Pursue accompanying table for the answer. The maximum petroleum reserves are lying under the soil of the Gulf countries. The reason for such copious reserves is that the Earth’s crust in this region comprises of sedimentary rocks which preponderantly comprise of the fossilized remains of the ancient creatures and vegetation. Constituting mainly hydrogen and carbon, these fossils have been subjected to tremendous heat and pressures of geological upheavals over the period of hundreds of million years to metamorphose into the hydrocarbon petroleum.
FYI: Mankind has already consumed 44% of the world’s petroleum reserves. Unless fresh petroleum reserves are discovered or alternative energy sources are harnessed on a large scale, the existing reserves will barely last a couple of decades.
Oil reserves (Wikipedia)