The laws of thermodynamics don't reverse under high temperature and pressure. Complex hydrocarbon molecules like octane and heptane break down into components like ethane and methane. They do not do the reverse.
This is why a refinery can crack or further process heavy hydrocarbons into lighter, but they do not take methane and create octane for transportation fuel.
This is not a system so The laws of thermodynamics do not apply here. Unless you count us burning it and somehow returning it to the depth of the earth. Which is not occuring.
The methane down there was deposited when the earth formed and is still there today: Hence NO SYSTEM=NO LAW OF THERMO-wotchacallits.
Under high temperature and pressure, carbon in the form of coal can phase shift into a diamond and still be carbon.
Why can’t the same logic apply here? High heat and high pressure chemically change metane into a carbon phase called oil and we just haven’t figured it out yet.
PS in a refinery the molecule chains are seperated according to their lengths according the amount of heat at that level in the cracking tower.
They don’t break down chemically they just seperate acording to heat applied