Origin and Evolution of Waters in the Hancheng Coal Seams, the Ordos Basin, as Revealed from Water Chemistry and Isotope (H, O, 129I) Analyses

Xingzhi Ma; Yan Song; Shaobo Liu; Lin Jiang; Feng Hong

The Hancheng region in the Ordos Basin, Northern China, is an important CBM development region of China National Petroleum Corporation (CNPC). It is one of the largest coalbed methane (CBM) production fields in China. By the end of 2010, more than 400 wells had been drilled in the area, and the total geologically controlled reserves were estimated to exceed 1000×10⁸ m³. The coal seams in the Permian Taiyuan Formation and the Carboniferous Shanxi Formation are the primary reservoirs for CBM the 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 of ¹²⁹I/¹²⁷I. Ten water samples were collected from 10 CBM production wells in the center and the south of the Hancheng region. The oxygen and hydrogen stable isotope compositions were determined using a Finnigan MAT253 mass spectrometer with analytical precision of <±0.20‰ for δ¹⁸O, and <±1‰ for δD. The ratio of ¹²⁹I/¹²⁷I of water was determined by accelerator mass spectrometry (AMS).

The result shows that the formation water is of NaHCO3 and 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. The ¹²⁹I/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, obtained through the ¹²⁹I 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 waters 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^-13 and δD, δ¹⁸O values of waters below the Global Meteoric Water Line. The other group is characterized by ¹²⁹I/¹²⁷I ratios in excess of 15×10^-13, which has undergone variable degrees of dilution by recent anthropogenic water.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013