Hydrogeochemical Characteristics And Water-Rock Interactions Of Coalbed-Produced Water Derived From The Dafosi Biogenic Gas Field In The Southern Margin Of Ordos Basin, China

The hydrogeochemical characteristics of coalbed-produced water can provide insights into the sources of ions and water, the groundwater environments, hydrodynamic conditions, and water-rock interactions of depositional basins. To study the water-rock reaction process and reveal whether there is a microbial activity in the groundwater, a case of the Dafosi biogenic gas field was chosen by testing the ionic concentrations and hydrogen and oxygen isotopic compositions of coalbed-produced water and employing R-type cluster and principal component analyses. The results showed that Na+, Cl-, and HCO3- are the principal ions in the coalbed-produced water, while the water type is mainly a Na-Cl. Due to the hydrolysis of HCO3-, the pH in this region was controlled primarily by HCO3-. As the main cation in water, Na+ contributed substantially to the total dissolved solids. Na+ is also related to the exchange between rock-bound Na+ and Ca2+ and Mg2+ in water or surrounding rocks. The coalbed-produced water's oxygen isotopes displayed a characteristic O-18 drift and enrichment, indicating that the O-16 isotope in the water was preferentially exchanged with the coal organic matter. Early evaporation is also contributed to the enrichment of TDS (total dissolved solids) and O-18 in the water. The central part of the study area, including the Qijia anticline, was affected by the Yanshanian uplift and denudation and subsequently developed a water-conducting fissure zone and was recharged atmospheric precipitation; these conditions were conducive to the formation of secondary biogenic gas.