Isotopic Composition of Hydrocarbon Gases
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 It is obsolete.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:
Interpretations Based on the Carbon Stable Isotopes
from: The Origin of Methane (and Oil) in the Crust of the Earth
Thomas Gold
U.S.G.S. Professional Paper 1570, The Future of Energy Gases, 1993
Carbon has two stable isotopes; 12C (6 protons, 6 neutrons) and 13C (6 protons, 7 neutrons). The natural carbon on the Earth contains predominantly 12C , and 13C is mixed in at a level of approximately 1 percent. This mixing ratio must have been determined in the nuclear processes in the stars that cooked up the elements and eventually supplied them to form the planets. There are no processes that could occur on the planets that would be able to change this ratio greatly. Only small variations can be produced, not by any effects on the nuclei themselves, but only by processes that show a slight preference and select in favor of either the light or the heavy isotope.
The study of the distribution of the carbon isotopes in relation to petroleum and natural gas has a very extensive literature. We shall discuss here only one aspect of it: can isotope measurements determine whether a hydrocarbon compound was derived from biological material or whether it is primordial? Because many petroleum geologists have considered that such a distinction can be made, and that petroleum and natural gas appear on that basis to be usually of biological origin, it is clear that we must address this aspect here.
A selection process that enriches one or other isotope is usually referred to as a process of "fractionation." The resulting fractionated material is referred to as isotopically light or isotopically heavy, depending on the ratio of the lighter to the heavier isotope. Measurements of the slight variations in the carbon isotope ratio in different samples is usually not done in absolute terms, but by comparison with a norm, and the small departures from this norm are then the quantities noted. The norm that has been selected for this purpose is a marine carbonate rock called Pee Dee Belemnite, or PDB, and this norm has a carbon isotope value that is about in the middle of the distribution of all the marine carbonates. The measurements are then quoted as the departure of the 13C content of the sample from that of the norm, and the figure is usually given in parts per thousand (permil) and referred to as the d13C value of the sample.
Unoxidized carbon in plants derives from atmospheric carbon dioxide by the process of photosynthesis. In this process the light isotope is slightly favored. As a result this carbon is slightly depleted in 13C relative to atmospheric carbon dioxide, and the effect is larger than that occurring in any other single nonbiological chemical process recognized in nature. When it was found that most of the deposits of unoxidized carbon, like petroleum, methane, coal and kerogen, show also a marked depletion of 13C, it was considered that this confirmed their biological origin. d13C for plant organic carbon is generally in the range of -8 to -35 permil (PDB standard). The atmospheric carbon dioxide from which that carbon derived is at -6 per mil, showing the possibility of a large fractionation effect. Marine carbonates laid down from atmospheric carbon dioxide dissolved in ocean water have d13C values ranging from about +5 to -5 permil (average 0) and thus evidently a fractionation averaging 6 permil occurs in favor of the heavy isotope during that process.
In the production of methane from plant debris a further fractionation takes place that again favors the light isotope, and plant-derived methane is therefore isotopically even lighter and its d13C plots at -50 to -80 permil. In the literature we now find that some arbitrary division has generally been assumed, so that carbon with d13C values lighter than -30 permil is regarded as of biological origin, while heavier carbon is taken to be from some other source.
There is no clear division in the actual data. Carbonaceous materials have d13C values spanning the range from +20 to -110 permil on the Earth, and they span an even larger range in the carbonaceous meteorites. There is no natural dividing point in the data and the choice of a particular figure in this continuous distribution, for making the distinction between organic and inorganic origin seems very arbitrary. The question is of course what other fractionation processes can select in favor of the light isotope.
Figures 1 and 2 above.
Figure 1:
Distribution of ratio (expressed as d13C) of the stable isotopes 13C and 12C in different terrestrial materials. Methane and carbonate cements span a much larger range of these isotope ratios than all other forms of terrestrial carbon. PDB, Peedee belemnite. A look at the distribution of the carbon isotope ratio in different natural forms of carbon gives immediately a strong suggestion (Figure 1). Just methane, the only carbon-bearing molecule that is light enough to suffer significant isotopic fraction, shows the largest spread of values. The atmospheric carbon dioxide from which marine carbonates have been deposited throughout geological time seems to have had a remarkably constant isotopic ratio, so that d13C values for nearly all these carbonates fall into the range of -5 to +5 permil. d13C in petroleum has a fairly narrow range, from -20 to -38 permil. Carbonate (calcite) cements in the rocks have d13C values spanning the second widest range. This fact by itself would suggest that the carbonate cements are generally produced from methane, and their shift of between 20 and 40 permil heavier than the range for methane suggests that a fractionation occurs when methane is oxidized in the ground and then combined with calcium oxide to produce the carbonate cement. Everything in the data points to such a process. These cements are found in great quantity overlying gas and oil fields. They are usually isotopically lighter than marine carbonates, the lightest among them as light as -65 permil. Where methane and carbonate cements are found in the same location the methane is usually isotopically lighter than the carbonate by between 20 to 60 permil. The overall isotopic distribution of methane and carbonate cements show a similar shift.
Figure 2. Carbon isotope ratios (expressed as d13C) of methane plotted against depth of occurrence (from Galimov, 1969). Although there is much variability in this relationship, it is almost always true that where methane is found at different levels in the same area, the methane is isotopically lighter (contains less 13C) the shallower the level.
Galimov (1969), a major contributor to the carbon isotope investigations, saw that in any vertical column methane tends to be isotopically lighter, the shallower the level. This appears to be true irrespective of the type or age of the formation from which the sample was taken.(Figure 2). It is most unlikely that in all those cases methane from two different sources mixed in such a fashion. A much better explanation is that a progressive fractionation of the methane had taken place in its upward migration. Some of this methane appears to be lost to oxidation, ending up as carbon dioxide (Figure 3), and a fraction of that in turn as calcite cements. This oxidation process seems to prefer the heavy isotope and so the remaining methane gets isotopically lighter on the way up. At each level the calcite thus derives from the already fractionated methane and so it also will become lighter, tracking the methane but always remaining heavier than the methane at that same level.
Figure 3. Comparison of the carbon isotope ratios (expressed as d13C) of methane and coexisting carbon dioxide in ocean-floor sediments (from Galimov and Kvenvolden, 1983). The carbon isotope ratios of the two gases seem to follow the same depth dependence, but with the CO2 always isotopically heavier than the methane. This is the pattern to be expected if progressive fractionation were happening, with the CO2 produced (probably by bacterial oxidation) from methane moving upward through the crust. Both the methane and the CO2 produced would become isotopically lighter on the way up, but the CO2 would be heavier by a constant amount than the methane from which it was derived. (This is not the interpretation given by the authors of the article.)
Progressive fractionation is an important process because it can drive the remaining material to a very much greater fractionation than could be done by any single chemical step. It is of course the technique used for commercial isotope separation, where extremely large fractionation factors are required. In our case two effects work in the same sense, helping to create the result. One is the tendency for the oxide to bind slightly more tightly with the heavier carbon isotope (in CO2), and in equilibrium conditions at a low temperature the heavy isotope will therefore be enriched in the oxide and depleted in the remaining methane. The other effect is the diffusion speed, which for methane with the heavier isotope and a mass number of 17, will be 3 percent slower than for the light methane with a mass number of 16. This means that in any circumstance where methane is diffusing through a barrier to fill a reservoir, the light isotope methane will enter initially with a 30 per mil enrichment. If this reservoir were to fill another, the light isotope enrichment would augment. Differential removal by oxydation, by different solubilities in water, by different adsorption on solids, by bacterial attack, will all affect the final result of a slow percolation of methane through diverse strata of rock. Since in any such progressive fractionation the final effect can be arbitrarily large, one cannot conclude that a large fractionation implies that of a single step process, namely the one that occurs in plants.
The constancy of the isotopic ratio of marine carbonates spanning all ages deserves further comment. If at any time between the Archean and the present a large change had taken place in the amount of plant debris that was buried, and if this plant debris was, as is usual, isotopically light, then more of the light isotope would have been taken out of the atmospheric-oceanic carbon dioxide reservoir. The remaining CO2 in that reservoir would have been driven to a slightly heavier composition. If as much as one-fifth of the buried carbon was in the form of such plant debris, then the shift in the remaining atmospheric carbon and the resulting shift in the carbonates laid down from it should have been readily observable. When land vegetation suddenly proliferated in Silurian times, for example, one might well think that twice as much unoxidized plant material was buried as before this event. Why is there no change in the isotopic ratio of marine carbonates in that epoch? An explanation that the primordial supply suffered a change in the isotopic ratio just sufficient to compensate is improbable. A more likely explanation is that the quantity of biological debris that is buried is a much smaller fraction than the one usually assumed. The reason for this may be that the identification of much of the buried carbonaceous material as plant debris is not correct and that a large proportion of this material derived directly from hydrocarbons ascending from the mantle. The extra contribution made by the time-variable burial of plant debris may then be so small an effect that it does not show in the isotopic ratio of the carbonates. Of course if all the dispersed kerogen and the oil shales, which have been regarded as source material for oil pools, were derived from the primary hydrocarbons, then this discrepancy would disappear. It is worth noting that the amount of carbon that would have been contained in certifiable fossils is a very small quantity by comparison.
The remarkable constancy of isotope values of marine carbonates also affects the question mentioned earlier, of the amount of carbon that may be coming up as a result of the heating of subducted sediment. In such a process some or all of the carbonates may be dissociated and the CO2 may come to the surface. Much of the unoxidized carbon that is in the same sediment, whether it derives from plants or from a primordial hydrocarbon supply, is known to be isotopically much lighter. Of that, only a fraction would be turned into liquids or gases, limited by the availability of hydrogen; the remainder would eventually turn into elemental carbon--graphite or anthracite--and in that form it would be insoluble and stable, and would not be returned to the atmosphere. A process of multiple recycling of sedimentary carbon would therefore always take away more of the light isotope than of the heavy, and this would drive the atmospheric-oceanic CO2 to a heavier isotope value. Recycling of sediments cannot account for a significant fraction of the resupply of atmospheric CO2 over geologic time.
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. |
NMOCs
(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 |
