Energy stocks are surging.

A set of coordinated attacks that took down more than half of Saudi Arabia’s oil supply caused the biggest spike in Brent crude prices in history over the weekend, sparking a widespread rally in the energy sector on Monday as the major averages turned lower for the first time in nine days.

Prices for Brent, the international oil benchmark, and U.S. West Texas Intermediate crude surged more than 12% on Monday. The Energy Select SPDR Fund, an exchange-traded fund tracking 28 large-cap energy stocks under the ticker XLE, climbed by more than 3%.

Out of all the subgroups in the energy space — which include oilfield service companies, refiners and producers, among others — one in particular stood out to CFRA and its head of ETF and mutual fund research, Todd Rosenbluth.

“We are more favorable on the exploration and production companies,” many of whose stocks can be found in the SPDR S&P Oil & Gas Exploration & Production ETF (XOP), Rosenbluth said Monday on CNBC’s “ETF Edge.” The XOP was up nearly 11% toward the end of Monday’s trading session.

“Some of these smaller-cap companies … [have] higher debt leverage, weaker cash flows,” Rosenbluth said. “This has really been a boost for them, a jolt that was unexpected from a supply perspective, and this could take a little while [to play out].”

At the same time, Rosenbluth urged investors to “make sure that you’re diversified,” particularly considering the pitfalls of a fund like XLE, which has 45% of its portfolio in two stocks: Exxon Mobil and Chevron.

But some investors are grappling with an even larger concern, said Tom Lydon, president of Global Trends Investments and the editor and proprietor of ETF industry website ETFTrends.com.

When the attacks occurred over the weekend, “investors were doing their homework, because we had a huge spike in not only long-term buy-and-hold ETFs, but also leverage and inverse ETFs,” Lydon said of his website’s tracking information in the same “ETF Edge” interview.

So, while some were interested in the longer-term prospects of investing in a fund such as XLE or the United States Oil Fund, ticker USO, which tracks crude oil itself, others were looking toward more aggressive directional trades, Lydon said.

“There was an equal balance between investors thinking there might be another long-term play here if we have more tension overseas, but also, if this thing is just a one-and-done opportunity, there’s an opportunity here to maybe short oil if it reverts back,” he said.

The Citigroup Global Markets VelocityShares 3x Inverse Crude Oil, a leveraged fund used to sell oil prices short, made a new 52-week low on Monday after falling almost 40%. The Citigroup Global Markets VelocityShares 3x Long Crude Oil — another leveraged ETF used to trade oil to the upside — rallied over 34%.

“The big question is, … going forward, is it one and done or are we going to see more?” Lydon reiterated. “A lot of investors out there are thinking that might be the case, so they’re actually leveraging up.”

And while he emphasized that those are primarily “short-term” trades, Lydon also had his eye on the long term.

“[Energy stocks are] undervalued for sure, and prices in general are low,” he said. “We’ve seen what can happen with a big catastrophe, this event that occurred. So, coming down to it, … we’re long-term investors for sure. We’re encouraging people to be long term, but there’s a trade on right now and people are surely taking advantage of it.”

And the health of the broader economy will determine the longevity of that trade, Rosenbluth said.

“Supply and demand is obviously the key thing that happens within the energy space,” Rosenbluth said. “Now, we’ve seen supply come off a bit. Demand is continuing to do well. If we see the global economy strong, we could see that continue.”

President Donald Trump said Monday afternoon that he was in no rush to respond to the attacks on Saudi Arabia’s oil reserves, which cut over 5% of the world’s daily crude oil production.

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