freudenrich, Craig. 2005. "How Oil Drilling Works". HowStuffWorks Inc.
|
|
|
|
