Six stratigraphic intervals representing one-third of Phanerozoic time contain petroleum source rocks that have generated more than 90% of world's discovered original oil and gas reserves. These stratigraphic intervals are (1) Silurian, (2) Upper Devonian-Tournaisian, (3) Pennsylvanian-Lower Permian, (4) Upper Jurassic, (5) middle Cretaceous, and (6) Oligocene-Miocene. Source rocks in these six intervals and associated oil and gas reserves are mapped world-wide in this study. The uneven stratigraphic and areal distribution of the source rocks is one of the main factors that led to the sharply uneven occurrence of oil and gas reserves. Study of the areal and stratigraphic distribution of source rocks and their effectiveness (the amount of generated oil and gas reserves) in the context of Phanerozoic plate-tectonic development indicates that the world may be divided into four petroleum realms, they are the Tethyan, Boreal, Pacific, and South Gondwana realms. The realms differ from each other in age, abundance, and quality of source rocks, reservoir rocks, and seals, and in structurai development of major petroleum basins. The differences in these petroleum geology factors resulted in marked variations in the effectiveness of source rocks. The causal connection of the hydrocarbon richness with the petroleum geology factors suggests that most undiscovered oil and gas will be found in the presently richest Tethyan and Boreal realms.