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Fluid-composition traps are controlled by the physical or chemical properties of the trapped fluids themselves. The insolubility, immiscibility, and density contrasts between hydrocarbons and water are major factors that allow hydrocarbons to concentrate into subsurface accumulations. Therefore, in its purest sense, all hydrocarbon accumulations have fluid-composition trapping elements. However, for classification purposes, fluid-composition traps are those in which unique properties of the hydrocarbons provide trapping mechanisms.

Fluid-composition traps are controlled by the physical or chemical properties of the trapped fluids themselves. The [[Wikipedia:Solubility|insolubility]], [[Wikipedia:Miscibility|immiscibility]], and [[Wikipedia:Density|density]] contrasts between [[hydrocarbon]]s and water are major factors that allow hydrocarbons to concentrate into subsurface accumulations. Therefore, in its purest sense, all hydrocarbon accumulations have fluid-composition trapping elements. However, for classification purposes, fluid-composition traps are those in which unique properties of the hydrocarbons provide trapping mechanisms.

==Classes and families==

==Classes and families==

Result from a chemical property of the trapped oil or gas.

Result from a chemical property of the trapped oil or gas.

:'''Class:''' Viscosity traps

:'''Class:''' Viscosity traps

:Petroleum can be trapped by the presence of barriers created by highly viscous oil (asphalt, tar) or solid hydrocarbons (albertite, gilsonite, or grahamite).

:Petroleum can be trapped by the presence of barriers created by highly [[Wikipedia:Viscosity|viscous]] oil ([[asphalt]], [[tar]]) or solid hydrocarbons ([[albertite]], [[gilsonite]], or [[grahamite]]).

::'''Subclass:''' Tar seals

::'''Subclass:''' Tar seals

:::'''Family:''' Bacterial degradation

:::'''Family:''' Bacterial degradation

:::Updip tar seal created by bacteria degradation of the hydrocarbons, preferentially removing the lighter fractions.

:::Updip tar seal created by [[bacteria degradation]] of the hydrocarbons, preferentially removing the [[lighter fractions]].

::'''Subclass:''' Disseminated

::'''Subclass:''' Disseminated

::Tar or asphalt fills the pores of shales, sands, or carbonates. These traps are often called [[tar sands]], oil shales, bituminous sandstones, or bituminous limestones.

::Tar or asphalt fills the pores of [[shale]]s, [[sandstone]]s, or [[carbonate]]s. These traps are often called [[tar sands]], [[oil shale]]s, bituminous sandstones, or bituminous limestones.

::'''Subclass:''' Veins

::'''Subclass:''' Veins

::The solid, essentially infusible form of petroleum is called pyrobitumen. It occurs in veins [[length::1 mm]] to [[length::8 m]] across and appears to have been injected or is a dead seepage.

::The solid, essentially infusible form of petroleum is called [[pyrobitumen]]. It occurs in veins [[length::1 mm]] to [[length::8 m]] across and appears to have been injected or is a dead seepage.

:'''Class:''' Coal-bed methane

:'''Class:''' Coal-bed methane

:Coal beds trap large volumes of methane, where most is adsorbed onto internal surfaces of micropores or along cleat faces. Coalification generates methane along with water, carbon dioxide, and carbon monoxide.

:[[Coal bed]]s trap large volumes of [[Wikipedia:Methane|methane]], where most is [[Wikipedia:Adsorption|adsorbed]] onto internal surfaces of [[micropore]]s or along [[cleat face]]s. [[Coalification]] generates methane along with water, [[Wikipedia:Carbon dioxide|carbon dioxide]], and [[Wikipedia:Carbon monoxide|carbon monoxide]].

==See also==

==See also==