Oil is not stable above a certain temperature. It breaks down in the earth to lighter and lighter components, just like what a refinery does. Granted, some methane may have come from the inards of the earth but not oil. Temperatures in the mantle are over 1,000 degrees.
Determining reserves, in the US, is a price dependent number.
The details are such that you can have a great deal of oil underground, but you have “run out”. The reason is that oil is in rock. It’s not an ocean down there. It’s pores in rock that contain it and the pores may (or may not be) interconnected.
If they are not interconnected, perhaps a hundred yards apart, well beyond any fracking distance, then that oil is there but at any reasonable price it is going to stay there.
So you can get oil reserves quoted (technically AND economically recoverable) that go up AND down, just based on price. Fracking is a technology that has been around for decades. No particular new technology made oil flow from shale, nor got those reserves quoted bigger. What made that happen was high price and low interest loans to fund the activity. With price crashed, provided it stays crashed, the shale industry will be destroyed.
To harvest unrecoverable oil (unrecoverable per price and technical ability), you’ll need Star Trek transporter beam technology. Of course, for the most part, if you had that, you would not need the oil.
:: Oil is not stable above a certain temperature. ::
Granted, at subsurface temp/press, crude can break down into lighter constituents but, that does not eliminate its physical properties (hydraulics) that would allow it to migrate to sedimentary formations in the form of liquid, gas or “heavy-gas”.
Current US production consists of not just crude (C11+) but of those lighter constituents: C2, C3, C4+, C5-C10+. Where did these come from?
NOTE:
C2-Ethane
C3=Propane
C4=Butane
C5-C10+=Natural Gasoline or “Condensate”
[Question: Do you use Saran Wrap?]
When it reaches the surface, your temp/press parameters are moot and the extraction from the formation is “fractionated”.