Why does very deep heating of kerogen mainly produce dry methane gas?

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Multiple large offshore petroleum oil rigs with cranes and equipment stand in the ocean, connected by walkways above the water. — petroleum Liquid petroleum (crude oil)—which is refined into gasoline, diesel, heating fuels, asphalt, plastics, and other useful chemical compounds—is often extracted from the ground using a series of drills and pumps.

At depths exceeding approximately 4,800 to 5,000 metres (16,000 feet), the extreme temperatures encountered in the Earth's crust render liquid crude oil unstable. In this postmature stage, the kerogen has already become condensed in structure and chemically stable.12

The production of dry methane gas at these depths occurs because any existing liquid hydrocarbons undergo thermal cracking. During this process, larger, more complex hydrocarbon molecules are broken down into simpler ones. Because hydrocarbons with chemical structures larger than methane are destroyed by heat much more rapidly than they can be formed, they are effectively eliminated, leaving methane as the primary stable end product.12