Natural gas is a byproduct of producing oil. Exploration for natural gas begins with geologists observing and examining the surface structure of the earth, and determining the areas where it is geologically likely that petroleum or gas deposits might exist. Natural gas wells are of different types and they include oil wells, gas wells, and condensate wells. Natural gas is known as sweet gas when it is relatively free of hydrogen sulfide. Natural gas, or any other gas mixture, containing significant quantities of hydrogen sulfide, carbon dioxide or acidic gases, is called acid gas.