WTI's May contract closed at -$37.63/barrel while the June contract closed at positive $20.43/barrel. Prices mainly dropped due to the COVID-19 pandemic, which reduced demand, with storage issues and the expiration of the May contract on the following day resulting in reduced trading. For the first time ever, the reduction in trading shut down the Trade at Settlement (TAS) mechanism of the May 2020 WTI contract 30 minutes before the end of trading due to a lack of buyers. The TAS shutdown signaled to market participants that all remaining open May 2020 WTI contracts that have to be sold, either outright or for the purposes of rolling to the June 2020 contract, have to be sold in the open market in the next twenty minutes. The speed at which the contracts had to be sold, and the closeness of that time to contract expiry meant that large physical oil traders, who could have transported and stored oil elsewhere given a low enough price, could not buy contracts due to operational, risk management, and position limit constraints. Any remaining traders who were willing to buy contracts gained enormous market power and pushed prices downwards into negative territory. This situation is an extreme case of contango. Assess WTI prices were affected, and the WTI spot price dropped to -$36.98 on April 20. At the same time, Mars crude oil produced in the US Gulf Coast (USGC) settled at -$26.63, and Middle East exporters who uses ASCI (of which Mars is a component) as the selling price benchmark had to settle for negative prices that day. Pricing premium/discount in relation to Brent Historically, WTI has generally traded at a premium to Brent before 2011, but since the shale oil boom in the 2010s it has been traded at a discount against Brent crude oil. Both the volatility of the WTI/Brent premium/discount, as well as its reversal from premium to discount in 2011, are studied and monitored by market participants as indicators for the functioning of WTI and Brent Crude as oil price benchmarks. US supply factors In February 2011, WTI was trading around $85/barrel while Brent was at $103/barrel. The reason most cited for this difference was that Cushing had reached capacity due to a surplus of oil in the interior of North America. At the same time, Brent moved up in reaction to civil unrest in Egypt and across the Middle East.