While I’m fast becoming an adherent to the abiotic oil theory, it is entirely possible that the Middle East was covered extensively in the past with lush vegetation.
Remember this when Gorebbel’s Warming enthusiasts complain about a warmer climate: in the past, when the earth was warmest, it was also most conducive to life. Plants grew abundantly when the earth was warmer and giant creatures (dinosaurs) thrived. Warmer earth = good for life. History has proven it.
You need to expand your thinking to include hundreds of millions of years of mostly plants and then animals.
Currently there is over 500 billion tons of carbon alone in living biomass on the earth. Over 10% of that is new each year, with similar amounts deposited.
Much of it decomposes, but some of it is trapped in sediment. Over 100's of millions of years, a tiny amount of sediment is a lot accumulated. If we get about and inch and a quarter every thousand years, that is 2 miles deep in 100 million years.
For years I've read several different articles regarding this, what I believe to be a complete falsehood, but the other side of the coin is that some researchers have indicated (and this is just one alternative theory...there are several but this one makes the most sense to me) that the sheer pressure of the tectonic plates creates this liquid we know as oil. Through the shifting throughout the millennium, oil has increased each time the earth shifted (earthquake) and the plates came/come together with such force that everything is liquidated.
I don't have any sources stored to cite but like you, I'm suspicious of the meme that (only) plants and animals rotting give us oil (fossil fuel). My personal (unscientific) opinion is that it is and has been BS.
Could there really have been that many rotting plants and animals with just the perfect conditions to create, capture and store that much gas there? Expand that thought to all the massive oil and gas being discovered all over the world and it almost makes the “fossil fuel” explanation one to be questioned.
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I’m pretty sure that phrase “fossil fuel” will disappear in my lifetime ,, in this galaxy we have methane clouds , planets with methane/butane/heptane oceans “gas giants” and much more ... I think this oil and gas was part of the planets founding ... from when it all coagulated ... hell the earth just might have a lead and gold center (heaviest items)...