The Ordos Basin, the second largest sedimentary basin in China, contains the broad distribution of natural gas types. So far, several giant gas fields have been discovered in the Upper and Lower Paleozoic in this basin, each having over 1000×10 8 m 3 of proven gas reserves, and several gas pools have also been discovered in the Mesozoic. This paper collected the data of natural gases and elucidated the geochemical characteristics of gases from different reservoirs, and then discussed their origin. For hydrocarbons preserved in the Upper Paleozoic, the elevated δ 13 C values of methane, ethane and propane indicate that the gases would be mainly coal-formed gases; the singular reversal in the stable carbon isotopes of gaseous alkanes suggests the mixed gases from humic sources with different maturity. In the Lower Paleozoic, the δ 13 C 1 values are mostly similar with those in the Upper Paleozoic, but the δ 13 C 2 and δ 13 C 3 values are slightly lighter, suggesting that the gases would be mixing of coal-type gases as a main member and oil-type gases. There are multiple reversals in carbon isotopes for gaseous alkanes, especially abnormal reversal for methane and ethane (i.e. δ 13 C 1 >δ 13 C 2 ), inferring that gases would be mixed between high-mature coal-formed gases and oil-type gases. In the Mesozoic, the δ 13 C values for gaseous alkanes are enriched in 12 C, indicating that the gases are mainly derived from sapropelic sources; the carbon isotopic reversal for propane and butane in the Mesozoic is caused by microbial oxidation and mixing of gases from sapropelic sources with different maturity. In contrast to the Upper Paleozoic gases, the Mesozoic gases are characterized by heavier carbon isotopes of iso-butane than normal butane, which may be caused by gases generated from different kerogen types.Finally, according to δ 13 C 1 -R o relationship and extremely low total organic carbon contents, the Low Paleozoic gases would not be generated from the Ordovician source as a main gas source, bycontrast, the Upper Paleozoic source as a main gas source is contributed to the Lower Paleozoic gases.Ordos Basin, natural gas, chemical composition, carbon isotope, genesisThe Ordos Basin, located in the middle of China, is the second largest sedimentary basin and one of the most tectonically stable basins in China [1] . Several giant gas fields, each containing over 1000×10 8 m 3 (bcm) of proven gas reserves, have been discovered in this basin, e.g. Sulige gas field as the largest gas field in China.The natural gas in the Ordos Basin is characterized by widespread, various types and multiple preservation, so that many scientists have carried on much research into gas sources, gas types and regional distribution. But
