I never thought that was the case.
There never could have been enough plant and animal mass in existence to account for all the petroleum.
I figured that way deep down in the earth, methane gas from the beginning of the solar system, was being compressed and heated to enormous temps and pressures and being liquified into petroleum.
After all, the gas giants Jupiter, Saturn, Neptune and Uranus and their moons are all full of methane and ammonia.....................
“The deepest humans have ever drilled into the Earth’s crust is 12.262 kilometres (40,230 feet) at the Kola Superdeep Borehole on the Kola peninsula of Russia. This drilling took 20 years to complete and is the deepest penetration of the Earth’s solid surface. The crust is the only layer of the Earth that humans have encountered, and it ranges from 3 to 40-plus miles thick. The Earth’s geothermal gradient means that the temperature increases by about 25 degrees Celsius for every kilometre drilled. This makes drilling deeper more challenging, and the pressure within the mantle may be far beyond the strength of any material known today.”
Source: https://quartzmountain.org/article/how-deep-have-we-travelled-into-the-crust
Space may or may not be “the final frontier”, but the fact is we have barely scratched the surface (pun intended) on the crust of our home planet...