The ultimate source of "abiotic oil" is postulated to be the primordial methane from the original gaseous constituents slowly percolating from the earth's deep interior to the surface. Helium is either primordial or results from various very long half-life primordial radioactives decaying (alpha particle decay). But both are gaseous, and get to the surface through the same "channels of porosity" until they hit some barrier (like a salt dome, or basalt layer), and are "trapped".
To me it seems the abiotic theories are given pretty short shrift. The biotic theory seems implausible. How do “vast sheets of primordial biological mats” get suddenly plunged from the surface to the depths required to create petroleum? Asteroids, plate tectonics seem unlikely to create the volume necessary to explain the huge amounts found. It perhaps could explain the formation of some rare gems but billions of barrels of oil are a different matter.