1. A method to determine viscosity of oil in a subterranean zone of interest, the method comprising: obtaining the oil from cuttings from drilling the zone or from a core sample of rock obtained when drilling the zone; determining a chemical signature of the oil in the zone; determining an amount of bacteria associated with the oil based on the chemical signature of the oil; identifying, from a relationship relating oil viscosities to amounts of bacteria, an oil viscosity for the amount of bacteria; and providing the oil viscosity for the oil.