When plankton die, their remains, along with soil and sand, gradually accumulate on the bottom of the ocean.
Over millions or tens of millions of years, the dead plankton accumulate sand and mud on top of the dead plankton. The dead plankton gradually burrow deep into the soil. At some point, the dead plankton turned into "oil" through the action of bacteria in the soil and the heat of the ground.
This oil then passes through cracks in the earth and collects in places where it is easy to accumulate, and these are oil fields. It is said that the reason why so much oil is found in the Middle East today is because there used to be a wide, shallow sea called the Tethys Sea in the region. It is thought that the sea was just right for dead plankton to accumulate and the conditions were right for oil to be easily produced.

There is another important condition for the formation of oil fields. This is a geological structure called an anticlinal structure. This is a structure in which the underground strata are convex and curved, and oil accumulates in the upper part of the convexity. Oil can only be found in areas where there is an anticlinal structure.