(Bloomberg) -- Activist investor Elliott Investment Management LP has amassed a $1 billion stake in Phillips 66 and is pushing for two board seats to improve the performance of its refineries and boost its stock price.

Elliott sent a letter to the Houston-based oil refiner’s board saying it sees an opportunity to increase Phillips 66’s stock price by about 75% by improving execution, restoring trust with investors and other measures, according to a statement Wednesday. Elliott didn’t call for ousting Phillips 66 Chief Executive Officer Mark Lashier.

“Mr. Lashier and the rest of the management team deserve investor support so long as they demonstrate meaningful progress against these targets,” Elliott wrote in the statement. “At the same time, we find the market’s skepticism to be understandable, and we believe the Board must take several steps to reassure investors that Phillips 66 is in the best possible position to achieve its value-creation potential.”

Phillips 66 was the only oil refiner to advance in the S&P 500 Index on Wednesday with a 3.6% increase as of 10:16 a.m. in New York. 

“We have engaged in discussions with Elliott Management, and we welcome their perspectives and the perspectives of other shareholders on our strategy and the actions we are taking to drive long-term sustainable growth and value creation,” Lashier said in an emailed statement.

The shares had advanced 13% this year before Elliott disclosed its stake, making it the fifth-best performer in the S&P 500 Energy Index. The company is the second-largest US fuel-maker by market value and its push into petrochemicals and natural gas liquids have helped insulate it during refining downturns, according to Bloomberg Intelligence.

Elliott, however, says the company’s performance has lagged in recent years as it has shifted its focus away from refining. As a result, Phillips 66 was poorly positioned to take advantage of the 2022-2023 boom for the refining sector, according to the statement. 

The West Palm Beach activist firm led by Paul Singer has a long record of taking stakes in energy companies and pressing for changes. The investor said it has identified multiple board candidates with refining experience to help boost Phillips 66’s performance.

If Phillips 66 fails to improve and meet its 2025 targets, Elliott said the company could follow a similar path as Marathon Petroleum Corp. In 2019, Marathon faced months of pressure from investors including Elliott and D.E. Shaw & Co. calling for sweeping changes. The campaign ultimately led to Gary Heminger stepping down as CEO after 45 years at the company.

Phillips 66 has been looking to expand its reach in natural gas liquids, key feedstocks for refineries and petrochemical plants. Earlier this year, the company agreed to buy $3.8 billion worth of common units in DCP Midstream LP to expand its overall stake in the pipeline operator to almost 90%.

--With assistance from Mitchell Ferman.

(Adds statement from Phillips 66 CEO in fifth paragraph)

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