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.