Inorganic formation

Some methane may have been produced by inorganic processes. The original source of Earth’s carbon was the cosmic debris from which the planet formed. If meteorites are representative of this debris, the carbon could have been supplied in comparatively high concentrations as hydrocarbons, such as are found in the carbonaceous chondrite type of meteorites. Continuous outgassing of these hydrocarbons may be taking place from within Earth, and some may have accumulated as abiogenic gas deposits without having passed through an organic phase. In the event of widespread outgassing, however, it is likely that abiogenic gas would be too diffuse to be of commercial interest. Significant accumulations of inorganic methane have yet to be found.

The helium and some of the argon found in natural gas are products of natural radioactive disintegration. Helium derives from radioisotopes of thorium and the uranium family, and argon derives from potassium. It is probably coincidental that helium and argon sometimes occur with natural gas; in all likelihood, the unrelated gases simply became caught in the same trap.

The geologic environment

Like oil, natural gas migrates and accumulates in traps. Oil accumulations contain more recoverable energy than gas accumulations of similar size, even though the recovery of gas is a more efficient process than the recovery of oil. This is due to the differences in the physical and chemical properties of gas and oil. Gas displays initial low concentration and high dispersibility, making adequate cap rocks very important.

Principal types of petroleum traps.

Natural gas can be the primary target of either deep or shallow drilling because large gas accumulations form above the oil window as a result of biogenic processes and thermal gas occurs throughout and below the oil window. In most sedimentary basins the vertical potential (and sediment volume) available for gas generation exceeds that of oil. About a quarter of the known major gas fields are related to a shallow biogenic origin, but most major gas fields are located at intermediate or deeper levels where higher temperatures and older reservoirs (often carbonates sealed by evaporites) exist.