The economics of producing a marginal Cardium sand oil field in west-central Alberta have been greatly improved by introduction of an inexpensive waterflood pressure maintenance and secondary recovery project. Canadian Gridoil Ltd. is now in full operation at its Willesden Green Cardium Unit No. 5. Of the 8.1 million bbl estimated original oil in place, only 9% would have been recoverable by primary depletion. The waterflood is calculated to add 13%, for ultimate recovery of 22% or 1.8 million bbl. This waterflood installation is considered a prototype of economical and profitable pressure maintenance systems which can be built to advantage in marginal oil fields in Alberta. Ultimate returns in the form of increased oil production and more than doubled oil recovery will be immensely greater than the capital investment of $195,000 in the facilities. Assuming GOR control and full well allowables, the entire capital cost should be paid out within 3 years. Life of the field is estimated at not less than 25 years.