There is no need for a "hidden reserve". Petroleum is the second-most abundant liquid on Earth and it is continually created (the Earth also releases hydrogen and oxygen and combines them on their journey to the surface to create new water- this is where spring water comes from. Look up "primary water"). Petroleum is not made from ancient fossils. Oil wells refill. The wealthiest oil companies in the world are those who manage their assets well and don't have to constantly shift their logistical operations because they extracted too much, too quickly.

Remember "Peak Oil", where the world was going to run out of oil in 5 years or 10 years or 1975 or 1997 or 2011 or any time at all? Now the narrative is "Just Stop Oil". Because all the predictions were nonsense. The goalposts have now moved to 2090 for Saudi Arabia running out of oil. And those are nonsense as well. Here's an intro if you're interested. https://youtu.be/hu_R3gp84TY