Also from plants
In the carbon age 350 to 299 million years ago, the climate was ideal for plants to flourish. Dead plants formed thick peat layers in vast bogs, providing perfect conditions for converting dead plants into coal. As the ground sank rapidly, the plant material descended into lower layers of the earth, where it successively degraded first into peat, then brown coal, bituminous coal and finally anthracite during what is known as the carbonisation process. During carbonisation, the plant material decomposes into gaseous reaction products, especially methane. Natural gas deposits produced during coal formation can be found in fields in the Netherlands and in the southern North Sea, for example.