Tar sands are reservoirs of sandstone filled with heavy oil at shallow depths of less than 2 km below the Earth's surface. The heat and bacteria at these shallow depths causes the oil to become biodegraded and thick, making it difficult to extract. Extraction methods aim to reduce the viscosity of the thick oil by heating the reservoirs, either through cyclic steam injection, burning some of the oil underground, or using electric heating powered by nuclear energy. Specifically, steam assisted gravity drainage uses two horizontal wells - steam is injected into the upper well to form a steam chamber that heats the oil, making it flow down into the lower well where it is pumped to the surface.