Hydrocarbons and Metallogenesis

Hydrocarbons and Metallogenesis


   There are several evidences of hydrocarbon association to metal ores. Black shales, specially those of high carbon content, have long been known to be enriched with a variety of transition metals, especially Mo, Zn, Vi, Cu, Cr, V, Co, Pb, U and Ag. The Kupferschiefer is associated with black shales and near Zechstein Salt in Germany. Miners try to found the black leader to prospect gold deposits. Mississipi Valley Type - MVT deposits are frequently associated with hydrothermal dolomite HTD and bitumen. In Australia, Proterozoic shales hosted Pb-Zn-Ag deposits, such as Mt. Isa, Hilton, McArthur River and Lady Loretta According. 


   The scientist Thomas Gold, remind that in geology there's no understanding about the role of hydrocarbon compounds and their capacity to transport metals. This is due few people reasoning with the possibility that hydrocarbons come from great depths as oil and natural gas and that they are primordial materials. Then, most part of Economic geologists still reasoning that metals are syngenetic with shale deposition and diagenetic process would be responsible for metal concentration.


   The understanding of real hydrocarbon origin and processes of hydrothermal salt formation maybe will be key to comprehension of certain metal ore accumulations in the Earth.


Metal Ores and Hydrocarbons

Thomas Gold, 1994

   The association of various metal ore deposits with hydrocarbons is a vast subject, but as yet very few people have worked on it. Many such associations have been seen, but as people did not recognize the possibility that hydrocarbons could come up from great depth, they could not see any reason for these effects. And people do not write papers to say they do not understand what they see.

   The general problems about concentrated mineral deposits are the following:

1.) The Earth formed by the collection of solids, mostly small grains, that had the elements pretty much mixed up. There may have been some layers that had a little more of this or that, but except for iron and nickel, there were no "clean" substances in this infall. We judge this from the great array of meteorites which are samples of the various contributions the Earth received. Many detailed trace element and isotope ratios show that this is true.

   What processes would single out a particular element and cause a deposition in a location which represents a concentration by a factor of one million or more from the original mix? A fluid that moved through a large amount of the mix, and picked up in solution the particular substance, and then shed it from solution as a result of changing circumstances such as temperature, pressure, ph, or the picking up into the solution of another substance that decreased the solubility of the first. All attempts at explanation assume processes of this kind and this seems inevitable. Water is generally considered the basic fluid, usually with aggressive contaminants like salts. But when it comes to the arithmetic of these processes, there is frequently serious trouble. Many metals , especially the heavy metals, are just not sufficiently soluble in brines. or in any aqueous fluids. The excerpt from Krauskopf (appended here) refers to this difficulty. Many other authors have also noted it.

   In my view hydrocarbons come towards the surface from depths between 150 and 300 km. They therefore leach through a very large amount of rock as they are driven up by buoyancy forces. Effective leaching requires powerful pumping action to drive fluids though fine pores and for a large distance: fluids coming up from great depth have of course this advantage. By comparison surface waters running through some crustal rocks have an incomparably smaller driving force. The leaching has to be due to fluids that originate at depth, because only those have the pressure differentials that are required for effective leaching.

2.) Which fluids have the capability to take into solution such substances as heavy metals or metal compounds?

   At high pressures and temperatures many metals will form organometallics, that means molecules that combine metal atoms with such elements as carbon and hydrogen, possibly with some nitrogen and oxygen also. Most organometallic compounds are soluble in hydrocarbon oils. Such oils, being forced through the rocks, will have a chance to combine with metals in the rocks to make organometallic compounds. In turn those that are soluble in the oils can then be transported by that same flow. This will be so also for many metals that have very low solubilities in aqueous liquids.

3.) What process can be so selective that it will deposit one metal ore in one location and another often nearby? What liquid stream will just leach out copper from the rocks, while another nearby stream will leach out zinc? Or why platinum here and gold there?

   The hydrocarbon flow, on the way up, will make a large array of molecules, in detail depending on such things as the carbon-hydrogen ratio, the ratio to other elements like nitrogen and oxygen, the catalytic action of specific minerals in the rocks, and the pressure-temperature regime it finds on the way. Among those molecules may be a class that is particularly favourable for forming a particular organometallic compound with one metal, another class with another. The great diversity of hydrocarbon molecules is thus the reason for the selectivity in the metal deposits. Certain groups of metals occur in close association, presumably because there exists a hydrocarbon stream there, and similar hydrocarbons that were abundant in that location have selected that group because these respond similarly. Thus lead and zinc are found together, gold and silver, etc.

   When these metal-laden streams come nearer to the surface, and reach lower pressures and temperatures, many of the compounds become unstable (many carbon compounds are stable at a high pressure only, like diamond). Also bacterial action may destroy them, as the bacteria will preferentially remove the hydrocarbon components. In this way the naked metal atoms remain.

   The close association of gold with carbon is well recorded in the literature. Conventional wisdom gives no hint of an explanation either for the association with carbon, or even for the occurrence of metallic gold altogether. It seems that carbon is an essential component in the laying down of gold. The gold miners of olden days knew this very well, and followed the "black leader", a trail of carbon black that led frequently to a gold deposit.

   It is interesting that the other substance that is commonly associated with gold is silicon dioxide. Silicon is in the same column, two below carbon, in Mendeleev's table of the elements and it has very similar properties. It will form oils that are quite similar to hydrocarbon oils, but frequently with higher thermal stability. I do not know (and possibly no one knows) whether at high temperatures and pressures, it will form silicon-metallic compounds, analogous to organometallics. An argument in favour of this would be the occurrence of gold in quartz veins rather than in quartz deposits, suggesting a common migration path. Mercury, found as the sulfide cinnabar, is often together with oil and tar.

   Many metals will of course make sulfides, if sulfur is available. Thus mercury may come up in a gas stream as mercury vapor or as dimethyl-mercury, but have enough sulfur to be turned into cinnabar. It is the same for many other metals, they would not resist being turned into the sulfide. For mercury it is particularly clear that it has come from great depths, as it is strongly associated with helium, in particular with helium high in helium-3, which is the marker for primordial helium, caught in the formation process of the Earth, and not merely derived from the radioactivity of uranium and thorium.

   In the drilling in the Siljan Ring structure in Sweden, large quantities of magnetite were found. Some twelve tons of a mix of very fine grained magnetite and natural petroleum were pumped up from one wellbore, and some kilograms of a similar paste were pulled up on the drillstring in a second hole. At the deeper levels, below 5 km, the magnetite paste impeded the drilling operation in both holes. It appears that it was this same paste that prevented any substantial inflow into the wellbores, necessary for any commercial production. Investigations by laboratories including that of the Danish Geological Survey, showed the oil to be an ordinary type of crude, somewhat biodegraded. In the second hole no drilling fluids were introduced that could possibly have resulted in the oils seen.

   The origin of such clean, concentrated magnetite and its very small grain size, much of it in the micron size range, certainly present a puzzle. Moreover the entire Siljan Ring structure displayed a positive magnetic anomaly, quite accurately centered in the ring. It therefore seems very likely that this same magnetite paste was the source for the magnetic anomaly, and that it was present in sufficiently large amount to account for it. If this is considered a possibility, then one may well wonder whether the various other large magnetite deposits of Sweden have a similar origin.

   The only clues we have about the origin of the Siljan magnetite come from the detailed trace element and isotope observations of it. Neutron activation analysis (done by the Los Alamos National Laboratory) showed a substantially different admixture of trace elements from the local granite or the much larger magnetite grains in it. For example the paste magnetite contained only 1/30th of the amount of Mg-27 as the magnetic grains of the granite; 1/7th of the Na-24; but 100 times as much Zn-65 (there is a commercial zinc mine in the region); 10 times as much Ba-131 and Ba-139; less than 1/10th the amount of Nd-148. Several other equally large differences were found. It does not seem probable that any iron oxide in the local granite can be the origin of the magnetite paste: no processes are known that could have separated these elements so sharply. One may therefore consider the possibility that all this magnetite has been brought up as an organometallic from a totally different chemical domain such as the mantle. It would be most illuminating to analyze some of the other magnetite deposits of Sweden for similar anomalies.

   From Introduction to Geochemistry, Konrad B. Krauskopf, McGraw Hill, 1982, p. 395.

   This is similar to the question we tried to answer in the last section, as to the minimum concentration of metal in a magmatic gas that would be significant for the formation of ore deposits. We proceed in the same way, using rough numbers to establish a limit of reasonableness. Suppose, for example, that an ore solution carried 10-7 g/liter of zinc. To deposit 1 ton of metal would require a minimum of 1010 cubic meters of solution, approximately the volume of water carried to the sea each year by the Hudson River (average flow approximately 10,000 sec-ft). Such a solution traversing a vein at a rate of 10 ft3/sec could deposit 1 ton of zinc in a thousand years, provided that all the dissolved zinc precipitates. The amount of water and the amount of time seem excessive, by comparison with scanty data on the flow of hot springs and on the geologic times required for the formation of ore bodies. Thus 10-7 g/liter can be taken as an absolute minimum, below which the concentration of metal is too small to be of interest. For most purposes a somewhat larger figure, say 10-5 g/liter, is a more reasonable minimum.

By this criterion the solubility of ZnS is barely high enough to be of interest at a temperature of 200° and a pH as low as 5.  The calculated solubilities of the sulfides of some other common metals (Mn, Fe, Co, Pb) have a similar order of the amounts of metal that can be carried by hot sulfide solutions seem far too small, except for a few metals under the most favorable assumed conditions, to account for the origin of ore deposits. This is the long-standing difficulty with the classical hydrothermal hypothesis.

Origin of Carbonate Rocks

Origin of Carbonate Rocks


   In geology, carbonates are a class of sedimentary rocks compose primarily of carbonate minerals. Two major types are limestone, which is composed of calcite or aragonite (different crystalline forms of CaCO3) and dolostone, which is composed of the mineral dolomite (CaMg(CO3)2).

   There are are many studies published in books, papers about carbonate rocks, mainly related to depositional environments, variations in textures, structures, facies, mineralogy, stratigraphy, diagenesis, deformation, formation of karst processes, paleontological content (fossils), isotopic studies, among others. The carbonate record is also relatively well documented by geological studies in many carbonate platforms from the Archean to recent.

   Nevertheless the problem lies in the fact that, in geology, there is little concern about the origin of this rock type.

The questions are:

•  Where does come from carbon present in the carbonate rocks?
•  What are the causes of the beggining of carbonate sedimentation?
•  What are the processes responsible for the formation of dolomite and dolostone?

   The ideas based on the principles of uniformitarianism have not resolved these issues. This problem is due geology not yet provided an understanding of the process of planetary formation, the origin of natural hydrocarbons and the carbon cycle, from deep within the Earth to ocean-atmosphere-biosphere systems.

   According to the scientist Thomas Gold, surface of the Earth is very rich in carbon and deserves an explanation. Four-fifth of this carbon is oxidized, mainly in the form of carbonates. Studies of Earth's carbon budget made by the Massachusetts Institute of Technology - MIT show that the carbonates represent about 5% of global carbon and about 83% of carbon in Earth's surface or near surface.

   The carbon present in carbonate rocks can be derived from excess methane in the ocean-atmosphere by outgassing of hydrocarbons and carbon dioxide from the primordial Earth's mantle. The dissociation of methane and its reaction with oxygen is then responsible for carbon oxidation and the formation of calcium carbonate salt would be common in this paroxysm, since calcium is an abundant element on Earth.  Methane is a greenhouse gas 20 times more potent than carbon dioxide and fixation in carbonates, its precipitation in marine and lake environments would be responsible by the removal excess of carbon of the oceanic-atmospheric pool. The carbonate rocks are highly chemically reactive and are reworked by sedimentary processes in the Earth's dynamic systems. Living organisms take advantage of the calcium carbonate to build their skeletons and structures and can also be entirely reworked and re-sediment bioclastic carbonate rocks.


   Cap carbonates occur after glacial periods, mainly in Neoproterozoic (Sturtian and Marinoan glacial events). Methane released from permafrost with excess in atmosphere could  be incresead to form overlying carbonate sequences.

   The process of dolomite formation is still an enigma for geology. However it is known that dolomite mineral do not precipitate in laboratory and the features in the process of dolomitization are best explained when related hypogene hydrothermal fluids from depth through deep faults, from which the magnesium that is incorporated into calcium carbonate. Primordial hydrocarbons sometimes occur associated to dolostones (hydrothermal dolomite - HTD) and frequently remain as bituminous material after intense biodegradation in carbonate vugs. 

   It is common association of carbonate sequences with sequences of halide salt as halite (traditionally the so-called evaporites). The gypsum and barite formed as sulphates, also associated with volcanic and hydrothermal systems that brings sulphur. Hydrothermal Salt theory and abiotic hydrocarbons are maybe a clue to understanding the process of dolomite formation.

   Indeed, understand release of primordial methane maybe would be the key for understand origin of carbonate rocks in the geological record. The dolomitization process it seems related to mantle deep fluids which bring hydrocarbons (oil and gas) and halogens similar to hydrothermal process of salt formation and its interaction with carbonate rocks.

Hydrothermal Salt

Hydrothermal Salt

   Most part of geologists still believe that salt rocks as halite NaCl, anhydrite CaSO4, gypsum CaSO4·2H2O, sylvite KCl, tachyhydrite CaMg2Cl6·12H2O, carnallite KMgCl3·6H2O, among others would be formed intrinsically by evaporation processes of shallow saline waters at surface environments and thus would configure sequences formed by the so-called "evaporites" which would be a class of chemical sedimentary rocks. The paradigm of the model related to formation of evaporites dates since 1849 by Italian chemist Usiglio and after proposals by Bischof  and Ochsenius, respectively in 1854 and 1877 based on previous ideas would take place through the model of restriction by a barrier (Barrier Model) of saline water bodies where the salt to precipitate and form layers according to its solubility, concentration and evaporation rates that are influenced by topography and arid climates.


   Would be traditional evaporite model the only real  explanation for the genesis of salt deposits? The traditional evaporite model is, however, not the only way of explaining all the salts appearing on Earth today. Thus, a group of Norwegian scientists have, over the past decade developed the new Hydrothermal Salt Theory (Hovland et al., 2006). Unlike the traditional model, this theory is more physically and chemically consistent, and over time, may take over parts of the current view. The new theory is not only valid on Earth, but also on planet Mars, where probably there are salt domes and salt deposits (Hovland et al., 2009)

   According to this new theory, Salt is originated from hydrothermal systems deep-through where supercritical water acts on stage at certain conditions of pressures and temperatures in a superior range of the superficial environments. The molecules of supercritical water has no polarity  and, different of non-critical water (normal water), indeed supercritical water can not dissolved salts and carrier them from deep sources brines to surface environments where then the effects of climate dry and restrictions areas in lakes or marine environments such salts are accumulated, reworked, precipitated, redissolved, reprecipitate forming spetacular sequences of salt rocks.


   It is noteworthy that there is no rock-forming minerals in the crust of the Earth with high content of chlorine or abundance to justify the occurrence of huge deposits of salt halite (NaCl). It makes sense then think that salt would not be derived primarily only from the dissolution of surface rocks. By the other hand, salt usually occurs associated with volcanic environments and hydrothermal systems as mud volcanism, for instance. There are many examples in the world, in marine, estuarine and continental environments and also a wider range of altitudes. 


   Good example of non-marine salt association is the salt of the Salar de Uyuni, in Bolivia. This occurrence is situated over 3,650 m altitude in the Andes, wide more than 100 km in area about 3,000 square miles. The salar is surrounded by several volcanic buildings. Another example is the Danakil Depression, in Ethiopia, Eritrea and the salts that occur in the Afar Triangle in Djibouti, where there is also intense volcanism. White Sands National Monument, in New Mexico, USA is another good example of salt related to volcanism. In this place there are spetacular dunes, not sand dunes, but gypsum dunes and the Carrizozo Malpais lava flow, over 70 km long, occurs closely towards north of White Sands. There also many similar occurences in the world.

Salar de Uyuni, Bolivia, South America (NASA GFSC Modis Terra image)

Salar de Uyuni is the world's largest salt flat, at larger than 3000 square miles in size. It is located near the crest of the Andes mountains, at an altitute of 3650 meters. It is estimated that Salar de Uyuni contains 10 billion tons of salt; its mineral content is mainly gypsum and halite. Around salar there are several volcanoes. 

Dallol is a unique crater in the world, located in Afar territory, in the Danakil desert (one of the most inhospitable deserts in the world), north-eastern Ethiopia, about fifteen kilometers from the border of Eritrea. This site is volcanic in the north end of a saline lake, Lake Karoum, in which salt is still mined today by the Afar. It follows from the explosion of a large magma chamber of the Great Rift Valley, over a large area west of saline Red Sea, and is - 136.8 meters below the level Sea, in the Danakil Depression. The heat regularly reaches 45 degrees in the shade. 

This vast area is known for its unusual geological formations: acidic hot springs, mountains of sulfur, salt columns, small gas geysers, pools of acid isolated by cornices of salt concretions and evaporites, sulfur chloride magnesium, sodium hydroxide and brine solidified. All on a white background, yellow, green and red ocher, due to the strong presence of sulfur, iron oxide, salt and other minerals. 

The site, like the volcanoes surrounding this area (Erta Ale volcano in Kenya, etc..) Is the result of the separation of the Arabian Plate and the African plate and the creation of the Red Sea rift. 

The large white area near the bottom of this image of New Mexico, USA, is the sand dune field known as the White Sands National Monument. The field has a surface area of 710 km² (275 mi²). White Sands is the world’s largest gypsum dunefield. The thick, dark brown line just north of the dunes is the Carrizozo Malpais, a large lava flow, one of the youngest volcanic features in the state of New Mexico. The 75 kilometer long Malpais, is composed of basaltic lava flows. (from Earth Snapshot)

   The questions are: where does Chlorine (Cl) come from for  to form the chloride salts  and sulfur (S) to sulphate salts? It is likely that chlorine migrate from great depths as organochlorine compounds, i.e., through its connection with primordial hydrocarbon molecules of the mantle. Salt  cations such as sodium, calcium and magnesium are abundant in the crust and mantle, but not chlorides and sulphates. Elements such as halogens Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) can have its origin related to primordial volatiles , including sulphur (S), which is likely to combine with oxygen to form sulphate compounds.


   Hydrothermal Salt Theory offers plausible explanation on the origin and evolution for the end-members of these salt sequences,  but  the dogmas of  geology, as the principle of actualism, are still very influential and difficult paradigm shift to an unconventional theory. This is due the difficult of most part of geologists still do not have comprehension of physical, chemical laws and mass balance of natural processes because they still remaining prisoner with their reasoning in context of restricted "a priori" theories.


   Indeed, understanding the whole process - from the origin to the formation of salt deposits is not  simple, since the elements and compounds have high chemical reactivity with many changes. Maybe Hydrothermal Salt Theory is the great light to the knowledge of salt rocks formation and evaporites are only extrinsic processes  of salt rework at surface shallow environments.


   Why are the oceans salty? That is a question that even children always ask and geologists and scientists still do not get an apropriated answer, obviously because they do not have enough understanding about what is salt and its origin but the Hydrothermal Salt Theory is probably a right way to answer that and other many questions about salt and its real origin.


   The traditional evaporite model could explains only certain situations after salt formed originally by hydrothermal systems reach to surface and reworked at this enviroment by normal evaporation processes. On the other hand, Hydrothermal Salt Theory also explain existence of deep-water salt sequences.

   Recently NASA found evidence of salt in Mars (see link below). Hydrothermal Salt Theory is suitable to explain origin of salt in Mars. We know that there was extensive volcanism in Mars. There, the highest volcan of solar system is present - The Olympus Mons (Mount Olympus) is a martian shield volcano which has base diameter about 624 km (374 mi) and 25 km (16mi) high. Volcanic outgassing brings methane and is oxidize to carbon dioxide to Mars atmosphere. Hydrothermal brines synchronous with volcanism could provide salt reach near surface of Mars. Natural oil-spills or oil seeps, seepages, are interpreted from Mars images also associated with salt. This phenomenon is very common on Earth.

Mons Olympus - Mars 

(ESA/DLR/FU Berlin - G. Neukum)

 References
Hovland, M., Rueslåtten, H., Johnsen, H.K., Kvamme, B., Kutznetsova, T., 2006. Salt formation associated with sub-surface boiling and supercritical water. Marine and Petroleum Geology 23, 855-869.

Hovland, M., Rueslåtten, H., ,Johnsen, H.K., Fichler, C., 2009.(Abstract) Hydrothermal evaporites - from the Conrad Deep, via Dallol, to Elysium Planitia. International Association of Sedimentologists (IAS) Annual meeting, Alghero, Sardinia, Book of Abstracts.

 

Links about Hydrothermal Salt

* http://martinhovland.weebly.com/the-hydrothermal-salt-theory.html

* http://science.tumblr.com/post/1499064075/the-hydrothermal-salt-theory

* http://meetingorganizer.copernicus.org/EGU2009/EGU2009-6707.pdf

* http://adsabs.harvard.edu/abs/2010AGUFMEP21A0732R

* http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2117.2006.00290.x/full

 Salt in Mars

* http://science.nasa.gov/science-news/science-at-nasa/2011/04aug_marsflows/

*Abiotic Hidrocarbons in Mars - Martin Hovland
*Crude Oil in Mars

Intracratonic Basins

Intracratonic Basins

   The intracratonic sedimentary basins consist  in fills of large areas within continental crustal stable masses. They are characterized by deposits related to continental and shallow marine paleoenvironments, with significant gaps, hiatus between depositional megasequences. They have generally round or oval shapes, and have long histories of relatively slow subsidence. Classic examples of such notable are the basins Williston, Michigan and Illinois, in North America, Paraná Basin, Parnaíba, Solimões, Amazonas, in Brazil, Murzuk and Al Kufra, in Libya, Karoo and Congo in Africa, Surat in Australia, among others.

   The origin of these basins has set up a puzzle in geology. The tectonic reconstructions at Plate Tectonics paradigm is very difficult. The Theory of Earth Expansion offers a plausible explanation. In the Paleozoic Era, Earth could have a radius smaller than the current one. There were no extensive processes of oceanic expansion, much less what is so-called subduction of plates. Dominated continental environments such as alluvial, fluvial and deltaic, lakes, deserts, glacial events, and shallow seas, never deep marine oceanic sedimentation, as seen during and after the Mesozoic Era. Formation of Intracratonic basins would promote processes of crustal uplift buckling and bending of the marginal arches and depressions with precursors rifts which later would then be filled by marginal erosion of these arches, with episodic sedimentation of clastic, carbonate, salt, ecc.

Tectonic provinces of South America (Mantovani et al., 2001).  AB, Amazon Basin; AC, Amazon Craton; CA, Central Andes; CT, Chilenia Terrane; MP, Mantiqueira Province; NA, Northern Andes; NME, Northeastern Margin of South Atlantic Shield; P, Patagonia; PB, Paraná Basin; PCB, Phanerozoic collisional belts; PNB, Parnaiba Basin; RLPC, Rio de la Plata Craton; SA, Southern Andes; SFC, São Francisco Craton; SP, Southern Province. In green major areas dominate intracratonic basins.

North-south cross section of Muruzq Basin. (Modified after Pallas, 1980). Typical profile of intracratonic basin. Large intracratonic Paleozoic Basin, straddling the Boundaries of Algeria, Niger and Chad. The Basin is filled with sediment ranging in age between Cambrian to Quaternary, with maximum total thickness of more than 3000 meter in the central part. 

Intracratonic basins frequently are filled by Paleozoic sediments. At that time Earth probably was much small and there wouldn't deep basins, no subduction, no pangea, just shallow seas and continental sedimentation. Understanding of Earth's evolution is crucial to solve many problems enrooted by geological dogmas including origin of Intracratonic Basins.

Isotopic Composition of Hydrocarbon Gases

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