Thomas Gold's work was focused on hydrocarbon deposits of primordial origin. Meteorites are believed to represent the major composition of material from which the Earth was formed. Some meteorites, such as carbonaceous chondrites, contain carbonaceous material. If a large amount of this material is still within the Earth, it could have been leaking upward for billions of years. The thermodynamic conditions within the mantle would allow many hydrocarbon molecules to be at equilibrium under high pressure and high temperature. Although molecules in these conditions may disassociate, resulting fragments would be reformed due to the pressure. An average equilibrium of various molecules would exist depending upon conditions and the carbon-hydrogen ratio of the material.[25]
[edit] Creation within the mantle

Russian researchers concluded that hydrocarbon mixes would be created within the mantle. Experiments under high temperatures and pressures produced many hydrocarbons, including n-alkanes through C10H22, from iron oxide, calcium carbonate, and water.[11] Because such materials are in the mantle and in subducted crust, there is no requirement that all hydrocarbons be produced from primordial deposits.
[edit] Hydrogen generation

Hydrogen gas and water have been found more than 6 kilometers deep in the upper crust, including in the Siljan Ring boreholes and the Kola Superdeep Borehole. Data from the western United States suggests that aquifers from near the surface may extend to depths of 10 to 20 km. Hydrogen gas can be created by water reacting with silicates, quartz and feldspar, in temperatures in the 25° to 270°C range. These minerals are common in crustal rocks such as granite. Hydrogen may react with dissolved carbon compounds in water to form methane and higher carbon compounds.[26]

One reaction not involving silicates which can create hydrogen is:

Ferrous oxide + Water → Magnetite + hydrogen

3FeO + H_2O \rarr Fe_3O_4 + H_2

The above reaction operates best at low pressures. At pressures greater than 5 GPa almost no hydrogen is created.[14]
[edit] Serpentinite mechanism

One proposed mechanism by which abiogenic petroleum is formed was first proposed by the Ukrainian scientist, Prof. Emmanuil B. Chekaliuk in 1967. He proposed that petroleum could be formed at high temperatures and pressures from inorganic carbon in the form of carbon dioxide, hydrogen and/or methane.

This mechanism is supported by several lines of evidence which are accepted by modern scientific literature. This involves synthesis of oil within the crust via catalysis by chemically reductive rocks. A proposed mechanism for the formation of inorganic hydrocarbons[27] is via natural analogs of the Fischer-Tropsch process known as the serpentinite mechanism or the serpentinite process [23][28].

CH_4 + \begin{matrix} \frac{1}{2} \end{matrix}O_2 \rarr 2 H_2 + CO
(2n+1)H_2 + nCO \rarr C_nH_{2n+2} + nH_2O

Serpentinites are ideal rocks to host this process as they are formed from peridotites and dunites, rocks which contain greater than 80% olivine and usually a percentage of Fe-Ti spinel minerals. Most olivines also contain high nickel concentrations (up to several percent) and may also contain chromite or chromium as a contaminant in olivine, providing the needed transition metals.

However, serpentinite synthesis and spinel cracking reactions require hydrothermal alteration of pristine peridotite-dunite, which is a finite process intrinsically related to metamorphism, and further, requires significant addition of water. Serpentinite is unstable at mantle temperatures and is readily dehydrated to granulite, amphibolite, talc-schist and even eclogite. This suggests that methanogenesis in the presence of serpentinites is restricted in space and time to mid-ocean ridges and upper levels of subduction zones. However, water has been found as deep as 12 km,[29] so water-based reactions are dependent upon the local conditions. Oil being created by this process in intracratonic regions is limited by the materials and temperature.
[edit] Serpentinite synthesis

A chemical basis for the abiotic petroleum process is the serpentinization of peridotite, beginning with methanogenesis via hydrolysis of olivine into serpentine in the presence of carbon dioxide[28]. Olivine, composed of Forsterite and Fayalite metamorphoses into serpentine, magnetite and silica by the following reactions, with silica from fayalite decomposition (reaction 1a) feeding into the forsterite reaction (1b).

Reaction 1a:
Fayalite + water → Magnetite + aqueous silica + Hydrogen

3Fe_2SiO_4 + 2H_2O \rarr 2Fe_3O_4 + 3SiO_2 + 2H_2

Reaction 1b:
Forsterite + aqueous silica → Serpentinite

3Mg_2SiO_4 + SiO_2 + 4H_2O \rarr 2Mg_3Si_2O_5(OH)_4

When this reaction occurs in the presence of dissolved carbon dioxide (carbonic acid) at temperatures above 500 °C Reaction 2a takes place.

Reaction 2a:
Olivine + Water + Carbonic acid → Serpentine + Magnetite + Methane

(Fe,Mg)_2SiO_4 + nH_2O + CO_2 \rarr Mg_3Si_2O_5(OH)_4 + Fe_3O_4 + CH_4

or, in balanced form: 18Mg2SiO4 + 6Fe2SiO4 + 26H2O + CO2 → 12Mg3Si2O5(OH)4 + 4Fe3O4 + CH4

However, reaction 2(b) is just as likely, and supported by the presence of abundant talc-carbonate schists and magnesite stringer veins in many serpentinised peridotites;

Reaction 2b:
Olivine + Water + Carbonic acid → Serpentine + Magnetite + Magnesite + Silica

(Fe,Mg)_2SiO_4 + nH_2O + CO_2 \rarr Mg_3Si_2O_5(OH)_4 + Fe_3O_4 + MgCO_3 + SiO_2

The upgrading of methane to higher n-alkane hydrocarbons is via dehydrogenation of methane in the presence of catalyst transition metals (e.g. Fe, Ni). This can be termed spinel hydrolysis.
[edit] Spinel polymerization mechanism

Magnetite, chromite and ilmenite are Fe-spinel group minerals found in many rocks but rarely as a major component in non-ultramafic rocks. In these rocks, high concentrations of magmatic magnetite, chromite and ilmenite provide a reduced matrix which may allow abiotic cracking of methane to higher hydrocarbons during hydrothermal events.

Chemically reduced rocks are required to drive this reaction and high temperatures are required to allow methane to be polymerized to ethane. Note that reaction 1a, above, also creates magnetite.

Reaction 3:
Methane + Magnetite → Ethane + Hematite

nCH_4 + nFe_3O_4 + nH_2O \rarr C_2H_6 + Fe_2O_3 + HCO_3 + H^+

Reaction 3 results in n-alkane hydrocarbons, including linear saturated hydrocarbons, alcohols, aldehydes, ketones, aromatics, and cyclic compounds.[28]
[edit] Carbonate decomposition

Calcium carbonate may decompose at around 500 °C through the following reaction:[14]

Reaction 5:
Hydrogen + Calcium carbonate → Methane + Calcium oxide + Water

4H_2 + CaCO_3 \rarr CH_4 + CaO + 2H_2O

Note that CaO (lime) is not a mineral species found within natural rocks. Whilst this reaction is possible, it is not plausible.
[edit] Laboratory experiments

Some research and laboratory experiments explore possible mechanisms, but there is little related geological evidence.
[edit] Carbonate reduction

Methane has been formed in laboratory conditions via carbonate reduction at pressures and temperatures similar to that in the upper mantle, but a large amount of water was provided to the reaction in excess of that which is typical in mantle lithology. No natural rocks with a wustite-calcite composition are known to exist, which precludes this reaction from occurring in nature. Likely reactions include:

Reaction 6a:
Ferrous oxide + Calcium carbonate + Water → Hematite + Methane + Calcium oxide

8FeO + CaCO_3 + 2H_2O \rarr 4Fe_2O_3 + CH_4 + CaO
and

Reaction 6b:
Ferrous oxide + Calcium carbonate + Water → Magnetite + Methane + Calcium oxide

12FeO + CaCO_3 + 2H_2O \rarr 4Fe_3O_4 + CH_4 + CaO

Methane formation is favored under 1,200 °C at 1 GPa. At 1,500 °C hydrogen production was prevalent. Methane production is most favored at 500 °C and pressures <7 GPa; higher temperatures are expected to lead to carbon dioxide and carbon monoxide production through a reforming equilibrium with methane.

This is cited as evidence of the plausibility of methanogenesis under mantle conditions.[14]
[edit] Calcite decomposition

One carbon compound, carbon dioxide, can be created by calcite decomposition at 1,500 °C:[14] Reaction 7:
Calcium carbonate → Calcium oxide + Carbon dioxide

CaCO_3 \rarr CaO + CO_2

Calcite is likely molten at these temperatures, being a mixture of CaO ions and CO2.
[edit] Ethane and Ethylene synthesis
Deep sea vent biogeochemical cycle diagram

The synthesis of ethane and ethylene has been done at 800 °C, using electric discharges in laboratory experiments. This experiment was in a hot gas, rather than hot mantle fluids. The calculated reactions are:[30]

Carbon dioxide + Methane → Carbon monoxide + Ethane + Water

CO_2 + 2CH_4 \rarr CO + C_2H_6 + H_2O
and

Carbon dioxide + Ethane → Carbon monoxide + Ethylene + Water

CO_2 + C_2H_6 \rarr CO + C_2H_4 + H_2O

[edit] Evidence of abiogenic mechanisms

* Scaled particle theory for a simplified perturbed hard-chain, statistical mechanical model predicts that methane compressed to 30 or 40 kbar at 1000 °C (conditions in the mantle) yields hydrocarbons having properties similar to petroleum [10][11]
* Experiments in diamond anvil high pressure cells have confirmed this theory