2. Geological and hydrogeological conditions
The Qinshui Basin is a Carboniferous-Permian Basin. The outcrops of
strata in Qinshui basin include the Cambrian, Ordovician, Permian,
Triassic and Quaternary deposits (Fig.1d) (Zhang et al., 2015).
The coal-bearing strata are the Taiyuan and Shanxi Formations in the
Upper Pennsylvanian and Lower Permian System (Fig.1e). The outcropping
strata in and surrounding Shizhuangnan block include Ordovic Majiagou
Group (O2s), Fengfeng Group (O2f);
Carboniferous Benxi Group (C2b), Taiyuan Group
(C3t); Permian Shanxi Group (P1s),
Shihezi Group (P1x, P2s) and Shiqianfeng
Group (P2sh); Triassic Liujiagou Group
(T1l), Heshanggou Group (T1h), Ermaying
Group (T2er) and Tongchuan Group (T2t)
and some loose Quaternary layers including Pleistocene
(Q1, Q2, Q3) and Holoce
(Q4) (Fig.3a). The adjacent strata exhibit conformity
contact, except the O2f and C2b, which form a parallel
unconformity contact. Shanxi Group consist fine-grained sandstones,
coal, carbonates and shale, the average thickness is about 150 m
(Fig.1e).
Shizhuangnan block is located in the northwest inclined slope belt of
Qinshui Basin. The structure of the study area is striking NNE and
dipping the west. The outcrop of the Jinhuo fault on the eastern edge of
the basin has a relatively high elevation. Besides, the normal faults
developed in the north of the block, including the Sitou Fault, the
water blocking fault cuts the coal measure strata and controls the
hydrodynamic field in the study area (Zhang et al., 2015).
There are five aquifers in Shizhuangnan block: the Ordovician aquifer,
C3t aquifer, P1x-P1s aquifer, T1l-P2sh-P2s aquifer and Quaternary
aquifer. The coal seam aquifers are confined by nearly impermeable
mudstones or shales. So generally, the water production rates of 3#
coal drainage wells are usually low. However, in the north of study
area, faults connect coal reservoir with the roof aquifer, resulting in
high water production rates in the north.
The elevation of the #3 coal seam gradually decreases from east to west
(Fig.3b). The Jinhuo fault makes the Carboniferous-Permian strata
exposed in the eastern part of the study area, where the atmospheric
precipitation supplies and seeps from east to west along the monoclinic
structure (Fig.3a). Because the Sitou fault in the west of the study
area does not conduct water, with the coal seam and sandstone strata
extending to the deep, the hydrodynamic conditions gradually weakened,
forming the unique hydrogeological conditions of the transition from the
east runoff area to the west stagnant area. The syncline in the western
stagnant zones is the major high yield area of high production wells,
with high salinity, high reservoir pressure and high gas content.
This study focused on the differences of C-N-S function genes between
different hydrological zones and the relationship between microbial
function structure and geochemical characteristics in the Southern
Qinshui Basin. The stable isotopic compositions, major and minor ions in
CBM co-produced water, were analyzed, the microbial species in the coal
reservoir water at varying depths by 16S rRNA sequencing were
investigated to evaluate microbial effects in the Shizhuangnan block.