The client drain continues at troubled JWT Group, as Chevron U.S.A. said yesterday it would pull its estimated $10 million Chevron brand gasoline account from the J. Walter Thompson advertising agency.

Chevron is the second major client to remove at least some of its advertising from the agency since the British marketing firm, WPP Group, agreed to purchase JWT Last week, Ford Motor yanked much of its overseas advertising -- valued at up to $140 million -- away from Thompson.

And Thompson has probably not seen the last of its client defections, industry analysts predicted.

"This could set the stage for even more departures," Emma W. Hill, an analyst at the New York securities firm of Wertheim & Co., said. "If you're a client that is dissatisfied with Thompson, certainly now is the time to leave."

One of the agency's biggest clients, Burger King, is reviewing its $200 million account. Goodyear Tire & Rubber Co. has threatened to pull its $30 million account. And Chicago-based Hilton Hotels is taking a second look at its $15 million account with JWT.

Amid the chaos, however, there are some rays of light. Thompson will continue to handle Ford's $200 million U.S. Ford Division account. It will also hang onto Chevron's $3 million corporate advertising. And despite Burger King's search for a new agency, most industry analysts also believe that Thompson will also hold onto that giant account.

"No one else wants Burger King badly enough," one analyst said. "The account is just too expensive to service." But a Burger King spokeswoman said yesterday that no decision would be made until September or October.

Chevron denied that the WPP purchase of JWT played any role in its decision to dump Thompson as its Chevron-brand agency. The New York ad firm Young & Rubicam, will take over the account Oct. 6.

Young & Rubicam already handles advertising for Gulf-brand gasoline. Gulf Corp. merged with Chevron in mid-1985 and Chevron has been looking to combine its gasoline advertising in one agency ever since