Crude oil and natural gas are mixtures of molecules formed by carbon and hydrogen atoms. There are many different types of crude oil and natural gas, some more valuable than others. Heavy crude oil is very thick and viscous and difficult or impossible to produce, while light crude oil is very liquid and relatively easy to produce. Acidic oils containing sulfur and acidic natural gases containing hydrogen sulfide are less valuable. Some natural gases produce more heat than others, contain natural gas condensates and gasoline fractions, and are more valuable.