Oil and natural gas were formed from the remains of prehistoric plant and animals. The oil and gas that formed in the source rock deep within the earth moved upward through tiny, connected pore spaces in the rocks. Reservoirs are not underground "lakes" of oil; reservoirs are made up of porous and permeable rocks that can hold significant amounts of oil and gas within their pore spaces. Most reservoirs contain oil, gas, and water. Technological innovation makes it easier to find new deposits of oil and gas and to get more oil or gas from each reservoir that is discovered. The reservoir performance is predicted by mathematical modeling of the reservoir or reservoir simulation. The initial oil, gas, water saturation and pressure are specified. Oil Shales are usually fine-grained sedimentary rocks containing relatively large amounts of organic matter from which significant quantities of shale oil and combustible gas can be extracted. Included in most definitions of oil shale, is the potential for the profitable extraction of shale oil and combustible gas or for burning as a fuel. Total world resources of oil shale are estimated to be 2.6 trillion barrels.