a specific is a sink of natural for people in an fuels
liquified_petroleum_gas is natural_gas. natural_gas is gas. oil_flow_station is not tangible_thing. natural_gas is not fuel. sink is natural_process. oil_flow_station is tangible_thing. natural_gas is fuel. liquified_petroleum_gas is not natural_gas. petroleum is fuel. petroleum is naturally_occurring_tangible_substance.