What happens to oil that remains in the ground

I tried to look this us but found nothing. My question is this. If oil was never harvested and it simply remained in the ground would it eventually break down into something else? If so what would that be?

If there’s no activity increasing the temperature of the deposit it stays as oil, if it heats up for some reason it can end up as natural gas.

Geologists often refer to the temperature range in which oil forms as an “oil window”. Below the minimum temperature oil remains trapped in the form of kerogen. Above the maximum temperature the oil is converted to natural gas through the process of thermal cracking.
Petroleum - Wikipedia

In addition, for conventional reservoirs the oil is trapped by a sealing formation over a porous rock formation where the oil or gas has migrated from its source rock.
Tectonic activity could over time break that seal and the oil migrate away to surface, but that would be very very very long time period effects.

I am thinking long term like up to a billion years. I am wondering if eventually it might be released as a methane or other gas.

If there is no cap rock and you have gas from the source rock, or break the reservoir seal , then yes it will just migrate to surface and head into the atmosphere as methane.
I was looking at a mass balance study of the Neuquen basin in Argentina which is a big gas basin , and it was estimated 90% of the hydrocarbons that had ever been produced by the source rock ( as opposed to produced by drilling a hole) had never made it into a reservoir trap and had just escaped to the atmosphere.

Oil will just migrate to surface and most likely get munched up by bacteria and I would guess those produce methane as any product, but that is a guess.