natural gas vs petroleum

natural gas

noun
  • A mixture of gaseous hydrocarbons associated with petroleum deposits; mostly methane with smaller amounts of ethane, propane and butane; principally used as a fuel. 

petroleum

noun
  • A flammable liquid ranging in color from clear to very dark brown and black, consisting mainly of hydrocarbons, occurring naturally in deposits under the Earth's surface. 

How often have the words natural gas and petroleum occurred in a corpus of books? (source: Google Ngram Viewer )