Seawater injection into oil reservoirs for secondary oil recovery is frequently accompanied by souring (increased sulfide concentrations) in crude oil. The hydrogen sulfide produced by microbiological sulfate reduction in the seawater causes various problems, including corrosion of tubing materials and deterioration of crude oil. Sulfate-reducing bacteria (SRBs) play major roles in souring. However, under high pH (>9), most microbes (including SRBs) cannot grow. Moreover, it is known that iron corrosion is theoretically negligible under the alkaline condition. To investigate new approaches to simultaneously control souring and metal corrosion, we analyzed souring under high-pH conditions. NaOH was added to adjust the pH clean seawater (ca. pH 8) to 11, or 13. Then, a carbon steel test coupon was incubated for 123 d and supplemented with microbes separated from oil field water (OFW) and crude oil. At pH 11 and pH 13, the corrosion rate of the test coupon was decreased. Additionally, souring did not occur at pH 11 and 13, although it took place at pH 8 with microbes. Next-generation sequencing analysis of the 16S rRNA gene revealed drastic changes in the microbial consortia for pH 8 after incubating for 111 d. Desulfotignum, which shows a high identity compared to that of toluene-utilizing SRB, became dominant. It is thought to contribute a biological souring by utilizing toluene in the crude oil at pH 8. On the other hand, at pH 11, the microbial consortia did not change significantly after 111 d of incubation. At pH 13, the microbial consortia drastically changed compared with that of initial condition (OFW) due to cell lysis. That is, even under strict conditions (e.g., pH 13), some bacteria are not lysed, increasing their relative ratio without growth. Alkaline addition could inhibit not only metal corrosion but also biological souring.