A naturally occurring subsurface pool of hydrocarbons trapped within rock formations in the Earth’s crust. The hydrocarbons may exist as crude oil or natural gas, and are the result of the high pressure and temperature decomposition of aquatic organisms that lived millions of years ago. Petroleum reservoirs are broadly classified as being oil or gas reservoirs and quantified in terms of the composition of the hydrocarbon mixture, their initial temperature and pressure, and surface production temperature and pressure. See well.