The Theory of Storage

For a financial asset with no storage cost, the no-arbitrage forward price is just spot plus the cost of financing the position to delivery — arbitrage keeps the curve pinned there. Commodities break this simple picture because holding a barrel of oil or a bushel of wheat costs money (storage, insurance, spoilage) and, more subtly, holding the physical commodity itself can be valuable in a way that holding a futures contract on it is not.

The theory of storage, developed by Holbrook Working, explains this second effect: a refiner or manufacturer that holds physical inventory gets convenience yield — the ability to keep running its plant without disruption if a supplier delay or local shortage hits, an option a futures contract on paper doesn't provide. When inventories are scarce, that convenience yield is high, because the marginal barrel in storage is doing valuable insurance work; when inventories are abundant, convenience yield falls toward zero, because there's little risk of running short regardless.

This is why commodity curves flip between contango and backwardation depending on inventory levels, unlike most financial futures curves which stay reliably in one shape. When storage is scarce and convenience yield is high enough to exceed financing and storage costs, the futures price sits below spot — backwardation — even though nothing about arbitrage in financial assets would ever produce that shape.