The process of merging hydrogen and carbon (from carbon dioxide and water) is accomplished through photosynthesis. Organisms like diatoms, a one celled plant, can start the process of creating organic matter ("A" in the diagram).

As the carbon and hydrogen are trapped in the food chain for nourishment of other organisms, crude oil is not produced immediately. However, as the death of these organisms and the burial of the organic matter in sediments is complete, crude oil will start to form (black layers in "B")

The different pathways that the organic matter will go through will determine the type of oil and gas produced.