Diamondback Energy Inc. (NASDAQ: FANG) shares fell 10% early on Aug. 15 following the U.S. oil and gas producer’s all-stock, $9.2 billion deal for shale rival Energen Corp. (NYSE: EGN).

On Aug. 14, Diamondback agreed to pay about $84.95 per share for Energen, a 16% premium to the Birmingham, Alabama, company’s close on Aug. 14. Energen shares rose 3.8% to $75.95 on Aug. 15 morning trade on the New York Stock Exchange (NYSE).

Diamondback was down 10.4% at $119.83 per share on the NYSE. U.S. crude futures fell $2.25 to $64.78 per barrel in New York on Aug. 15. The Dow Jones industrial average was down about 1% on Aug. 15.

Activist investor Corvex Management LP, which holds about 8.3% of Energen and had pushed for a sale of the company, signaled it intends to support the deal. The acquisition must be approved by regulators and shareholders of both companies.

“The transaction is strategically compelling and delivers significant financial accretion to Energen shareholders,” Keith Meister, managing partner of Corvex Management, said. “The pro forma company should be poised for many years of growth at an industry leading cost structure.”

The transaction, which includes assumption of $830 million in Energen debt, is expected to close by the end of year, Diamondback’s CEOTravis Stice said on a call with investors on Aug. 15.

Stice said the combined company could achieve $2 billion in synergies starting in 2019 and has the potential to add another $1 billion in cost savings and higher revenues thereafter.

“This transaction represents a transformational moment for both Diamondback and Energen shareholders as they are set to benefit from owning a milestone large-cap independent that is uniquely positioned to deliver unmatched growth and returns in the Permian basin,” Stice said.

Activist investor Carl Icahn, who owns about 8.9% of Energen, was unavailable to comment Aug. 15 on whether he will support the sale.

The drop in Diamondback’s share price reflected oil investors’ worry the transaction could dampen the company’s debt-adjusted production growth per share, said Michael Kelly, an analyst at investment bank Seaport Global Solutions LLC.

“Investors in the oil and gas space are a little skeptical of the synergies that can come from this and whether the share dilution is warranted,” Kelly said, adding he views the transaction as a buying opportunity.