## 4 Discussion  
MEOR is regarded as a low-cost, simple and environment-friendly technique for improving oil recovery in low permeability reservoirs. After mixing with appropriate nutrients, the production water is injected into the formation. Water-soluble nutrients can enter the cracks of the rock and activate indigenous microorganisms, and their metabolites drive the oil to the bottom of the production well to improve the recovery yield of several materials, including biosurfactants, organic acids, and gases [16; 30]. The oil reservoirs studied in this research are located at Xingzichuan oil field, which is an ultra-low permeability oil reservoir. After two rounds of activator injection in the W226 well using the slug alternate injection method, the oil production of W226 increased to a rate 3.55 times more than that of the control well W202. To reveal the potential mechanism of MEOR by nutrient injection in the W226 well, a comparison of the microbial community and functional microorganisms was carried out between the production water samples from the production well W226 and the control well W202. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Well No.** & **Functional Bacteria** & **0 d (cells-L\({}^{-1}\))** & **30 d (cells-L\({}^{-1}\))** \\ \hline \multirow{4}{*}{W226} & Total bacteria & 2.54 \(\pm\) 0.42 \(\times\)10\({}^{7}\) & 4.32 \(\pm\) 0.78 \(\times\) 10\({}^{7}\) & 1.22 \(\pm\) 0.24 \(\times\) 10\({}^{9}\) \\  & HOB & 9.78 \(\pm\) 1.15 \(\times\) 10\({}^{6}\) & 1.16 \(\pm\) 0.10 \(\times\) 10\({}^{7}\) & 5.34 \(\pm\) 0.62 \(\times\) 10\({}^{8}\) \\  & NRB & 6.54 \(\pm\) 0.22 \(\times\) 10\({}^{5}\) & 1.14 \(\pm\) 0.24 \(\times\) 10\({}^{6}\) & 9.73 \(\pm\) 1.23 \(\times\) 10\({}^{7}\) \\  & SRB & 7.54 \(\pm\) 0.39 \(\times\) 10\({}^{6}\) & 4.71 \(\pm\) 0.63 \(\times\) 10\({}^{6}\) & 8.76 \(\pm\) 0.92 \(\times\) 10\({}^{5}\) \\ \hline \multirow{4}{*}{W202} & Total bacteria & 1.94 \(\pm\) 0.35 \(\times\) 10\({}^{7}\) & 9.16 \(\pm\) 0.85 \(\times\) 10\({}^{6}\) & 2.31 \(\pm\) 0.42 \(\times\) 10\({}^{7}\) \\  & HOB & 8.14 \(\pm\) 0.92 \(\times\) 10\({}^{6}\) & 3.38 \(\pm\) 0.41 \(\times\) 10\({}^{6}\) & 4.54 \(\pm\) 0.68 \(\times\) 10\({}^{6}\) \\ \cline{1-1}  & NRB & 4.98 \(\pm\) 0.39 \(\times\) 10\({}^{5}\) & 6.58 \(\pm\) 0.54 \(\times\) 10\({}^{4}\) & 4.92 \(\pm\) 0.72 \(\times\) 10\({}^{4}\) \\ \cline{1-1}  & SRB & 5.93 \(\pm\) 0.42 \(\times\) 10\({}^{6}\) & 2.74 \(\pm\) 0.63 \(\times\) 10\({}^{6}\) & 6.78 \(\pm\) 0.65 \(\times\) 10\({}^{6}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Enumeration of functional microorganisms in oil reservoirs. Figure 5: Comparative bacterial (**a**) and archaeal (**b**) community analysis of the production and control well at the genus level. The analysis was conducted among the genera with the abundance of more than 1%. Data were analysed using the results from triplicate independent experiments by _t_-test. \(p\) values less than 0.05 were considered significant. The addition of activators increased the richness but reduced the evenness of the bacterial community, whereas it increased both the richness and evenness of the archaeal community, thus indicating the significant impact of activators on the archaeal community, especially at the genus level (Figure 5b). The hydrogenotrophic methanogens of the mesophilic _Methanocalculus_ and thermophilic _Methanothernococcus_ genus (from phylum Euryarchaeota) were predominant in production well W226, which has been detected in most oil reservoirs as the dominant archaea [31]. The thermophilic methanogen _Methanothernococcus_ spp. appeared to conduct nitrate and methane metabolism in the production water samples, where they were predominant [32]. Consequently, the increased diversity of archaea, especially halophilic archaea in the W226 well, indicates that the archaeal community is more adaptable in high salinity conditions after adding the activator.