Recent advances in hydraulic fracturing, which is an old process with new capabilities due to advances in drilling and production techniques allow for production from source rocks that have generated large volumes of gas that has not been able to migrate from the source into a reservoir where conventional production would have been used. Opening migration pathways and controlling the pressure in the source allows the gas to change phase and release from the source through the created pathways to the well for production.
kwinkunks comment is right, vertebrates don't contribute to oil formation in any significant way.