NEW YORK (Reuters) - Royal Dutch Shell RDSa.L is in talks with several potential buyers for its refinery outside of San Francisco, but the Anglo-Dutch oil giant is reluctant to part with its last asset in California, three people familiar with the process say.

Shell's company logo is pictured at a gas station in Zurich April 8, 2015. BG Group's bonds traded up strongly on Wednesday on the back of Royal Dutch Shell's US$70bn offer to buy the UK firm. REUTERS/Arnd Wiegmann

The company is in the midst of a massive asset sale, shedding properties from Thailand to the North Sea to pay down debt following its $54 billion purchase of smaller British rival BG Group last year.

Shell, Europe’s largest oil company, has sold around $15 billion of assets over the past year as part of a planned $30 billion in asset sales to trim debt incurred from the transaction.

Bidders for Shell's 158,000 barrel-per-day Martinez refinery, located 30 miles (48 km) northeast of San Francisco, include PBF Energy PBF.N and NTR Partners III LLC.

Still, sources familiar with the issue say the company wants to sell for a higher price, with one saying the plant could be valued at about $900 million.

Shell, which barred potential buyers from hiring advisors during a first round of the auction, has since allowed third parties to review materials related to a sale, according to one person familiar with the negotiations.

Shell declined to comment. PBF referenced its quarterly calls with analysts, where it has said it considers all refining and logistics assets that come on the market, but declined to comment on interest in the specific plant. NTR did not respond to requests for comment.

Shell retained Lazard last year to advise on the overall asset sale program. In the fall, Shell retained Deutsche Bank to find a buyer for the Martinez facility.

EXIT FROM CALIFORNIA?

Over the past 15 years, Shell has sold refineries in Bakersfield and Wilmington, California. Selling the Martinez plant would mark its exit from the state.

While state-specific emissions regulations and fuel standards make it more expensive to operate a refinery in California, the plant still drew interest because of its location and ability to process local crude.

Among the bidders, PBF bought a refinery in Torrance, California last year, while privately held NTR Partners has bid on other California plants.

California’s environmental regulations and pipeline connections make the state an island, with few sources for gasoline imports.

As a result, when one plant in California is shuttered, margins at other refineries in the state surge.

Most operators in the state own more than one plant. PBF, one of the only California refiners with a single operation, would consider buying a second to hedge against disruptions at its troubled Torrance refinery, Jeff Dill, PBF’s president for West Coast operations said last month.

The Martinez refinery, which has been operating since 1915, processes crude into gasoline, jet fuel, diesel and other refined products and has a coker unit for processing heavy crude.

The potential sale would include a pipeline that brings crude produced in California’s San Joaquin Valley to the refinery.