Many conventional interpretations of carbon isotopic ratios of natural hydrocarbon gases consider that very negative isotopic values of carbon (delta C13) would suggest "biogenic" origin while values that tend to be less negative as source "thermogenic", in the latter case as if the gases were from thermal fractionation of liquid hydrocarbons deposits such as oil. This interpretation has become dogma and too little to discussed about its validity.
As the scientist Thomas Gold affirmed, there is a favorability in isotopic fractionation with respect to carbon 12C, lighter, faster diffusion. Life uses preferentially the carbon 12C. In the migration of methane, this tends to get more and more negative dC13 when it reaches shallow levels (low pressure and temperature) and the reverse at deeper levels in the crust.
Studies of carbon isotopes, of course, are very important but the interpretation of the fractionation is still questionable, especially by the fact that, traditionally, many still continue to imagine that hydrocarbons such as oil and natural gas were intrinsically derived from biological detritus source, i.e., fossil - which is therefore nonsense. Another major problem lies in the confusion caused by use of the expression "organic carbon", "organic chemistry", when dealing with studies of hydrocarbon molecules. Hydrocarbons are primordial compounds and abiotic in origin, most of which was trapped deep in the Earth after the process of accretion. Many chemical reactions occurred on the primary compounds and some of them to reach levels of low pressure and temperature in migration process with different configurations and isotopic fractionation.
It is interesting to note that hydrocarbons are abundant in the solar system and universe. Satellites of Saturn like Titan has huge lakes of liquid hydrocarbons (methane and ethane, mainly).
It is also not recommended using and enjoying graphics that show fields as thermogenic gas / biogenic and mixed. It is obsolete. A more appropriate discussion can be found in work of Thomas Gold (see below) and link to Thomas Gold - Professional Papers:
U.S.G.S. Professional Paper 1570, The Future of Energy Gases, 1993
The table below shows typical landfill gas components. It's interesting to note that ethane or propane gases does not occur in landfill waste although these gases are common in natural gas pools. Methane in landfill is produced by fermentation but microrganisms can not produce gases heavier than methane. This is important evidence to consider about origin of natural hydrocarbons because methane in landfill waste is the unique hydrocarbon gas produced by biological interaction. Natural gases are, of course, abiogenic in origin. The classification of gases such as thermogenic and abiogenic is not suitable and must be abandoned.
Typical Landfill Gas Components
|Component||Percent by Volume||Characteristics|
|methane||45–60||Methane is a naturally occurring gas. It is colorless andodorless. Landfills are the single largest source of U.S. man-made methane emissions|
|carbon dioxide||40–60||Carbon dioxide is naturally found at small concentrations in the atmosphere (0.03%). It is colorless, odorless, and slightly acidic.|
|nitrogen||2–5||Nitrogen comprises approximately 79% of the atmosphere. It is odorless, tasteless, and colorless.|
|oxygen||0.1–1||Oxygen comprises approximately 21% of the atmosphere. It is odorless, tasteless, and colorless.|
|ammonia||0.1–1||Ammonia is a colorless gas with a pungent odor.|
(non-methane organic compounds)
|0.01–0.6||NMOCs are organic compounds (i.e., compounds that contain carbon). (Methane is an organic compound but is not considered an NMOC.) NMOCs may occur naturally or be formed by synthetic chemical processes. NMOCs most commonly found in landfills include acrylonitrile, benzene, 1,1-dichloroethane, 1,2-cis dichloroethylene, dichloromethane, carbonyl sulfide, ethyl-benzene, hexane, methyl ethyl ketone, tetrachloroethylene, toluene, trichloroethylene, vinyl chloride, and xylenes.|
|sulfides||0–1||Sulfides (e.g., hydrogen sulfide, dimethyl sulfide, mercaptans) are naturally occurring gases that give the landfill gas mixture its rotten-egg smell. Sulfides can cause unpleasant odors even at very low concentrations.|
|hydrogen||0–0.2||Hydrogen is an odorless, colorless gas.|
|carbon monoxide||0–0.2||Carbon monoxide is an odorless, colorless gas.|
Source: Tchobanoglous, Theisen, and Vigil 1993; EPA 1995