Page 29 - Handbook of Natural Gas Transmission and Processing Principles and Practices

1.3. Natural Gas Origin and Sources
There are different theories as to the origins of fossil fuels. The most widely accepted theory of the
origin of natural gas asserts that it came from organic matter (the remains of land and aquatic
plants, animals, and microorganisms) that was trapped within sediments as they were deposited
and transformed over long periods of time into their present form. Two main mechanisms, namely
biogenic and thermogenic, are responsible for the degradation of the original fossil organic material
in sediments (Rojey et al., 1997). Biogenic gas is formed at shallow depths and low temperatures
due to the action of bacteria on the organic debris accumulating in the sediments. In contrast,
thermogenic gas is formed at greater depths by degradation of organic matter, called kerogen,
accumulated in fine-grained sediments, especially clays and shales. This degradation occurs
through the combined effects of temperature and pressure. Thermogenic gas is believed to be
produced through two mechanisms, namely, direct thermal cracking of sedimentary organic matter
and secondary thermal cracking of oil that is formed in the first stage. The former is called the
primary thermogenic gas that coexists with oil, while the latter is called secondary thermogenic gas
that coexists with an insoluble solid matter, called pyrobitumen. Both mechanisms involve thermal
cracking with some degree of sustained pressure, mainly through the weight of the sedimentary
formation. Little information is available on the time required to generate thermogenic gas other
than the general belief that it is a very long time.
Hydrocarbons are generated in “source rock” and often migrate to subsurface formations called
“reservoir rock.” The quality of a reservoir rock is determined by its two principal properties:
porosity and permeability. Porosity is the void space between the grains, and indicates the rock's
capacity to contain liquid or gaseous hydrocarbons. Permeability is the measurement of the rock's
ability to transmit the oil or gas.
Natural gas resources differ by the geological characteristics of their reservoir rock. In fact,
natural gas comes from both “conventional” and “unconventional” geological formations (Fig. 1.1).

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