How hydrocarbons were formed

Oil and gas are derived from organic materials that were deposited millions of years ago. Over time, these materials were buried and the soft sediments around them became rock layers. Under the right conditions of heat and pressure, those organic materials can be transformed to oil and gas in what are known as source rocks.




Some of the oil and gas generated in the source rock escaped and flowed through layers of porous rock, such as limestones and sandstones. Individual pockets of hydrocarbons accumulated in porous reservoirs developed when the hydrocarbons were trapped under impermeable rock layers.
Understanding subsurface structures
During the early stages of searching for new fields, Shell explorationists must build up a thorough understanding of the overall basin to evaluate and grade the potential of the petroleum systems. Across the globe, we use consistent and highly effective methods to analyse these systems in which each play is unique. A technique we use to develop this understanding is known as play-based exploration – a technique that has increased our ability to make timely and high quality decisions


Source : http://lout-de-chevalier.blogspot.com/2009/06/how-hydrocarbons-were-formed.html