Certain strains of algae contain high amounts of vegetable oils. It is estimated that the algae in a 1 acre pond will have up to 15000 gallons of oil in it. The problem is that other bacteria gets into the water and greatly reduces the yield. The challenge is to find a way to keep the algae and get rid of the bacteria without creating a super strain of algae that would choke rivers and streams and lakes to death.
That would be about a third of a gallon per square foot. That would be the equivalent of a layer of oil about 1/2" thick.