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Oil may strengthen, then fall on Iran deal: Gartman

There may actually be strength in the oil market before the final deal with Iran is reached, investor Dennis Gartman told CNBC on Thursday. but once that deal happens expect crude prices to hit new lows.

Oil closed down Thursday after global powers negotiating with Iran announced they had reached a framework of agreement on the country's nuclear program. If Tehran implements the framework measures, it would eventually result in the United States and the European Union easing sanctions, originally implemented to squeeze Iran's nuclear program. The end of those sanctions could bring millions of additional Iranian crude barrels into the market.

A final deadline for the talks is set for June 30, which could be a testing time for crude markets.

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"Between and now and June, you may actually see the front months WTI strengthen up simply because … the market will know [while] there will be plenty of crude after June, there may be less supply before June," the editor and publisher of The Gartman Letter said in an interview with "Closing Bell."

However, after once the deal is completed, "you are going to see new lows in crude oil because the Iranians are going to be extraordinarily aggressive in selling. They have no choice."

Defending the market

An oil tanker is seen off the port of Bandar Abbas, southern Iran.
Atta Kenare | AFP | Getty Images
An oil tanker is seen off the port of Bandar Abbas, southern Iran.

In addition, Gartman said, Saudi Arabia will be sure to defend its market share and put pressure on crude prices.

John Kilduff, founding partner of Again Capital, agrees oil is headed lower, noting that Asian countries have already been aggressively buying the Iranian oil that has been allowed in the market. He expects that to continue.

"This is going to have another tremendous downward push for oil prices once it becomes realized that this oil is going to get sold fairly rapidly," said Kilduff, also a CNBC contributor.

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He also thinks if Congress votes down the agreement, the current global coalition will come to an end. "The rest of the world is going to move on without us and normalize relations with Iran," he predicted.

Meanwhile, the trade that most interested Gartman wasn't whether to go long or short on oil. Instead, he wants to own tankers.

"There is a lot of crude oil that is going to be out there. It's going into storage, it's filling up in Cushing, it's filling up in the Gulf, and it's going to fill tankers," he said.

"If you take a look at tanker stocks, it's been one of the strongest areas of any stock market in the world," Gartman added. "That's the only thing I'm certain of at this point."

Read MoreAnalyst: We've got an oversupplied oil market

—Reuters contributed to this report.