Abstract The present volume and composition of trapped hydrocarbons are temporary events controlled by the interacting volumes of trap, oil, and gas, all of which change through time. Thus, both the volume and type of trapped hydrocarbon result from the interacting effects of kerogen amount, type, and maturity; gas and oil migration efficiencies; trap growth; and the possibility that free gas may displace trapped liquids. By estimating the historical evolving effects of each factor, we can estimate trapped hydrocarbon volume as well as trap content (i.e., only oil, oil leg and gas cap, or only gas). The procedures that lead to such estimates are: (1) estimate the quantities of oil and gas provided by the source within the drainage area; (2) estimate the quantities of gas lost by dissolution and diffusion; (3) estimate the trap volume; and (4) compare hydrocarbon volumes with trap volume to find the limiting volume.