natural gas vs oil

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. 

oil

noun
  • petroleum-based liquid used as fuel or lubricant. 

  • petroleum 

  • Liquid fat. 

  • An oil painting. 

  • Oil paint. 

  • Containing oil, conveying oil; intended for or capable of containing oil. 

verb
  • To lubricate with oil. 

  • To grease with oil for cooking. 

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