## 1 Introduction  
The oil recovery process is complex and expensive. Particularly for low permeability reservoirs, with permeability less than 50 mD, it is difficult to effectively increase oil production using conventional oil recovery techniques because of small pores, large filtration resistance, and low water absorption capacities [1, 2]. Microbial enhanced oil recovery (MEOR) is an effective alternative to tertiary oil recovery techniques; it involves the use of indigenous or exogenous microorganisms and their metabolic products, including biogas, biosurfactants, biomass, and acids, to retrieve residual crude oil from reservoirs [3, 4]. Since it is difficult for exogenous microbes to migrate into the reservoir strata, injecting exogenous functional microorganisms does not significantly improve oil production [5, 6]. To overcome the limitations of exogenous microbial flooding, indigenous microbial enhanced oil recovery (IMEOR) technology has been developed to stimulate the metabolism of beneficial microbes on-site by introducing nutrients into the recovery reservoir [7]. Primarily,however, nutrient flooding focuses on medium- and high-permeability reservoirs. Reports on the selective activation of microbial metabolism in low-permeability oil reservoirs are limited. [PERSON] et al. evaluated the stimulation of indigenous microbes by introducing an organic nutrient activator and found that activated microbes after the injection of nutrients shift the wettability of rock surfaces from oil-wet to water-wet, thus contributing to enhanced oil recovery [8]. Similarly, [PERSON] et al. found that Fe(III)-reducing microbes and their metabolites could improve reservoir permeability by reducing Fe(III) content and inhibiting clay swelling, thereby enhancing oil recovery in low-permeability oil reservoirs [9]. However, there are only few studies on the changes in microbial community structure and diversity in oil reservoirs after in situ indigenous microbial flooding. Many indigenous microorganisms in oil reservoirs play a crucial role in improving oil production, including _Pseudomonas_ spp. [10], _Arobacter_ spp. [11], _Bacillus_ spp. [12]_Acineto-bacter_ spp., _Rhodococcus_ spp. [13; 14], and _Arthrobacter_ spp. [15]. Most of those populations can survive in the harsh environment of oil reservoirs and are capable of hydrocarbon degradation, biosurfactant synthesis and emulsification. These species belong to groups with diverse functions, such as hydrocarbon oxidizing bacteria (HOB), nitrate reducing bacteria (NRB), methane producing bacteria (MPB), and ammonium oxidizing bacteria (AOB) [16]. During the MEOR process, injected nutrients selectively activate beneficial microorganisms in oil reservoirs and prevent the stimulation of unfavourable microorganisms, such as sulphate reducing bacteria (SRB). During their growth, SRB produce the metabolite H\({}_{2}\)S, which reduces oil quality and corrodes metal pipelines and oil storage tanks in the subsequent production process. Thus, it is necessary to investigate shifts in microbial community structure corresponding to the injection of nutrients during MEOR. Recently, with the emergence of high-throughput next generation sequencing technology, researchers can accurately and quickly obtain the microbial community structure from production water samples of oil reservoirs. Compared to traditional culture-dependent methods, this culture-independent approach is superior in estimating the overall composition of microbial communities in oil reservoirs, including both cultivable and uncultivable microbes. Increased accuracy and coverage have greatly advanced the comprehensive and in-depth analysis of the microbial community structure in the complex oil reservoir environment. In addition, high-throughput sequencing approaches based on the 165 rRNA gene of bacteria or archaea could help us identify the microbes that belong to different domains. [PERSON] et al.