Petroleum
Because most hydrocarbons are lighter than rock or water, these often migrate upward through adjacent rock layers until they they either reach the surface or become trapped beneath impermeable rocks, within porous rocks called reservoirs. However, the process is not straightforward since it is influenced by underground water flows, and oil may migrate hundreds of kilometres horizontally or even short distances downward before becoming trapped in a reservoir. Concentration of hydrocarbons in a trap forms an oil field, from which the liquid can be extracted by drilling and pumping.
Three conditions must be present for oil reservoirs to form: first, a source rock rich in organic material buried deep enough for subterranean heat to cook it into oil; second, a porous and permeable reservoir rock for it to accumulate in; and last a cap rock (seal) or other mechanism that prevents it from escaping to the surface. Within these reservoirs fluids will typically organize themselves like a three-layer cake with with a layer of water below the oil layer and a layer of gas above it, although the different layers vary in size between reservoirs.
The vast majority of oil that has been produced by the earth has long ago escaped to the surface and been biodegraded by oil-eating bacteria. Oil companies are looking for the small fraction that has been trapped by this rare combination of circumstances. Oil sands are reservoirs of partially biodegraded oil still in the process of escaping, but contain so much migrating oil that, although most of it has escaped, vast amounts are still present - more than can be found in conventional oil reservoirs. On the other hand, oil shales are source rocks that have never been buried deep enough to convert their trapped kerogen into oil.
