4.2 Division of runoff area and stagnant area
The sampling was mainly in the south of Shizhuangnan block with the purpose of comparing the distribution differences of C-N-S functional genes between stagnant area and runoff area, so one of the most import issues in this study was to divide different hydrodynamic zones. Sever factors that change significantly were considered, as shown in the contour map.
The direct manifestation of hydrodynamic field is water pressure, so the three-dimensional hydrodynamic field in the sampling area was firstly modeled, and clearly the the water pressure rose sharply from east to the west on the edge of the western syncline (Fig. 5b), which was caused by the convergence of water flow in the low-lying area of the western stagnant area due to the Sitou fault which blocked water at the boundary.
The edge of the syncline in the west of the study area could be used as the boundary line to divide the hydrodynamic zones, as indicated by the black dotted line (Fig.4). Through this boundary, the atmospheric precipitation transit from the flowing state to the stagnant state, that was, from the runoff area to the stagnant area. The stagnant environment was conducive to the enrichment and preservation of coalbed methane. Indeed, the measured gas content had obvious changed across this line, from 7~12 m3/t to 14~20 m3/t (Fig.4). The stagnant environment made the interaction between water and rock stronger, resulting in higher mineralization in the stagnant area, which was also confirmed by the KDS concentration, as shown in the Fig. 6d. As there was transition from oxidation environment in runoff area to reduction environment in stagnant area, the geochemical data also support the above view, the concentration of NO3-, SO42- , Fe3+ and δ13CDIC, had obvious changed on both sides of this line, NO3-, SO42- and Fe3+ were anaerobic electron acceptor in involved in several important anaerobic respiration processes, such as denitrification, sulfate reduction and iron reduction, their concentration were all reduced in the stagnant area’s anoxic environment , shown in Fig. 6a~6c.
It could be predicted that with the precipitation moving from east to west, the dissolved oxygen in water was gradually consumed, the aerobic respiration was weakened while the anaerobic respiration was enhanced, resulting in the consumption of anaerobic electron acceptors, such as NO3-, SO42- and Fe3+ in the stagnant area. As the environment in the stagnant area was lack of oxygen supply, the anaerobic respiration was stronger than that in the runoff area, and the consumption of the electron acceptors was stronger in stagnant area.
Anaerobic respiration such as methanogenesis has isotope fractionation effect (Wang et al., 2016), resulting biogenic methane enriches lighter carbon and then aggravates dissolved inorganic carbon isotope (McCalley et al., 2014). If the methane production in stagnant area was stronger, the dissolved inorganic carbon isotope in stagnant area would be more positive than that in runoff area. The dissolved inorganic carbon isotope test results supported this view the C pool was isotopically fractionated by microbial methanogenesis or other microbial carbon cycling effect(Fig.6e).
Considering the structural location (Fig.6f), water pressure (Fig.5b), gas content (Fig.4) and geochemical data (Fig. 6a~6e), the edge of syncline structure was selected as the boundary of dividing runoff area and stagnant area, as shown in Fig.5a, the red area was stagnant area, the blue area was runoff area, and the gray area in the north was fault developed deep coal.
In general, the structural location determined the hydrodynamic conditions, and then affected the distribution of hydrochemical field. This study pedicteded that the microbial functional genes involved anaerobic respiration such as denitrification, sulfate reduction were stronger in stagnant area. Next, this prediction would be proved by the gene sequencing results.