The phrase "marginal barrel" refers to the production of an additional barrel of oil, which is considered to have higher costs or lower profitability compared to the other barrels already being produced. Full definition
What had previously been a market priced at a premium to world prices because the marginal barrel was moving in was now priced at a significant discount because the marginal barrel was moving out, and by expensive means. (canadianbusiness.com)
By late 2010, the marginal barrel in the U.S. Midwest was being moved out to the US Gulf Coast by barge or by rail — both more expensive than pipeline shipment. (canadianbusiness.com)