From Halopedia, the Halo wiki
Contains Precinct 13
Tyumen is a city on a human colony. Its downtown area contains Precinct 13.
It is unspecified which Tyumen this is, as it could either be the Tyumen on New Harmony, or the real life Tyumen in Russia if it still exists in the 26th century. Alternatively, it could also be a Tyumen from a unspecified colony.