In a move that will cost Montgomery County a projected 10,000 jobs and $70 million in anticipated revenues over the next two decades, Mobil Corp. has scrapped plans to locate a huge, multi-million-dollar complex in the county's saturated Shady Grove area and is seriously considering Fairfax County as its location instead.

Montgomery and Fairfax have been engaged in a kind of bidding war for national corporations looking to locate major facilities in the metropolitan area.

Mobil's decision was relayed to county officials Friday and announced yesterday. It highlights Montgomery's troubles at a time when county officials may be forced to halt development in the lucrative Shady Grove area because of a 1973 county ordinance that prohibits new development in an area unless existing public facilities are adequate to sustain it. Fairfax has no such law.

The problem is that existing road facilities in the Shady Grove area near I-270 are saturated, according to county officials. And a proposed 2 1/2-mile highway called I-370, designed to alleviate the same Shady Grove road problems that are hampering development, is buried in a financial quagmire because of the state's current inability to help fund it.

When Mobil decided to acquire the Washingtonian golf course on I-270 for its 212-acre complex, the company saw the site as "the best development opportunity in the whole Washington area," according to Robert B. McLean, search manager for the Mobil Land Development Corp.

"But the reason we are not pursuing the acquisition of the Washingtonian property is because we feel the transportation issue doesn't fit into our timetable for development," McLean said yesterday, adding that Mobil might not have changed its plans had they "some reasonable assurance" that I-370 would be constructed soon.

The result is that Montgomery has lost a $360 million investment of office, hotel, retail and residential space that would have meant jobs for as many as 7,000 county residents by 1994, according to county officials.

Stating that I-370 is also needed to allow businesses already located on I-270 to expand, County Executive Charles Gilchrist said "we cannot attract new economic development or encourage business presently located in Montgomery County to expand without adequate transportation facilities."