Billionaire oilman Dan Wilks increased his stake in NexTier Oilfield Solutions, according to a regulatory filing on Thursday, his latest purchase in the hard-hit oilfield services industry.

Oilfield equipment and service companies were among the worst performing sector of the energy business as the COVID-19 pandemic cut oil demand. Wilks, who made his fortune on the U.S. shale boom through a Texas oil trucking firm with his brother is betting the sector has bottomed, according to analysts.

Wilks and his brother, Farris, last month took a roughly 10% stake in hydraulic fracking firm U.S. Well Services. Wilks also snapped up shares in rival shale fracking firm ProPetro Holding Corp, reporting a roughly 7.5% stake in January, and has been engaged in a lengthy court battle to take over Canadian pressure pumper Calfrac Well Services Ltd.

Wilks first took a 5% active stake in NexTier in February and now holds a nearly 8% stake in the company, according to Thursday's U.S. Securities and Exchange Commission (SEC) filing.

Representatives for NexTier and the Wilks family did not immediately respond to a request for comment.

The Wilks Brothers own hydraulic fracturing firm ProFrac Services. In addition to the family's equity stake in U.S. Well Services, ProFrac recently invested roughly $22.5 million in the company as part of a licensing agreement that allows them to build and operate electric frac fleets.