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The microbial community in oil reservoirs plays important roles in carbon, sulfur, and nitrogen cycling. We identified the microbial composition, abundance, and diversity in two heavy oil wells (W1 and W2) located at Liaohe oilfield, China, that were flooded with higher temperatures, forming a decreasing gradient-temperature field (DGTF) from the steam chamber to the original surrounding formation. The two wells were connected to each other at a depth of 650 m and with temperatures of 249 °C (W1) and 259 °C (W2) in the steam chamber. Water and oil samples were collected from both wells, and 16S rRNA genes were sequenced using Illumina MiSeq (Illumina, San Diego, CA). The number of operational taxonomic units (OTUs) was 5499 and 5938 for W1 and W2, respectively, at 97% clustering. In both wells, the dominant bacterial phylum was Proteobacteria. Euryarchaeota and Crenarchaeota were the only two archaeal phyla detected. Terrimonas and Fluviicola were more abundant in the water sample W1–W, whereas Thermomonas, Nitrosomonas, and Sphingobacterium were more abundant in the oil sample W2-O. Aquificae was the only phylum detected in these wells that had not been detected in other oilfields. Excluding exogenous contamination, the results of this study showed abundant in-situ microbial diversity. The heavy oil reservoir has a large DGTF in higher temperature heavy oil reservoirs, potentially affecting the inner environment. [ABSTRACT FROM AUTHOR]