ExxonMobil Corp. on Tuesday struck a deal valued at close to $6.6 billion to more than double its Permian Basin resource and add about 275,000 acres total, mostly in New Mexico, signaling that it will give no ground in the hottest play in the U.S. onshore.

The transaction with the legendary Bass family of Fort Worth, TX, would add an estimated resource of 3.4 billion boe in New Mexico’s Delaware sub-basin. Once completed, ExxonMobil would have about 6 billion boe of resource total in the Permian, considered its No. 1 U.S. onshore target.

The properties, within Bass Operating Co. (Bopco) entities, would be a “major addition” to ExxonMobil’s unconventional liquids portfolio, said CEO Darren W. Woods. ExxonMobil’s U.S. onshore resources are operated by subsidiary XTO Energy Inc.

The Bass operations currently produce more than 18,000 boe/d net, 70% weighted to liquids. Included in the deal are close to 250,000 acres in the Permian alone, the bulk of which is contiguous, held-by-production units in the New Mexico Delaware formation, with an estimated 60 billion boe-plus in place.

“This acquisition strengthens ExxonMobil’s significant presence in the dominant U.S. growth area for onshore oil production,” Woods said. “This investment gives us an exceptional Delaware Basin position in a proven multi-stacked play that can generate attractive returns in a low-price environment.”

The mega-deal came one day after Noble Energy Inc. agreed to pay $2.7 billion for Permian-focused Clayton Williams Energy Inc., which would expand its position in the West Texas portion of the Delaware, and it continued a string of Permian acquisitions since the start of the year.

ExxonMobil currently is producing 140,000 boe/d net in the Permian in a leasehold that stretches across West Texas and southern New Mexico. Since early 2014, ExxonMobil has snapped up at least five Permian-related deals to build a position estimated at more than one million acres. Before making the deal with Bass, XTO Energy operated in four New Mexico counties with a leasehold estimated at more than 685,000 acres.

“The highly contiguous position will provide significant cost advantages in developing 3.4 billion boe of resource, of which 75% is liquids,” Woods said. “By utilizing ExxonMobil’s technological strength coupled with its unconventional development capabilities we can drill the longest lateral wells in the Permian Basin, reducing development costs and increasing reserve capture.”

The Bass family, notoriously private, is selling Bopco for an upfront payment of $5.6 billion in shares, as well as a series of contingent cash payments totaling up to $1 billion that would begin in 2020 and end by 2032. Bopco companies also hold producing acreage in other areas in the United States.