Microorganisms in petroleum reservoirs play significant roles in hydrocarbon degradation, and through the terminal electron-accepting process of methanogenesis, they also contribute to microbially enhanced oil recovery (MEOR) worldwide, with great economic and environmental benefits. Here, a molecular investigation, using the 16S rRNA and mcrA gene profiles based on MiSeq sequencing and clone library construction methods, was conducted on oil and water (aqueous) phases of samples of high (82-88 degrees C), moderate (45-63 degrees C), and low temperatures (21-32 degrees C) from seven petroleum reservoirs in China. A core bacterial microbiome with a small proportion of shared operational taxonomic unit (OTU) values, but a high proportion of sequences among all reservoirs was discovered, including aerobic degraders, sulfate- and nitrate-reducing bacteria, fermentative bacteria, and sulfur-oxidizing bacteria distributed mainly in Proteobacteria, Bacteroidetes, Deferribacteres, Deinococcus-Thermus, Firmicutes, Spirochaetes, and Thermotogae. Their prevalence in the previously reported petroleum reservoirs and successive enrichment cultures suggests their common roles and functions involved in aliphatic and aromatic hydrocarbon degradation. The methanogenic process generally shifts from the dominant hydrogenotrophic pathway in the aqueous phase to the acetoclastic pathway in the oil phase in high-temperature reservoirs, but the opposite was true for low-temperature reservoirs. No difference was detected between the two phases in moderate temperature reservoirs. Physicochemical factors, including pH; temperature; phase conditions; and nitrate, Mn2+, and Mg2+ concentrations were the main factors correlated to the microbial compositional and functional profiles significantly. Linear discriminant analysis (LDA) effect size (LEfSe) analysis shows distribution differences of microbial groups towards pH, temperature, and the oil and aqueous phases. Using the software Tax4Fun for functional profiling indicated functional metabolism differences between the two phases, including amino acids, hydrocarbons in the oil phase, and carbohydrates in the aqueous phase.