The U.S. Energy Information Administration published a detailed explanation this week, saying that crude oil inventories at the Cushing, Oklahoma storage hub may have fallen to what the industry calls “tank bottom” levels. At that threshold, reported stocks no longer accurately represent barrels that can actually be withdrawn and delivered to market.
Four weeks below 20 million barrels
Crude oil inventories at Cushing fell below 20 million barrels during the week ending June 19 and remained there through the week ending July 10, according to the EIA’s Weekly Petroleum Status Report.
Stocks at the hub fell sharply after the Strait of Hormuz closure forced global buyers to seek alternative oil supplies and draw more heavily on U.S. crude stocks.
The EIA said it was reviewing the results of a pilot study to better understand the minimum working inventory levels at select petroleum inventory facilities. Cushing serves as the delivery point for U.S. West Texas Intermediate futures and is one of the world’s most closely watched oil storage hubs.
What tank bottoms are and why they matter
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 that minimum volume, pump suction can become ineffective and storage facilities cannot function. These minimum volumes are sometimes called tank bottoms.
Tank bottoms are a 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, while working storage capacity refers to the maximum safe fill volume of storage available, minus the tank bottom capacity.
As GCN has reported, the EIA published a detailed explainer on tank bottom risk as inventories at Cushing crossed below the critical threshold.
Storage facilities must generally operate within their working storage capacity limitations, which prevents inventories from truly falling to zero under normal conditions. This means that not every barrel of crude oil or petroleum in inventories is necessarily accessible.
In extreme cases, a storage 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.
WTI briefly flipped 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, as the WTI-Cushing price rose 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.
For traders, the distinction matters because the weekly inventory figures published by the EIA are among the most closely watched indicators in global oil markets. If a growing share of reported inventories consists of operationally unavailable crude, headline stock figures may overstate the volume that can respond to immediate supply disruptions.
EIA pilot study underway
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’s “What are tank bottoms?” explainer, published July 16, 2026, draws a distinction between reported inventories and deliverable supply, a nuance that the EIA notes carries an important operational caveat when stocks approach these thresholds.
Cushing’s role extends well beyond storage: it serves as the primary delivery and pricing point for West Texas Intermediate crude oil futures on the NYMEX, where physical settlement and delivery of U.S. benchmark crude occurs. For broader context on national crude supply trends, GCN’s earlier report on record U.S. crude output in 2025 underscores why Cushing’s operational constraints carry such wide market significance.
The EIA’s pilot study findings are expected to inform how the agency reports and contextualizes storage data going forward.
