oil rig vs oil slick

oil rig

noun
  • A rig, consisting of platform and machinery, used to drill for oil and other petroleum products like natural gas. 

oil slick

noun
  • A thin film of oil floating on the surface of water, especially one that has leaked or been discharged from a ship etc 

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