By Eric SharpeTo understand why shale oil plays have become the driving force in the American oil industry, a brief primer on “conventional” oil needs to be understood. For a century, most oil came from the familiar drilling process where a rig bores into the earth until it “strikes” oil, then a pump jack is put into place to draw the oil from the ground. More specifically, “conventional” oil reservoirs come about when three conditions are present: 1. a “source rock” rich in hydrocarbon material –oil or gas, created over millions of years of pressure to organic material, like trees and plants. 2. A “porous rock” above this source rock where oil can seep into. 3. A “cap rock” that prevents hydrocarbons from seeping any further upward. If these conditions are met, a “hydrocarbon trap” is created. A large “trap” is basically an oil field.