Crude oil reservoir.

           Crude oil reservoirs form: a source rock rich in hydrocarbon; a porous and             permeable reservoir; and a cap rock.                                                                           With these reservois, fluid will typically organize themselves like a three-layer cake with a layer or water below the oil layer and a layer of gas above it.                                        The reactions that produce oil and gas are often modeled as first order breakdown          reactions, where hydrocarbons are broken down to oil and natural gas by a set of reactions.                                                                                                                      "Natural lift" production methods that rely in a  natural reservoir pressure to force the oil to the surface. The natural pressure in most reservoirs eventually dissipates. Then the oil must be extracted by "artificial lift".