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.