Shale investor Dan Wilks on Feb. 1 reported a 5.13% active stake in NexTier Oilfield Solutions Inc., saying the shares of the U.S. provider of hydraulic fracturing services are "undervalued" and that he may push for a sale or other alternatives.

The billionaire Wilks brothers have been acquiring stakes in hard hit services firms across North America, with Dan Wilks last month reporting a 7.5% active stake in troubled oilfield services firm ProPetro Holding Corp.

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Wilks Brothers LLC already owns hydraulic fracturing firm ProFrac Services and had recently engaged in a lengthy takeover battle for Canada's Calfrac Well Services Ltd.

Dan Wilks, who acquired the stake in NexTier through an investment vehicle, said he might propose capital allocation strategy, ownership structure and strategic transactions, including a sale as a whole or in parts as well as acquisitions or investments by NexTier, among others.

Dan Wilks and his brother Farris Wilks made their fortune during the shale boom through a company they founded called Frac Tech Services, which provided trucks to frackers.

NexTier, which recently changed its name following the combination of Keane Group and C&J Energy Services, was not immediately available for comment.