All hydrocarbons (crude oil and natural gas) are created from a process of decay of microscopic plants and organisms that lived in the oceans millions of years ago. When these organisms died, they sank to the bottom of the ocean and were gradually covered by several layers of sediment (deposits left by water or wind). The heat and pressure between these layers increased as they were more deeply buried and compacted. Over a long period, the combination of this heat and pressure eventually turned the decomposing materials into the hydrocarbons we know today.
The rock layers rich in organic matter in which the hydrocarbons formed are called "source rocks". Once the hydrocarbons are formed, the surrounding pressure usually forces the hydrocarbons to migrate upward from the compact source rock to more porous and permeable rock layers, known as "reservoir rocks". Usually, reservoir rocks are rocks such as sandstones and carbonates. These rocks have sufficient porosity and permeability to allow the movement of fluids such as crude oil, natural gas and water through the rock. If there are no permeable rocks, the hydrocarbons often continue their migration to the surface and escape. However, it is more usual for this migration to the surface to be blocked by an impervious rock layer or other geological formation. The hydrocarbon becomes trapped and accumulates to form a hydrocarbon deposit. The stored hydrocarbon deposits, in rocks that have sufficient permeability to allow them to migrate naturally in and through them, are often referred to as basins or conventional deposits.
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