How is oil formed?
There are two theories as to how oil is formed. The most widely accepted theory, the biogenic theory, begins with plants using solar energy to convert carbon dioxide and water into oxygen and carbohydrates through a process known as photosynthesis. When the plants die, the sediments containing them become buried and, as the depth of burial increases, heat and pressure transform the carbohydrates into hydrocarbons. This takes place in source rocks, usually very fine-grained rocks known as black shales. Coal is a solid hydrocarbon derived from land plants. Oil is a liquid hydrocarbon derived primarily from simple marine plants and animals, and natural gas is a gaseous hydrocarbon derived from either terrestrial or marine materials at a higher temperature and pressure than coal or oil.
The other theory of oil formation, the abiogenic theory, speculates that hydrocarbons were trapped inside the earth as it formed and are migrating to the surface.
Once the oil is formed, continued pressure from overlying rock strata forces the oil to migrate through permeable rock layers until it is trapped in reservoirs of porous sedimentary rock such as sandstone or limestone, or until it escapes at the surface. There are several types of traps:
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Normal fault – Normal fault traps occur where reservoir rock on one side of the fault is positioned against impermeable rock on the other side of the fault.
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Thrust fault – Thrust fault traps occur where reservoir rock overlain by impermeable cap rock has first been folded, then thrust-faulted over itself.
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Stratigraphic pinch-out – Stratigraphic pinch-outs occur where reservoir rock loses its porosity due to cementing or decreased grain size, or where reservoir rock gradually thins out and is surrounded by impermeable rock.
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Reef – Ancient reefs built by corals and other communal organisms often develop porosity that, if sealed by impermeable rock, forms prolific reservoirs. Porous rocks draping over the reef may form separate reservoirs.
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Anticline – Compression folds rocks into anticlines (hills) and synclines (valleys). If reservoir rock is overlain by impermeable rock, traps form at the crests of the anticlines.
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Salt dome – Salt domes occur when salt at depth is forced toward the surface by the weight of surrounding rock. As the salt deposits bulge upward, traps are formed in upturned reservoir rocks flanking the domes and folded reservoir rocks overlying the domes.
In a reservoir containing more than one fluid, natural gas overlies crude oil which overlies water because of density stratification.
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