From Oil Refineries:

shipping yards or piers occasionally and sometimes continuously leak oil into the water. This discharge or waste oil from ships is not negligible.

Storage tanks: there is always some continuous lead-age or vaporization into the air. About 0.5% of the yearly through-put is lost into the air through evaporation.

Refinery plant consisting of topping tower, vacuum distillation tower, with many joints, pumps, valves etc. These are all small sources of leakage, whose total quantity of leakage is significant.