The coal seams in the Permian Taiyuan Formation and the Carboniferous Shanxi Formation are the primary reservoirs for the coalbed methane(CBM)in the Hancheng region in the Ordos Basin.In this paper,the origin and evolution of waters associated with CBM production were studied on the basis of water chemistry and isotopes including the chloride and iodine compositions,oxygen and hydrogen stable isotopes,and radioactive isotope ratio of129I/127I.The ratio of129I/127I of water was determined by accelerator mass spectrometry(AMS).The result shows that the formation water is of NaHCO3and NaCl types with the total dissolved solids(TDS)varying from 1532.29 mg/L to 7061.12 mg/L.The values of129I and I/Cl ratio indicate that the formation waters were diluted by meteoric water.The129I/127I ratios range from 6.6×10 13to 1459.5×10 13.The129I/127I ratios for most of the samples are between the129I/127I initial value and that of recent anthropogenic water.This age of the formation water samples,obtained through the129I decay curve method,ranges from 0 Ma to 18.5 Ma,suggesting that the waters from the Taiyuan Formation and the Shanxi Formation are very young.Two different origins of water are identified in the Hancheng region.One group is dominated by pre-anthropogenic meteoric water,and is characterized by129I/127I ratios lower than the initial value of 15×10 13andδD,δ18O values of waters below the Global Meteoric Water Line.The other group is characterized by129I/127I ratios in excess of 15×10 13,which has undergone variable degrees of dilution by recent anthropogenic water. The coal seams in the Permian Taiyuan Formation and the Carboniferous Shanxi Formation are the primary reservoirs for the coalbed methane (CBM) in the Hancheng region in the Ordos Basin. This paper, the origin and evolution of waters associated with CBM production were studied on the basis of water chemistry and isotopes including the chloride and iodine compositions, oxygen and hydrogen stable isotopes, and radioactive isotope ratio of 129I / 127I.The ratio of129I / 127I of water was determined by accelerator mass spectrometry (AMS). The result shows that the formation water is of NaHCO3and NaCl types with the total dissolved solids (TDS) varying from 1532.29 mg / L to 7061.12 mg / L.The values ​​of 129I and I / Cl ratio indicate that the formation waters were diluted by meteoric water.The129I / 127I ratios range from 6.6 × 10 13 to 1459.5 × 10 13. The 129I / 127I ratios for most of the samples are between the 129I / 127I initial value and that of recent anthropogenic water. This age of the formation water samples, obt suggesting that the waters from the Taiyuan Formation and the Shanxi Formation are very young. Two different origins of water are identified in the Hancheng region. One group is dominated by pre- anthropogenic meteoric water, and is characterized by 129I / 127I ratios lower than the initial value of 15 × 10 13and δD, δ18O values ​​of waters below the Global Meteoric Water Line. The other groups are characterized by129I / 127I ratios in excess of 15 × 10 13, which has undergone variable degrees of dilution by recent anthropogenic water.