NEWARK, June 28— The decomposing body unearthed from a shallow grave in a southern New Jersey state forest Saturday night was identified today as that of Sidney J. Reso, the Exxon executive kidnapped outside his home two months ago.

The authorities said that Mr. Reso had been shot in one arm and that he appeared to have died shortly after being abducted on April 29. They said the cause of his death had not been officially determined, and they refused to speculate on whether the gunshot wound played a part.

Mr. Reso's body was found about 8 P.M. Saturday in a secluded corner of the Bass River State Forest, about 20 miles north of Atlantic City. It had been buried about three feet beneath dense underbrush. Led by a Suspect

The discovery ended an eight-week search that F.B.I. officials today described as the most intensive kidnapping investigation since the abduction of Patty Hearst in 1974. One of the suspects in the kidnapping, Irene J. Seale, told Federal authorities of Mr. Reso's death and then helped a team of 40 law-enforcement officials find the grave Saturday morning, an official close to the investigation said.

The Morris County Prosecutor, W. Michael Murphy, said at a news conference today that he planned to file a felony murder charge in the case on Monday. He refused to say whether he would charge both Mrs. Seale and the other suspect in the case, her husband, Arthur, or just one of them. Arthur Seale, an ex-police officer, is a former security official at Exxon.

Michael Chertoff, the United States Attorney for New Jersey, declined to speculate at the news conference when and where Mr. Reso died. In New Orleans, where Mr. Reso grew up, the wife of his cousin Jerome Reso said that Mrs. Seale had told the authorities that Sidney Reso died on May 3, four days after the abduction.

Mr. Chertoff also refused to discuss any negotiations or legal deals that might have prompted Mrs. Seale to tell authorities about the death and the body.

But he seemed incensed at news reports that the 57-year-old man may have died of natural causes. "Nobody can say that a man who dies in captivity -- having been abducted from his family and having been held against his will -- died of natural causes," he said. "I cannot imagine causes that are less natural than the causes that apply in this case."

New Jersey's Medical Examiner, Dr. Robert Goode, began an autopsy at the state morgue here this morning and was to continue the examination on Monday, officials said.

Law-enforcement officials speculated, pending completion of Dr. Goode's autopsy, that Mr. Reso may have had a heart attack, perhaps fatal, during captivity. He suffered a heart attack three years ago, said Gary Penrith, the head of the Federal Bureau of Investigation's New Jersey office. Mr. Penrith said he understood that Mr. Reso was not required to take prescribed medicine for his condition, but took a daily aspirin as a blood thinner.

The search team arrived at the forest with Mrs. Seale about 8 A.M. on Saturday and scoured the wooded terrain for several hours before locating the grave, with the assistance of a state police search dog, in the late afternoon, said the official close to the investigation. Mr. Reso's remains were removed about 8 P.M.

The body of Mr. Reso, who was president of Exxon's international division, was positively identified today, Mr. Penrith said. He called the death "unbelievably despicable."

In a statement today, Exxon's chairman, Lawrence G. Rawl, said Mr. Reso had made an "invaluable contribution" to the company in his 35-year career. "Sid was one of the finest men I have known who combined outstanding professional competence with humanity and warmth," Mr. Rawl said. Describing Murder Charges

Mr. Reso vanished about 7:30 A.M. on April 29, moments after leaving home for the 10-minute drive from his home in Morris Township to his office in Florham Park. His car, its engine idling, was found at the foot of his 250-foot driveway.

The Seales, both 45 and unemployed, were arrested about 1 A.M. on June 19, after an aborted attempt to collect an $18.5 million ransom for Mr. Reso. Through a series of telephone calls and letters, the kidnappers led an F.B.I. team on a four-hour drive through four towns in Morris and Somerset Counties. The string of communications broke off abruptly after a rented Oldsmobile that other agents had been following near Far Hills, N.J., encountered a roadblock -- which was not related to the Reso case -- and drove off, officials have said.

The Seales were arrested about an hour later at a car depot in Hackettstown, N.J., where Mrs. Seale had rented the Oldsmobile on June 18.

The Federal Government has charged the Seales with kidnapping, extortion, conspiracy, using the mail to transmit ransom letters and interstate travel in aid of extortion.

State charges of kidnapping, extortion and conspiracy have also been filed by Mr. Murphy's office. The felony murder charge Mr. Murphy described applies when a crime victim dies during the commission of a felony.

The indictment Mr. Chertoff filed against the Seales on Thursday asserted that others had joined the couple in the kidnapping. No other suspects have been identified. But Mr. Chertoff vowed today to press on with the investigation.

"I promise that we will not in any way abate or relax our efforts until we have successfully completed the investigation," he said.

For Mr. Reso's wife, Patricia, the discovery of his body ends an anguishing 59-day vigil she kept at their home in Morris Township. She has said she prayed daily for his safe return and kept a journal that, she said, enabled her to speak directly to her husband. 2 Televised Appeals

Twice during the 59 days, Mrs. Reso made televised appeals for the kidnappers to free her husband. The first, on May 15, came in response to a message the kidnappers left a day earlier on a light pole at the Rockaway Townsquare Mall in Morris County, officials have said.

If the authorities are right, the kidnappers' demand came even though they knew that Mr. Reso was dead.

Mrs. Reso's second appeal came June 16, the same day the kidnappers directed F.B.I. agents to a letter in Morris County that gave instructions for bundling the ransom -- $18.5 million in old $100 bills -- into laundry bags, officials have said.

In this appeal, Mrs. Reso said she was praying that her husband would be freed in time for Father's Day, June 21.

Mrs. Reso remained secluded at her home today.

S J Reso