There is a spectrum of fuels from peat to various types of brown coal, to bituminous coals to anthracite.
It appears pretty clearly that the harder and more energy-containing types are formed from peat by the action of heat and pressure underground.
As far as animals and plants being unable to form thick deposits, there are numerous layers of limestone around the world hundreds of feet thick. It is pretty clear that limestone is primarily formed from the shells of marine organisms.
Which means if a oil deposit were the size of, lets say Manhattan, and was 1000 ft deep, it would have had to have been 10,000 feet deep, or more, with plant and animal carcasses, preserved, encapsulated, and then crushed to its current size, and then pressurized and heated, to produce crude.
Now a tree that dies, the root system underground decays in about 50 years, or less. Somehow that decay has to be stopped, in order for 100's or 1000's of feet of plant and animal material to be deposited. Then it all has to be encapsulated together. Then taken 1000's and 1000's of feet below ground. Where over millenia, heat and pressure do their thing.
Mess up one of the processes and it would wipe out the entire production. Unless of course there was even more material that was part of this process, that never was changed over, and what we have is just a small fraction of what "could have been". Meaning that either the initial material was the size of Manhattan and 100,000 ft deep. Or it was the size of New York state 10,000 ft deep. Or it was the entire eastern half of the US, from the Mississippi to Atlantic, 100 feet deep.
Now add up ALL the oil pockets in the world, and figure how much material it would take to produce that. Plus coal and peat.
I don't know, seems kinda a push to think that ALL the crude came from plants and animals.
I mean if we were able to take all the vegetation and animal life on earth, push it into the Grand Canyon, bury it, pressurize it, and heat it, how full would the Grand Canyon be with oil? Now do that REPEATEDLY for EVERY single deposit.
How do you get vegetation to all gather in one spot and not decay. When plants die they decompose and provide nutrients for new plants. Now you have to get MASS animal congrigation, in one spot, then suddenly die, and not decompose, and become encapsulated, along with all the vegetable material.
So that happens in Texas, then Pennsylvania, then Michigan, then Alaska, then Saudi Arabia, Kuwait, Indonesia, and all the ocean reserves.