9781422285596

12 Natural Gas

to form, compared to petroleum oil. Then, over millions more years, great earth movements, drifting continents, erosion, mountain building, and similar actions altered the overlying rocks. This is why natural gas and petroleum oil are found at varying depths today. Because they formed from the preserved or fossilized remains of living things, they are known as fossil fuels. Other terms include hydrocarbon fuels and organic fuels, since they were made naturally or organically. Where natural gas and petroleum oil would collect depended on the nature of the rocks. These needed to be porous , with tiny channels and holes. The natural gas and oil could ooze through, like water soaking through a sponge. Being light in weight, or low density, natural gas and oil tended to flow or migrate upward through porous rock, usually until they reached a rock layer that was nonporous or solid. Here they stayed, trapped under this layer.

A cross-section view of shale.