The cargo ship departs automatically when there is enough cargo to collect on the world map. Each journey of the ship has a fixed cost of fuel that is based on the size of the ship. Larger ships are more efficient in fuel consumption (they get upgraded automatically with cargo depot upgrade). The ship doesn't need to be assigned to individual mines / oil rigs, it collects cargo automatically. It can transport multiple types of products at the same time and will make sure all of them are supplied (it will depart anytime any of its products need to be transported).
TassoleHoff