Average porosity values for the producing zones of oil and gas fields worldwide are examined as a function of the present depth for sandstone and carbonate lithologies divided into 10 groupings by reservoir depositional age (Precambrian-Silurian to Pliocene-Pleistocene). The wide variations in average reservoir porosity within each depth range reflect the extreme ranges in porosity-controlling factors such as depositional facies, early diagenetic histories, geothermal gradients, and degrees of uplift from previous maximum burial that exist in the Earth's petroleum reservoirs. Median porosity for a given depth nevertheless decreases with both increasing depth and age in most age and lithology categories examined. Maps of reservoir geographic distributions corresponding with each porosity-depth plot show the Earth's petroleum provinces in terms of reservoir ages and lithologies. The results demonstrate quantitatively and empirically the degree to which porosity is related to depth, lithology, and geological age on the global scale of observation.