Abotic methane fron mantle rocks is not common but also not all that rare. Methane doesn’t polymerize to higher hydrocarbons methane is CH4 higher hydrocarbons are CH2(n)+CH2(n)+.... Methane does exactly the opposite under high heat and pressure forming hydrogen and solid carbon the hydrogen being so reactive it hydrates any mear by minerals.
Like I said in over 20+ years doing thousands of geochemical analyses on oils form nearly every basin in the world. I nor any other geochemistry expert has ever seen a liquid hydrocarbon sample that didn’t have at least one of the three biological markers. Not a single sample anywhere EVER. its a great theory that has no peer reviewed examples by any professional scientist who do these analysts every day for every oil company on earth. Outer space hydrocarbons are all in the outer solar system why? Because the temperature needed to liquidfy methane , ethane or propane is below negative 50C those are the only liquid hydrocarbons in the outer solar system only trace amounts of butane, heptane or pentane have ever been spectrographed. C1,C2 being the bulk with tiny amounts of C3 and trace amounts of C4,5,6 there is no mechanisms to break the higher energy carbon carbon bonds nor a catalyst to cause double carbon single hydrogen bonding to form higher hydrocarbons chains. All liquid hydrocarbons on earth started as much longer chain organics that under heat and pressure were split into smaller chains breaking at the single carbon carbon bonds. Cellulose, hemicellulose ,lignin all have thousands to hundreds of thousands of carbon carbon bonds those when subjected to heat and pressure under ground break into chains of 5 to 25 in length to be liquid at STP conditions. The Germans figured out if you use platinum under low pressure and just high enough temps you can take carbon monoxide and hydrogen break the much easier to breal carbon oxygen bond and add hydrogen this is FT synthesis. The pressures in the crust vastly exceed what FT would work at at high pressures FT yields methane only this says nothing of platinum being at part per trillion or less concentrations in the crust.