1 Introduction
Understanding the mechanisms of biospheric characteristics in coal reservoir is important to CBM exploration (Tang et al., 2012; Wei et al., 2014; Guo et al., 2015; Wang et al., 2019). C-N-S microorganisms such as methanogens, nitrifying bacteria, denitrifying bacteria and sulfate reducing bacteria, play an important role in controlling the accumulation of coalbed methane, especially biogenic gas (Guo et al., 2012; Pashin et al., 2014; Schweitzer et al., 2019). Although such microorganisms regulate most biosphere processes related to fluxes of greenhouse gases like CO2, CH4 and N2O, little is known about their role in coal reservoir. Through the systematically collection of water samples from coalbed methane wells in Southern Qinshui Basin for microbial sequencing and geochemical testing, it will show whether microorganisms’ distribution changed in different hydrogeochemical zones of the coal seam.
Microorganisms mediate biogeochemical cycles of carbon (C), nitrogen (N), phosphorus (P), sulphur (S) in ecosystem dynamics. Fast developing metagenomic technologies allow us to explore the distribution characteristics and mechanism of microorganisms (Zhang et al., 2015; Wong et al., 2018). Integrated metagenomics technologies such as 16s RNA sequencing was used to determine the microbial community structure and functions structure in high rank coal ecosystem in Qinshui Basin.
Environmental factors such as redox conditions, pH, temperature and ion concentrations play an important role in controlling the distribution of various types of microorganisms, especially C-N-S microorganisms (Schlegel et al., 2011; Barnhart et al., 2013). These environmental factors are significantly changed in different hydraulic zones of Southern Qinshui Basin (Zhang et al., 2015). Therefore, this study predicts that hydrological conditions have important controlling effect on C-N-S microorganisms. The aim of this study is to find whether there is shifting microbial function composition from different hydrological zones, such as from runoff area to stagnant area. This study also tries to explain that if different microbial populations in different hydrological zones would have differential geochemical responses and that if weakened hydrodynamic conditions would greatly stimulate the functional genes involved in nutrient-cycling processes.
Southern Qinshui Basin located in north China craton is one of the most highly developed CBM region (Fig.1a) (Wang et al., 2016). Shizhuangnan block, in adjacent to Fanzhuang block and Zhengzhuang block (Fig.1b), is located in the transition area from the southeast edge of the basin to the deep part (Fig.2b). 3# coal seam is the main gas producing layer. The elevation of 3# coal is 223~597m (Fig.2b), and the coal thickness is 4.45~8.75m (Fig.2d). 3# coal seam in the study area has the highest coal rank (Romax 2.92~3.02%) (Fig.2c) and the highest gas content (8~25 m3/t) in Qinshui Basin (Fig.2a). Now there are more than 1000 drainage wells in this block (Fig.1c). The daily average gas production of 570 drainage wells in Shizhuangnan block was counted and the gas production of most wells exceeded 1000 m3/d. Moreover, high production wells were mainly distributed in the west of the block (Zhang et al., 2016).
Hydrogeological condition is an important controlling factor of coalbed methane enrichment (Xu et al., 2015; Yao et al., 2014). As the coal bearing strata in the study area are monoclinic structures extending from east to west, the synclines in the west weaken the hydrodynamic strength, the temperature and redox environment of the study area change dramatically from the oxidation runoff belt in the east of the basin edge to the deep stagnant area in the west (Zhang et al., 2015), making it an ideal area to research microbial distribution difference, the hydrogeological conditions of the study area will be introduced below.