Oil recovery is a biological based technology consisting in manipulating function or structure, or both, of microbial environments existing in oil reservoirs. More efficient than other EOR methods when applied to carbonate oil reservoirs. The activity increases with microbial growth. This is opposite to the case of other EOR additives in time and distance. Microbial nutrients are biodegradable and therefore can be considered environmentally friendly. Enhanced oil recovery (abbreviated EOR), also called tertiary recovery, is the extraction of crude oil from an oil field that cannot be extracted otherwise. EOR can extract 30% to 60% or more of a reservoir's oil, compared to 20% to 40% using primary and secondary recovery. According to the US Department of Energy, carbon dioxide and water are injected along with one of three EOR techniques: thermal injection, gas injection, and chemical injection. More advanced, speculative EOR techniques are sometimes called quaternary recovery.