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11 Chapter One: How Natural Gas Formed

Breakdown into Natural Gas What happened to the kerogens depended on depth of burial, which determined the pressure and temperature. From about 120 up to 300°F (50–150°C), they became petroleum oil. This temperature range is known as the oil window , with most oil formation at 140–250°F (60–120°C). Being buried even deeper meant the kerogens got hotter still. From 212 to 390°F (100–200°C), equivalent to being about 4 miles (6.4 kilometers) deep, the kerogen breakdown yielded natural gas. In North America, about one-quarter of natural gas reserves are known as associated gas , meaning the gas occurs together with petroleum oil. The other three-quarters are nonassociated, existing mainly without oil. Natural gas, like petroleum oil, varies in its contents. Usually, it is mainly methane, ranging from 75 to 98 percent, but more generally it is 90–95 percent methane. Most of the rest is the gases ethane, typically at levels of 2.5–3 percent; propane, at 0.2 percent; and butane, at less than 0.1 percent. The amount of carbon dioxide varies greatly but is usually less than 1 percent. There are also small quantities of nitrogen (1–5 percent), as well as hydrogen sulfide, oxygen, hydrogen, and water vapor. The Passing of Time The processes that made natural gas and petroleum oil took millions of years and happened only in certain places. Burial to greater depths caused more natural gas Types of Natural Gas • “Dry” natural gas is nearly all methane, whether natural from the ground or having been purified. • “Wet” natural gas is less pure, with the other gases like ethane, propane, and butane mixed in, as natural gas liquids, or NGLs. • “Sour” natural gas contains significant amounts of hydrogen sulfide, which can eat away or corrode equipment and produce polluting sulfur-containing gases when it is burned. • “Sweet” natural gas has very little hydrogen sulfide.