Effective Petroleum Source Rocks of the World: Stratigraphic Distribution and Controlling Depositional Factors

<jats:title>ABSTRACT</jats:title> <jats:p>Six stratigraphic intervals, representing one-third of Phanerozoic time, contain petroleum source rocks that have provided more than 90% of the world’s discovered original reserves of oil and gas (in barrels of oil equivalent). The six intervals are (1) Silurian (generated 9% of the world’s reserves), (2) Upper Devonian-Tournaisian (8% of reserves), (3) Pennsylvanian-Lower Permian (8% of reserves), (4) Upper Jurassic (25% of reserves), (5) middle Cretaceous (29% of reserves), and (6) Oligocene-Miocene (12.5% of reserves). This uneven distribution of source rocks in time displays no obvious cyclicity and the factors that controlled the formation of source rocks vary from interval to interval.</jats:p> <jats:p>Maps that show facies, structural forms, and petroleum source rocks were prepared for this study. Analysis of the maps indicates that several primary factors controlled the areal distribution of source rocks, their geochemical type, and their effectiveness (i.e., the amounts of discovered original conventionally recoverable reserves of oil and gas generated by these rocks). These factors are geologic age, paleolatitude of the depositional areas, structural forms in which the deposition of source rocks occurred, and the evolution of biota. The maturation time of these source rocks demonstrates that the majority of discovered oil and gas is very young; almost 70% of the world’s original reserves of oil and gas has been generated since the Coniacian, and nearly 50% of the world’s petroleum4 has been generated and trapped since the Oligocene.</jats:p>