Why are most of the world’s known oil reserves concentrated in a relatively small number of fields?

The concentration of the world’s oil reserves in a small number of fields is a fundamental characteristic of petroleum production. While approximately 600 sedimentary basins are known to exist, only a fraction are significant producers, and the vast majority of the world's known recoverable oil is contained within a relatively small number of giant and supergiant fields.12

This distribution occurs because petroleum accumulation is closely tied to specific, favorable geological conditions found in only a few basins. Large fields are typically discovered early in the exploration process, and while thousands of oil fields have been identified since 1859, more than 90 percent are considered insignificant to global production.12

Key factors influencing this concentration include:
* Geological Favorability: Significant petroleum generation requires specific conditions within sedimentary basins, where organic matter is buried, heated, and pressurized over millions of years.13
* Trapping Mechanisms: Oil must be captured in petroleum traps—such as structural traps or stratigraphic traps—to accumulate in exploitable quantities.4
* Reservoir Quality: The porosity and permeability of reservoir rocks determine how effectively oil is stored and transmitted.5