Reference

Several mechanisms have been proposed for the abiotic chemical synthesis of hydrocarbons within Earth's mantle. These include:

1. Serpentinization: This process involves the hydration of olivine and/or pyroxene, which produces hydrogen gas (H2). This hydrogen can then react with carbon dioxide (CO2) or carbon monoxide (CO) to form methane (CH4) and other hydrocarbons. This mechanism is often associated with the Fischer-Tropsch-like (FTT) process occurring under high-pressure and high-temperature conditions within the mantle.

2. Reduction of Carbonates: Under low oxygen fugacity, carbonates can be reduced to form abiotic hydrocarbons and graphite or diamond. This process is significant in the formation of abiotic hydrocarbons.

3. Hydrothermal Synthesis: In alkaline hydrothermal systems, abiotic long-chain hydrocarbons can be produced. The specific mechanisms for this synthesis are not fully understood, but it is known that short-chain hydrocarbons can also be produced in these environments.

4. Deep Fault Migration: Hydrocarbons generated in the asthenosphere can migrate through deep faults into the Earth's crust.

These mechanisms suggest that hydrocarbons can be synthesized abiotically and then migrate to the crust, contributing to the formation of oil and gas deposits. However, the abiotic origin of hydrocarbons remains a subject of scientific debate, with the biological origin being more widely accepted. For more detailed information, you can refer to sources such as Wikipedia, ResearchGate, and ScienceDirect.