Seeing as how petroleum is a finite resource, it will likely be largely replaced by alternative energy sources within the next 40 years. One barrel is equal to 159 liters. The volume of crude oil reserves worldwide is estimated at 160 trillion barrels, of which some 4 trillion barrels are to be found in the United States. The majority of the U.S. reserves are located in Alaska, California, and Texas. In time, the petroleum reserves of the United States may actually be increased by oil shale, a rock plentiful in Colorado, Wyoming, and Utah.
Oil shale contains a waxy substance known as kerogen, which yields oil when heated.