Origin of oil, natural gas and carbonates

D.Anton
There are two main theories about the origin of the carbon compounds on the planet Earth. These compounds are diverse but predominantly expressed in the oxidized compounds (p.ej.el carbon dioxide and carbonates) and non-oxidized compounds (e.g. hydrocarbons). A majority hypothesis currently holds that carbon compounds originally come from a very rich atmosphere of carbon dioxide, while another theory considers these compounds originated deep in the crust and upper mantle and then rose in a slow and widespread degassing process.
Here we transcribe a text of "The Deep Hot Biosphere" by Thomas Gold (2001)

"According to the first theory on the earth 's near-surface enrichment in carbon, the initial blanket of carbon dioxide in the earth  atmosphere would have to have been very substantial . The figure implied  by the mass of carbonate rock would require a mass of carbon dioxide in the early atmosphere about  eighty times greater than the whole of our planet atmosphere and about as massive as that of our  sister planet Venus. In contrast, today's proportion of carbon dioxide in the earth's  atmosphere  is only 3.5 parts per ten thousand  by volume.

However, there is good reason to believe that the early earth did not acquire much material in the form of gases, because there is a very low abundance of gases such as neon, non-radiogenic argon, krypton, and xenon in the atmosphere today. No physical processcould have sorted out these inert gases from the solar system's gaseous mix,  where they are known to be consideerably more abundant. And because all these inert gases are heavy atoms, they would not have escaped the earth's gravity and drifted offf into space at a greater rate than other gaseous elements.
The only sound explanation, in my view, is that atmospheric gases have derived mainly from outgassing of volatiles derived at depth from buried solid materials- not from an initial large artmosphere acquired at the earth's formation or by later capture of gases from space.
The theory that the earth started out with a massive CO2, atmosphere fails in yet another way.-the pattern of carbonate rock deposition  through geological time does not support it.
Rather than a skewing of carbonate deposition to earlier times, the sedimentary records show a rather continuous accumulaton of such oxidized carbon, as well as unoxidized carbon over the last two billion years, which is the period of time over which the sedimentary record  is usefully intact. Indeed, the total carbon excess of the surface layers is clearly shown to have been increasing  since early times. Recycling cannot account for that. Rather a continuous addition drawn from sources upwelling  from within the earth must be held responsible.
Strangely, although most of the oxidized carbon that is in the carbonate deposits is derived from the atmospheric-ocenic pool  of carbon dioxide, the present content of carbon in this pool represents only about onepart in 740 of the known deposited amounts  (using the estimated total deposited carbon over thecourse of two billion years  and the measured cO2 content of atmosphere and oceans). What is the origin of the supply that maintains atmospheric CO2 at levels that result in the deposition ofcarbonates through all geological epochs and that maintains a supply ratesufficiently constant to keep plants alive?

If outgassing of carbon-containing volatiles from the depths of the earth were responsible, what mean rate of outflow would be implied?  Using the figures presented above, this global average rate of outgassing would have to be sufficient to replace the amount equal  to the present atmospheric-oceanic content ofcarbon dioxide every 2.7 million years. In other words, the carbon must have been replaced in those surface reservoirs 740 times in two billiion years.
As already mentioned, the chemistry of meteorites indicates that carbonates or other forms of oxidized carbon were not constituents of the materuals that formed the solid planets. Most of the carbon was initially in unoxidized form, primarily as hydrocarbons. The evidence from deep boreholes that are not too close  to active volcanic regions shows, in accordance with the meteorite evidence, that  hydrocarbons are the dominant carbon-bearing fluids there. At still deeper levels, where the pressure is so great that damonds are the stable form of carbon, unoxidized  carbon again evidently dominates and formss these crystals of pure carbon.
Some fraction of these upwelling carbon fluids, starting out largely in the form of CH4 and other light hydrocarbon molecules , will be oxidized  during the ascent.  The oxygen availabillity from the rocks, the temperature and pressure along the pathways of flow, and the action of subsurface microbial life will deteermine the ration of methane to carbon dioxide emerging from the ground
in any one region. Any methane that reaches the atmosphere without being oxidized to carbon dioxide in the oxygen-rich atmosphere and there join the pool of atmospheric-oceanic CO2. What fraction of all the upwelling carbon upwelling carbon volatiles would be delivered to the atmosphere as methane, and what fraction as carbon dioxide?
 The carbon dioxide coming from volcanoes is well studied , whereas the large quantities of methane that emerge from non-volcanic ground go mostly unnoticed. The (superficial) impression created by this is that carbon dioxide is the principal source of the source of the surface carbon excess, and that it also the main carbon-bearing gas in the ground.
An analysis of the isotopes of carbon, however, reveals an error in this dominan tivew. The study of the isotopes of carbon is a large and complex field. I will mention here only one aspect tthat bears directly on the subject  under discussion, but even that is necessarily rather technical. It has to be addressed becausethere hasbeen much debate about its interpretation and insignificance.
(to be continued)

Where does Earth carbon come from?

D,Anton

The importance of the element carbon in planetary and geological processes has been sometimes underestimated. In the Solar System carbon is found in unoxidized form in rhe outer giant planets such as Jupiter, Saturn, Uranus and Neptune. In those planets it appears as methane, ethane and other hydrocarbons. In Venus and Mars carbon is oxidized into CO² representing above 95% of the atmospheric masses. In the Earth, carbon is found in unoxidized forms (coal, crude oil, natural gas, etc) and in oxidized forms as CO² in the atmosphere (400 ppm) and as carbonates in limestones and other calcaire geological formations.

The question thar arises is the following: Where does the carbon on earth come from? The answers are several but two possibilities are prevalent. It may be an original compound from the moments in which the planet.was formed or resulted from the ascent of methane (and other hydrocarbons) through the crust. The first theory relates to the biogenic theory of petroleum formation and the second to the abiotic hypothesis. Thomas Gold was a energetic supporter of the second theory. Here we reproduce his views on the carbon cycle on earth.

Clues about Earth carbon in the carbonate record

T.Gold

"The surface and subsurface sediments on the earth contan approximately one hundred timesas much of the element carbon as would have derived from the grinding up of the basementrocksthat contributed to the sediments. This surface is thus enormoously enriched in carbon. This enrichment requires an explanatiion.

The total quantity of carbon contained in the sediments and on the surface isestimated to average about 200 tons per each square meter of the earth surface area.

One-fifth of all this carbon is in unoxidized form, including various forms of coal, crude ols, kerogen (carbonaceous compounds diffusely distributed in the rocks), and natural gas, either as free gas or in the form of methane hydrate ices. In addition, there is the thin veneer of living and not-yet-decomposed biological material. This latter category- in my opinion the only demonstrably biological component-represents only a very small fraction of the total unoxidized carbon.

The other four-fifthof the carbon is the oxidized form, mostly limestone (calcium carbonate), and dolomite (a blend of calcium and magnesium carbonate). Much of this carbonate was deposited in oceansm, having derived the carbon from the atmospheeric-oceanic pool of carbon dioxide. Carbonate precippitates naturally out of the water column from dissolved carbon dioxide and calcium and magnesium oxides. It can also be precipitated out of the water biologically, by organisms that build carbonate shells or skeletons."

Reproduced from "The Deep Hot Biosphere", Thomas Gold,  2001.
More information in "Inexhaustible? Petroleum and Natural Gas",  D.Anton, Piriguazu Ediciones