When initially developed, conventional petroleum reservoirs depend on the pressure of their gas cap and oil-dissolved gas to lift the oil to the surface. Water trapping the petroleum from below also exerts an upward hydraulic pressure. In the late 1940s, drilling companies began inducing hydraulic pressure in wells to fracture the producing formation. Combining hydraulic fracturing with directional drilling has opened up the production of tight petroleum and natural gas reservoirs, particularly unconventional gas shales such as the Marcellus Shale formation. In rural areas, storage pits may be used to hold freshwater for drilling and hydraulic fracturing. In urban settings, due to space limitations, steel storage tanks may be used to hold drilling fluids as well as to store water and fluids for use during hydraulic fracturing. Water storage pits used to hold water for hydraulic fracturing purposes are typically lined to minimize the loss of water from infiltration.