Abstract: A fluid impermeable bag for enclosing a lower portion of a residential heating oil reservoir. The secondary containment has an open upper end, a stiff rim around the open upper end and straps extending from the stiff rim above the open upper end for circling an upper portion of a reservoir. The bag is made of a flexible material whereby the bag is collapsible for installation on a new tank or for retrofitting a reservoir in service. In a second aspect of the invention, a flap is detachably affixed to the rim of the bag and is extendible for covering the upper portion of a reservoir installed outdoors. The bag of the present invention is mountable under a heating oil reservoir and is particularly appropriate for preventing environmental spills of all sizes due to common deteriorations of these reservoirs.