I'm sorry if that is stupid question. Geology is not my thing, but I'm curios about this.

Let's assume that mankind has never extracted fossil fuels (or existed in the first place) and for millions of years fossil fuels just stacked up in the earth's crust. Then, let's fast forward time absurdly huge amount of time ahead, assuming there's no astronomical catastrophes.

What will happen with the pockets of fossil fuels when they grow absurdly huge? Would they surface earth to form seas of oil or release all the gas into atmosphere? Or would they grow in opposite direction - to the center of earth, then what would it become?

  • $\begingroup$ How do fossil fuels get created? $\endgroup$
    – user253751
    Mar 4 '20 at 12:49
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    $\begingroup$ Yes, it has happened - before the industrial age, there was oil even on the surface or near it. It was rare, but they existed. All these resources were quickly depleted. $\endgroup$
    – peterh
    Mar 4 '20 at 12:57
  • $\begingroup$ @user253751 that's a different question, which you are welcome to ask as such! Although you might want to try to narrow it down a little. $\endgroup$ Mar 4 '20 at 13:50
  • $\begingroup$ @SemidiurnalSimon As far as I know, new fossil fuels are not created because biological processes degrade the feedstock that would have become fossil fuels otherwise. Hence, there is not really any risk of the entire world being converted into fossil fuels. $\endgroup$
    – user253751
    Mar 4 '20 at 14:55

Obviously if all fossil fuels had been left in the Earth it would greatly lessen the climate warming problem. You ask what would happen to these fossil fuels in the course of time if we left them alone. The coal would remain inert and fairly well behaved; some would be exposed by erosion, but would do no harm. Some would be buried even deeper as new deposits of sediment were laid on top of them.

Most of the oil would remain buried out of harm's way, but some would be exposed by erosion and geological faults to form tar pits or asphalt lakes (which are both much the same thing). Oil seepages are a nuisance, because they pollute the sea and fresh water on land. Fortunately they do not occur very often. On land, the volatile fraction evaporates quickly and leaves a thick, sticky residue called tar or asphalt, the latter being slightly thicker.

Well known examples are the La Brea tar pits in Los Angeles, famous for the fossils of extinct animals trapped by the tar, and the asphalt lake in Trinidad which has long been mined for surfacing roads etc. There are several other tar pits in California, and a large asphalt lake in Venezuela.

Looking into the future, a few more oil reservoirs would be caused to leak in the course of time by geological faults, but most would stay safely in the ground. Meanwhile the old processes would be at work, transforming vegetable matter into peat, peat and wood into lignite, and lignite into coal. Plankton and vegetable matter in the ocean would form more oil reservoirs.

Present fossil fuel deposits will become buried more and more deeply in the course of millions of years, when man has vanished from the scene, with new layers of sediment and new fossil fuels laid down on top of them, but they will never be buried deep enough to reach the asthenosphere, never mind the centre of the Earth. They will just, in some cases, have another kilometre or two of rock placed on top of them.

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    $\begingroup$ Let's remember that in an extremely long time, a billion years or more, the shortage of CO2 will eventually lead to the extinction of plant life. And most of that CO2 will be interred in the crust. $\endgroup$
    – Ginasius
    Mar 5 '20 at 9:50
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    $\begingroup$ Volcanoes will replenish it, and evolution will adapt plants to cope with reduced CO2 if it actually happens. But of course life must end on Earth eventually, first the advanced lifeforms and then, in about 4 - 5 billion years, all life forms. $\endgroup$ Mar 5 '20 at 10:00

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