Lawyers for Mobil Oil Corp. president William P. Tavoulareas and The Washington Post questioned Post reporter Patrick Tyler for 6 1/2 hours yesterday about how he pieced together two 1979 stories that the oil company executive and his son Peter claim were libelous.

Tyler, testifying on the third day of the Tavoulareases' $50 million libel suit against the newspaper, recounted for the six-member federal court jury exactly where in the Nov. 30, 1979, story--the first of the two articles--there was information that Mobil officials had given him about the creation of Atlas Maritime Co., the London-based firm that the newspaper said the elder Tavoulareas set up with his son as a partner.

Under cross-examination by Post attorney Irving Younger, the 30-year-old reporter told how he had included the elder Tavoulareas' denial through a spokesman that he had urged a shipping executive to include his son as an executive. But Tyler said that despite his repeated requests to talk with key Mobil officials, including Tavoulareas, the company did not set up the interviews.

Earlier, under questioning by one of the Tavoulareases' attorneys, John J. Walsh, Tyler acknowledged that he was given a memo before the stories were published from a Post copy editor, Cass Peterson, questioning the merit of the first story about the Tavoulareases' business dealings.

While saying that she was impressed with the extent of Tyler's reporting, Peterson said in the memo, written to another editor, that she was "still left with an overwhelming sense of So What? . . . . It's impossible to believe that Tavoulareas alone could put together such a scheme for the sake of his son's business career, or that he would want to."

Tyler responded to Peterson's memo with a four-page memo to William Greider, then the Post's assistant managing editor for national news, saying that while Peterson's remarks disturbed him, the Tavoulareas story, among other things, presented "a rare occasion when our readers can get a peek at how decisions are sometimes made at the top of the largest and most important industry in the world."

Walsh said he plans to call Peterson as a witness later in the trial.

In other testimony, Tyler said that Sandy Golden, who was a special correspondent for the Nov. 30 story, had misconstrued comments he made to him about a meeting Tavoulareas had with Post Executive Editor Benjamin C. Bradlee in early December 1979, after both stories were published.

Tyler said that a reference he made to Golden about Tavoulareas being "blown out of the water" referred to information about the younger Tavoulareas' summer home rentals on Long Island. Tyler said he did not recall saying that the oil company executive returned to New York after the meeting with Bradlee "with his tail between his legs," as Golden testified on Thursday.