You obviously didn't look your shit up hard enough. The first hit on Google for "What is crude oil made from" says specifically "Crude oil is a mixture of (sic) plants and ANIMALS which died millions of years ago." https://revisionworld.com/gcse-revision/chemistry/carbon-compounds/what-crude-oil
Wow, that is such bull crap. When you spend tons of time writing, you know, like for a dissertation, proper english becomes a habit that is just ingrained. Like driving a stick shift, it doesn't take long before you don't even think about what gear you're in, or need to be in.