An anaerobic environment is one that lacks oxygen, which can significantly affect the types of chemical reactions that occur, as well as the types of organisms that can survive. In geology, anaerobic conditions are often found in deep sediments, swamps, and some groundwater systems. These environments are important in the study of biogeochemical cycles, particularly the formation of organic-rich sediments and fossil fuels.
Reference: Berner, R. A. (1981). “A New Geochemical Classification of Sedimentary Environments.” Journal of Sedimentary Research, 51(2), 359-365.