A steam supply apparatus (1) for enhanced oil recovery from an oil field (10). The steam supply apparatus includes a water circulating portion (50) providing a continuous flow of water to a water heater (4) for the production of a mixture of steam and water, and a separator (5) for directing the steam to the oil field. Water returned from the oil field is purified in a water treatment apparatus (3) to provide make-up water to the water circulating portion. A buffer volume (3b) is provided in fluid communication between the water treatment apparatus and the water circulating portion. The buffer is sized to provide a time lag between a degradation of the quality of the purified water and an occurrence of an unacceptable scale production in the water heater, thereby allowing continued safe system operation until the water quality problem is corrected.