Aug 19 (Reuters) - Marathon Petroleum Corp and Archer-Daniels-Midland Co announced on Thursday a joint venture to produce soybean oil that will be exclusively sold to Marathon as a renewable diesel feedstock.

Refiners are on the hunt for secure access to feedstocks for renewable fuels amid supply constraints and soaring prices for fats, greases and oils.

The JV soybean processing complex in Spiritwood, North Dakota, is expected to produce about 600 million pounds of refined soybean oil annually, enough feedstock for about 75 million gallons of renewable diesel per year when complete in 2023, the companies said.

That is approximately 40% of the feedstock needed to supply Marathon's nearby Dickinson, North Dakota, plant, which can process about 180 million gallons of renewable diesel annually.

Marathon also plans to convert its idled Martinez, California refinery to produce 730 million gallons per year of renewable fuels, one of a number of refiners ramping up green fuel production.

The project creates a domestic buyer for the state's export-dependent soybean producers, who saw demand from top buyer China, evaporate during the U.S.-China trade war.

ADM executive Ken Campbell said he believes renewable diesel demand may be as much as 5 billion gallons by 2025.

But Marathon told investors last quarter that soybean oil's economics were "challenged" because higher prices coupled with the relatively higher carbon intensity of the oil limits refiners' ability to profit on production.

Margins to produce renewable diesel from soybean oil so far this quarter have averaged about $1.35 per gallon, more than $1 lower than processing used cooking oil, according to data from investment bank Tudor, Pickering, Holt & Co.

Carl Icahn's CVR Energy paused plans to produce renewable fuels at its Wynnewood, Oklahoma, facility due to high soybean oil prices.

ADM announced in May its plan to build the soy processing plant in Spiritwood, located in the heart of the soy farming belt in eastern North Dakota.

The JV will be majority-owned by ADM with a 75% stake, while Marathon Petroleum will own the rest. (Reporting by Arunima Kumar in Bengaluru, Laura Sanicola in New York and Karl Plume in Chicago; Editing by Aditya Soni, Dan Grebler and Lincoln Feast.)