Here's a breakdown:

* Petroleum Formation: Petroleum is formed over millions of years from the decomposition of tiny marine organisms (plankton and algae) that sink to the ocean floor. This organic matter gets buried under layers of sediment, and under heat and pressure, it transforms into hydrocarbons (crude oil and natural gas).

* Reservoir Rocks: These hydrocarbons accumulate in porous and permeable rock formations called reservoir rocks, often sandstone or limestone, beneath the ocean floor.

* Traps: To be commercially viable, these reservoirs need to be trapped by impermeable rock layers that prevent the oil and gas from escaping. These formations are called traps.

Therefore, while petroleum originates in the ocean, it's found in the layers of rock beneath the ocean floor, not directly in the ocean itself.