Since no one seems to want to jump in, I'll give it a shot. I've taken one petroleum geochemistry class, but it was a few years ago. I thought it was fascinating. So I'm going to use this as a way to learn more about the subject as well as play the devil's advocate to a point. I hope you don't mind.
This important change occurs only when kerogen is subjected to high temperature over long periods of time.
How was this determined?
Can you increase the pressure and temperature without requiring long periods of time and still get the same product (i.e., hydrocarbon)?