An important step in the evaluation of a play or prospect is to consider the potential supply of petroleum charge, which is ultimately constrained by masses and volumes supplied by the source bed. The two factors limiting the mass of petroleum expelled from the organic matter in the source bed are (1) its initial expulsion potential and (2) the cumulative fraction of potential that has been expelled up to its maximum state of maturation. To evaluate the initial expulsion potential, we introduce a workflow to estimate the ultimate expellable potential (UEP), divided into an oil potential (UEO) and a gas potential (UEG), which represent the cumulative masses of oil and gas that can be expelled upon complete maturation of the source rock. For use in resource estimation, these masses can be converted to surface volumes of oil and gas per unit area (million stock tank barrels per square kilometer and billion standard cubic feet per square kilometer or million barrels of oil equivalent per square kilometer, respectively). The UEP (UEO, UEG) can be mapped across the depositional extent of the source bed, just as a reservoir depositional system is mapped. These potentials per unit area constitute the first level of quantitative resource estimation in the evaluation of a play or prospect. We show examples of UEP (UEO, UEG) mapping based on available public data. Three of the example source rocks are aquatic organofacies that have charged major conventional petroleum systems: the Callovian to Oxfordian marine organofacies of the Arabian Basin, Saudi Arabia; the Volgian marine organofacies Bazhenov Formation of the West Siberian Basin, Russia; and the Eocene-Oligocene lacustrine freshwater organofacies Shahejie Formation of the Bohai Basin, China. We also include an unconventional system: the uppermost Devonian-lowermost Mississippian marine organofacies Bakken Formation of North Dakota, United States. The UEPs of the studied source rocks in the Arabian Basin and West Siberian Basin, with tens of millions of barrels of oil equivalent per kilometer over large areas, define world class in marine source rocks since these basins are ranked number one and number two in the world by oil endowment. Until more data are available on other lacustrine basins, we offer the UEP of the studied Bohai Basin source rock as an example. In contrast, the UEP of the Bakken Formation source rocks (combined Upper and Lower Members) is relatively modest despite its world class unconventional oil endowment. The Bakken's effectiveness, despite its relatively low UEP, reflects the negligible migration saturation losses involved in charging the Middle reservoir Member. This illustrates that the commonly touted term world class can be rather meaningless. It needs to be considered in context given the task in hand: the greater the (vertical) migration losses incurred in charging reservoirs, the higher the UEP needed to create the charge needed to overcome them.