Natural gas is primarily methane or CH4, with smaller quantities of other hydrocarbons. It was formed millions of years ago, when dead organisms sunk to the bottom of the ocean and were buried under deposits of sedimentary rock. Subject to intense heat and pressure, these organisms underwent a transformation in which they were converted to gas over millions of years. Natural gas is found underground rocks, called reservoirs. The rocks have tiny spaces called pores that allow them to hold water natural gas and sometimes oil. The natural gas is trapped underground by impermeable rock, called a cap rock, and stays there until it is extracted. Natural gas can be categorized as dry or wet. Dry gas is essentially gas that contains mostly methane; wet gas, on the other hand, contains compounds, such as ethane and butane, in addition to methane. These natural gas liquids, or NGLs for short, can be separated and sold individually for various uses, such as in refrigerants and to produce products like plastics. Conventional natural gas can be extracted for drilling wells; unconventional forms of natural gas, like shale gas, tight gas, sour gas, and coal bed methane, has specific extraction techniques. Natural gas can also be found in reservoirs with oil and is sometimes extracted alongside oil. This type of natural gas is called associated gas.