Shaking off oil price loses brought by Brexit concerns, West Texas Intermediate futures jumped about 4% to $49.88/bbl on June 29.

The move is a sign of steady gains seen since oil prices plummeted to a 13-year low of $26.68/bbl in January.

But as U.S. E&Ps, which have slowed production, await the desired price that will trigger a production ramp-up and global supply disruptions reduce the global crude stockpile, questions still remain.

“Is the rally sustainable and is the worst behind us?” Thomas Watters, managing director for S&P Global Ratings, asked earlier that day during a webcast on the U.S. oil and gas market outlook.

S&P has evaluated factors underpinning a rebound, but it is too early for the company to call a further rally. This is despite global supply outages peaking at a record 3.6 million barrels per day recently as wildfires in Canada, infrastructure-damaging militias in Nigeria and turmoil in Libya put a dent in global production.

“It looks like a good portion of production outages could come back online relatively soon,” Watters said, referring to operators restarting in Canada and negotiations between the Nigerian government and militia groups.

Another factor that could contribute to near-term oil prices are drilled but uncompleted wells in the U.S.

“It has been estimated that of the 4,000 or so of these wells that about 90% are economical at $50/bbl,” Watters said. “Once tapped these wells could potentially cap any significant price movement.”

Underlying fundamentals that will ultimately drive oil prices include: U.S. shale production; the resolve and ability of OPEC members to preserve market share; potential production increases from Iran, Iraq and Libya; and global oil demand, Watters said.

“We’d like to say that $50 to $60 is the new $90 to $100,” he added. “We believe that at $60 to $65 much of the production from U.S. shale becomes economical.”

Several companies have indicated plans to boost spending on new drilling with futures for the balance of the year and into 2017 topping $50 a barrel.

Analysts and producers have said U.S. crude prices at more than $50 were keys to triggering a return to the well pad. Some believe that number needs to be higher.

Regardless, S&P expects more oil price volatility. This could be the case whenever U.S. shale players increase production.

“Shale players can turn oil production on and off rather quickly, creating a shorter-term investment cycle—the likes of which we haven’t seen before,” Watters said. “Shale in effect is becoming an indirect swing producer, one based on economics and not subject to geopolitical factors that the Saudis have historically utilized the surplus capacity for. Despite concerns about Brexit, we don’t believe it will significantly disrupt the underlying fundamentals that ultimately drive oil prices.”

Fading concerns over Brexit, potential for an oil workers’ strike in Norway and a crisis in Venezuela’s energy sector were among factors supporting the June 29 rally.

Brent crude surpassed the $50 a barrel mark.

While spot contracts in Brent and U.S. crude surged, the premium for longer-dated oil spiked, too, as traders bet crude in storage will fetch better prices in coming months.

The U.S. Energy Information Administration reported that crude stockpiles fell 4.1 million barrels in the week to June 24, the sixth consecutive week of drawdowns. That was more than the 2.4 million barrels expected by analysts in a Reuters poll.

Velda Addison can be reached at vaddison@hartenergy.com. Reuters contributed to this report.