Formation of natural gas

[Richard D + 86]

In the absence of air,the microbial decomposition of organic matter produces approximately equal volumes of methane and carbon dioxide. Can anyone explain to me how it is that natural gas fields usually contain little carbon dioxide? The carbon dioxide might possibly be removed by the slow reaction with minerals such as olivine. At high pressure,carbon dioxide might dissolve in the water content of sediments. In that case,reduction in pressure as the natural gas was taken out would result in increasing carbon dioxide content of the produced gas.

[Meredith Poor + 895]

A keyword search on 'wellhead gas composition' brings up these comments:

Important impurities found in natural gas:

Water: Most gas produced contains water, which must be removed. Concentrations range from trace amounts to saturation.

Sulfur species: If the hydrogen sulfide (H2S) concentration is greater than 2 to 3%, carbonyl sulfide (COS), carbon disulfide (CS2), elemental sulfur, and mercaptans may be present.

Mercury: Trace quantities of mercury may be present in some gases; levels reported vary from 0.01 to 180 μg/Nm3. Typically, the mercury level in pipeline gas should be reduced to 0.01 μg/Nm3.

Diluents: Although the gases shown in Figure 12.2 are typical, some gases have extreme amounts of undesirable components. For example, some wells in Colorado contain as much as 92% carbon dioxide. High hydrogen sulfide contents (e.g., in Alberta, Canada), and nitrogen contents (e.g., in Texas) have also been observed.

Oxygen: Some gas-gathering systems in the United States operate below atmospheric pressure. As a result of leaking pipelines, open valves, and other system compromises, oxygen is an important impurity to monitor. A significant amount of corrosion in gas processing is related to oxygen contamination.

 

When CO2 is mixed with water, it is mildly acidic. This may cause it to react with minerals in rocks, as you point out, although in some cases these might simply be carbonates (CaCO3, for example). Reactions with Olivine 'reduce' the CO2 to form methane and Serpentine.

[turbguy + 1,543]

Since planetary missions observe some moons to have seas of liquid methane, with no apparent biological origin, I wondered if some earth-bound methane might be of similar "origin".

This idea does not seem to have much "internet-support".

 

[Symmetry + 109]

18 minutes ago, turbguy said:

Since planetary missions observe some moons to have seas of liquid methane, with no apparent biological origin, I wondered if some earth-bound methane might be of similar "origin".

This idea does not seem to have much "internet-support".

 

Some early methane perhaps, but the carbon cycle has probably chewed up an reused that carbon many times over since then.

The associated fossils and other complex hydrocarbons pretty much rule out extraterrestrial origin.

[turbguy + 1,543]

2 hours ago, Symmetry said:

Some early methane perhaps, but the carbon cycle has probably chewed up an reused that carbon many times over since then.

The associated fossils and other complex hydrocarbons pretty much rule out extraterrestrial origin.

I get to wonder "how deep" the carbon cycle go go below the current surface. 

[Meredith Poor + 895]

2 hours ago, turbguy said:

I get to wonder "how deep" the carbon cycle go go below the current surface. 

Deep into the mantle. Sedimentary rock contains a lot of calcium carbonate (CaCO3) from seashells. This sinks into the mantle in subduction zones. The heat in the mantle breaks up the CaCO3 the same way cement plants 'reduce' the CaCO3 to to CaO (lime) and CO2.

At high enough temperatures carbon and oxygen dissociate. If water is broken up into H2 and O2 radicals, some hydrogen will react with carbon to form methane or other alkanes. If this makes it to the surface, it is most likely to be released via volcanoes. 

[Gerry Maddoux + 3,627]

^

A+, Meredith!