Coal-derived gas theory and its discrimination

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The formation of coal-derived gas theory experienced three stages. In 1940s, a simple coal-derived gas theory was established. It was found that coal could generate gas and form reservoir, but no attraction was paid to oil generation from coal. In 1960s, it was proposed the exinite in coal had great contribution to the generation of coal-derived oil. With large proportion of exinite, it could even form a coal-derived oil field. Thus the coal-derived oil theory came into being; however, no attention was paid to the oil/gas ratio during the hydrocarbon generation process. In 1970s, the Chinese scholars found that hydrocarbon generation from coal measure was dominated by gas generation and supplemented by oil generation. Therefore, a complete coal-derived gas theory was established. The coal-derived gas theory was supported by the following evidences. (1) Pores indicative of gas generation products and traces were found in 8 kinds of coals; (2) cellulose and lignite with low H/C ratio account for 60%–80% among the original substances of woody plant in coal, which is dominated by gas generation; (3) humic kerogen is dominated by gas-prone methyl and condensed aromatic ring; (4) pyrolytic experiments on peat and anthracite get abundant gas products (346−422) m3/t, with a small amount of light oil. Therefore, a number of coal-derived gas fields were discovered in coal-bearing basins in the world. In the past, distinguishing of natural gas type is dominated by gas composition. In this study the distinguishing index was developed into three facies, i.e., gas, liquid and solid (kerogen), which broaden the distinguishing scope and index. Combined with carbon isotopes, light hydrocarbons and biomarkers, 26 indexes were proposed to distinguish the coal-derived and oil-derived gases, and a series of plots and δ13C- Ro relationships were compiled, which greatly improve the precision and reliability of gas distinguishing. The coal-derived gas theory opened a new natural gas exploration field, which leads to the change of guidance theory of natural gas exploration from monism (oil-derived gas) to dualism (coal-derived and oil-derived gases), providing a theoretic basis for the rapid development of the Chinese natural gas industry. In 2016, the reserves of natural gas and coal-derived gas and production of natural gas and coal-derived gas increased by times of 52 and 408, 10 and 216.6, respectively compared to that before the occurrence of coal-derived gas theory in 1978, which make China from a gas-poor country into the sixth major gas-producing country in the world.