Abstract: In view of the low degree of thermal evolution and complicated shale gas occurrence of the Chang₇ Member in the central Ordos Basin, a comparative study was conducted with basic geochemical methods and gas analysis experiments to discover the geochemical characteristics of shale gas and explore the occurrence process of shale gas. The results showed that the shale mainly contained Type Ⅰ and Type Ⅱ₁ organic substances, with moderate thermal evolution, average organic carbon content of 4.52%, free hydrocarbon content of 4.09 mg/g, and pyrolysis hydrocarbon content of 8.70 mg/g; the shale gas in the study area was petroliferous gas, and its geological origin was thermal decomposition; the occurrence state of shale gas was coexistence of free gas, dissolved gas and adsorbed gas, and the sum of adsorbed gas content and dissolved gas content was greater than free gas content; the adsorption of shale for different gases was quite different, and its adsorption for nitrogen was the weakest; as for the hydrocarbon gases, the initial migration capacity of small molecule gases was greater than that of macromolecular gases. Due to the low thermal evolution, on the one hand, oil, gas and water coexisted in the shale, generally low gas content in shale is not high. On the other hand, a large amount of oil and gas was preserved in geological movement. In the development process of Chang₇ Member, joint development of shale oil and gas is of the most practical and commercial significance. There is much for reference of the study results to the further exploration and development of Chang₇ Member.

Key words: shale gas, thermal evolution degree, geochemical characteristics, Chang₇ Member, Ordos Basin