Biogeochemical Assessment of the Coalbed Methane Source, Migration, and Fate: A Case Study of the Shizhuangnan Block, Southern Qinshui Basin.

Abstract

The exploration and exploitation of coalbed methane (CBM), an essential unconventional gas resource, have received much attention. In terms of shallow groundwater assessment during CBM production, biogenic methane natural formation in situ and methane migration from deep sources into shallow aquifers need to be of most concern. This study analyzes geochemical surveys including ions, isotopes, and dissolved methane concentrations in 75 CBM coproduced water samples in the southern Qinshui Basin. Most of these water samples are weakly alkaline. Some samples’ negative oxidation/reduction potential (ORP) values reveal that the CBM reservoir water samples are mainly produced from reductive groundwater environments. Cl–, Na+, and HCO3– are the dominant ionic constituents of the water samples, which are usually associated with dissolved methane concentrations. The biogeochemical parameters and isotopic features provide an opportunity to assess the origin, migration, and oxidation of biogenic or thermogenic methane. Some water samples suggest biogenic methane formation in situ characterized by negligible SO42– and NO3– concentrations and low δ13CCH4. Only a few water samples indicate the migration of biogenic methane into shallow aquifers without oxidation based on elevated SO42–, NO3–, and δ13CDIC and low δ13CCH4. A few cases characterized by elevated δ13CCH4, negative δ13CDIC values, and negligible SO42– and methane concentrations suggest the oxidation of biogenic methane rather than the migration of thermogenic methane. A significant number of cases mean methane migration to shallow aquifers. Partial oxidation of thermogenic or mixed methane is evaluated by negligible SO42–, NO3–, and methane concentrations and elevated δ13CCH4. Dissolved methane isotopic compositions and aqueous biogeochemical features help study methane formation and potential migration in shallow groundwater.