In three sentences or less explain why predicting our future oil supply is controversial and involves some uncertainty.

Most of our oil supply, if not all, comes from non-renewable sources like petroleum and natural gas. Before they can be even used as raw materials with calorific values that are used for energy supply, they take billions of years to develop. They all start from rocks that are compressed deep deep down in the Earth's crust. Due to high pressure from all the weight of the Earth above it and the high temperature near the Earth's core, it slowly stores high chemical energy. As mentioned previously, this process takes a very long time. In this modern times, we are very dependent on petroleum for our fuels that it is likely that we may run out of oil supply. That is why scientists are finding alternative ways to save some reserves of the limited supply. We can never say how long the reserves may last. It really depends on the rate of our usage.