The U.S. Energy Information Administration (EIA) has published an explainer on crude oil stocks at the Cushing, Oklahoma storage hub, noting that inventories may be approaching their operational floor, a threshold the agency calls “tank bottoms,” after inventories fell below a critical level for several consecutive weeks.
What triggered the explainer
Crude oil inventories held at storage facilities in Cushing, Oklahoma, fell below 20 million barrels during the week ending June 19 until the week ending July 10, according to the EIA’s Weekly Petroleum Status Report.
The sustained drawdown prompted the agency to publish a dedicated Today in Energy explainer defining what tank bottoms are and why they matter for the broader U.S. crude market.
Cushing has been running out of oil because America has become the supplier of last resort for regions of the world that typically get their oil and fuel from the Middle East, with U.S. exports surging to a record during the Iran conflict and crude flowing out of Cushing faster than domestic drillers could replenish it. Tanker flows through the Strait of Hormuz, a vital chokepoint accounting for roughly one-fifth of global oil shipments, effectively stalled amid heightened security risks. Brent crude jumped from around $72 a barrel on February 27 to nearly $120 at its peak, as fears mounted over supply disruptions through the Strait of Hormuz. Those Hormuz disruptions scrambled supply chains across the mid-continent market.
How tank bottoms work
Storage facilities for crude oil and petroleum products require a minimum volume of product in their tanks and pipe infrastructure to remain operational, which can vary between different storage facilities or tank systems.
If storage levels fall below their minimum volume, pump suction can become ineffective and storage facilities cannot function. These minimum volumes are sometimes called tank bottoms.
Tank bottoms are part of what distinguishes working storage capacity from shell capacity. Shell capacity refers to the total volume of material that might be held in a given storage facility by design. Working storage capacity refers to the maximum safe fill volume of storage available, minus the tank bottom capacity.
Storage facilities must generally operate within their working storage capacity limitations, which prevents inventories from truly falling to zero under normal conditions. This means not every barrel in inventories is necessarily accessible.
In extreme cases, an inventory facility may be unable to provide a product even though it is not technically empty, because it is effectively at the bottom of its working storage capacity.
Market impact: WTI flips above Brent
In mid-June, when storage at Cushing fell below 20 million barrels, the spot price differential between international benchmark Brent crude oil and U.S. benchmark West Texas Intermediate (WTI) priced at Cushing fell just below $0 per barrel.
The five-day rolling average differential became negative from June 18 to June 24, and again from July 2 through July 8.
The negative differential occurred as the WTI-Cushing price increased above the Brent spot market price, putting the spread at its lowest point since January 2022.
The relatively high WTI-Cushing price suggests that recent low inventory levels may be near tank-bottom levels, contributing to extreme tightness for Cushing storage operators and the mid-Continent crude oil market.
Cushing is one of the world’s largest oil storage hubs, and stock levels at the site are closely watched because they directly influence WTI prices and provide a key measure of the U.S. oil market’s supply-demand balance.
EIA study and what comes next
The EIA is currently reviewing the results of a pilot study to better understand the minimum working inventory levels at select petroleum inventory facilities.
The agency said it was examining those results after stocks reached levels that may constrain normal operations, signaling that a more detailed accounting of usable storage capacity could follow.
The development adds a new dimension to an already tight domestic supply picture. As GCN has reported, U.S. crude output outpaced Russia and Saudi Arabia by roughly 40 percent in 2025, yet the Cushing episode underscores that raw production figures do not capture how much oil is actually deliverable once storage systems approach their physical minimums.
Analysts and storage operators will be watching the EIA’s pilot-study findings closely for any revision to official working-capacity benchmarks.
