Oil source rocks are sedimentary rocks usually with large...

Oil source rocks are sedimentary rocks usually with large amounts of shale .
These rocks are deposited in water as clays and silts in lakes or more commonly in the ocean.
They are not spontaneously formed deep under the earth.

When they they are deposited there is considerable organic material that is the remains of plants and animals , that were once alive , buried with the inorganic clay and silt .
Subsequent deep burial increases the temperatures and pressures such that the organic material can sometimes transform into oil and gas .
There have been inorganic theories for the generation of hydrocarbons and some could be have been generated that way but the mainstream thinking is that most if not all oil and gas was created from once living animals and plants entrapped in sedimentary rocks .

Many sedimentary basins have depths much deeper than 16000 ft .
I think part of the Sacramental Basin is much deeper than that.
If you understand how sedimentary rocks are formed you will have to know that dead animals and plants HAVE to be buried in them .
I am not sure even if it is true that no fossils have been found in any rocks more than 16000 ft below the surface.
If that is true it only means that the lithification process that has changed the once soft mud and silt into hard shale has destroyed the fossils.

If you look hard enough on the Internet you can find almost anything .