## 1 Introduction  
Since the discovery and use of oil, oil production has undergone significant long-term development. The global demand for oil has continued to increase, without abating. Chemical enhanced oil recovery (EOR) has been recognised as an effective technique for the recovery of bypassed oil, as well as residual oil trapped in the reservoir [1]. Alkaline-surfactant-polymer (ASP) flooding technology has been widely used in the Daqing field, China. ASP flooding technology increases the sweep volume and improves the oil recovery efficiency through the synergistic effects of alkali, polymer, and surfactant [2]. However, almost half of the crude oil still remains in the subsurface after the ASP flooding process. Microbial enhanced oil recovery (MEOR) technology has emerged as one of the most promising methods for oil recovery due to its simple process, low investment, and minimal impact on the environment. MEOR involves the injection of nutrients and suitable bacteria that are able to grow under anaerobic reservoir conditions into a reservoir [3]. MEOR bacteria are carried by injected water and accumulate in porous oil/rock and oil/waterinterfaces. Microbes can contribute to EOR in two main ways: by growing in the reservoir rock to produce gases, biosurfactants, biopolymers, and other non-toxic biochemicals to recover trapped oil; and by selectively plugging high-permeability channels to increase the sweep efficiency of the recovery process. Selective plugging approaches use microbial cell mass and/or biopolymers to plug high-permeability zones and redirect the flow [4; 5]. Other processes produce biosurfactants in situ, leading to an enhanced mobilization of residual oil and a reduced oil viscosity [6; 7; 8]. Most importantly for the petroleum industry, these microbial products should induce a number of highly desirable changes in the physicochemical properties of the crude oil and significantly improve or almost completely restore the reservoir rock lithological properties [9]. The prolonged interaction of the metabolites with the oil in the reservoir changes the properties of the oil in such a way that the immobile, non-recoverable oil is converted into mobile oil that can flow to the production wells and increase oil production accordingly [10]. Successful in situ MEOR operations depend on the development of microbial consortia capable of surviving and producing the desired metabolites in hydrocarbon-bearing and saline reservoirs [11]. Research activities are continuously focusing on anaerobic extremophiles, including halophiles, barophiles, and thermophiles, for better adaptation to reservoir conditions [12]. Currently, with the use of molecular and culture-based methods in petroleum microbiological analyses, more studies have shown microbial community diversity in different reservoir environments [13]. This is strongly in support of the hypothesis that certain microorganisms have a wide distribution in oil reservoirs. The different types of microorganisms in the reservoir environment interact through material and energy metabolism, forming a stable community structure. This structure is the material foundation of microbial oil recovery technology. However, there are significant differences in the structures of microbial communities due to the different types of oil reservoir environments and different exploitation methods. [PERSON], [PERSON] et al. compared the microbial community structures of oil and water phases in a low-permeability reservoir after water flooding [14]. [PERSON], [PERSON] et al. investigated the effect of polymer flooding technology in the Daqing oil field on the microbial community in the oil reservoir system [15]. Furthermore, studies on the microbial diversity and community composition in reservoirs flooded with ASP are limited.