How is natural gas formed?
There are two theories as to how natural gas is formed. The most widely accepted theory, the biogenic theory, maintains that natural gas formation begins with photosynthesis, where plants use energy from the sun to convert carbon dioxide and water into oxygen and carbohydrates. The remains of these plants and the animal forms that consume them are buried by sediment and as the sediment load increases, heat and pressure from burial converts the carbohydrates into hydrocarbons. Natural gas formation takes place in source rocks, usually fine-grained black shales. Continued pressure from burial forces the natural gas to migrate from source rocks into more porous and permeable rock such as sandstone and limestone, which, if overlain by impermeable strata such as shale, form reservoirs that contain the gas.
The other theory of natural gas formation, the abiogenic theory, speculates that hydrocarbons were trapped inside the earth as it formed and are migrating to the surface.
There are several types of traps.
- 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. - Thrust fault
Thrust fault traps occur where reservoir rock overlain by impermeable cap rock has first been folded, then thrust-faulted over itself. - Stratigraphic pinch-out
Stratigraphic pinch-outs occur where reservoir rock loses its porosity due to cementing or other diagenetic processes, or where reservoir rock gradually thins out and is surrounded by impermeable rock. - 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. - 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. - 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 oil which overlies water because of density stratification.
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