The layers of rock gave a lot of pressure on the dead organism and also heat them up. Layers of materials built up over time and sediments continue to wash in. Together, it turns the remain to crude oil.
Yes, and the whole process will repeat again. And each time creating more layers as more and more layer piled up with the temperature and pressure increase.
That is the end?
This is because under the ocean, when sediments and mud keep flowing over and deposit. When the deposition is higher than the sea level, it becomes land.