Published reports that the administration has ordered a halt in purchases for the nation's oil stockpile and plans to sharply cutback the size of the petroleum reserve has prompted an inquiry from Congress on the status of the reserve.

Sens. Henry M. Jackson (D-Wash.) and Bill Bradley (D-N.J.), in a letter to the White House made public yesterday, called on Zbigniew Brzezinski to review the status of the oil stockpile.

Energy Department officials over the last week have been saying the department is considering sharply reducing the size of the 750-million-barrel planned reserve by about a third.

President Carter two years ago committed the country to ultimately having 1 billion barrels in storage by the early 1980's.

DOE is now more than a year and a half behind schedule in filling the reserve, and has only 94 million barrels in storage. Because of the Iranian oil shortfall earlier this year, and as a result of urgings from allies and Arab oil producing states, the administration has not purchased oil for the reserve since last November.

"Disruptive events in the Middle East in the 1970's suggests a serious risk of recurrent and drastic oil supply interruptions," the letter said, adding, "However, the administration decision to discontinue oil purchases for the strategic petroleum reserve indicates that the U.S. policy has discounted these risks."

Earlier this week, Energy Department officials said the administration was considering plans to begin purchasing oil for the reserve later this year or early next year from the Naval Petroleum Reserve or from the government's own royalty oil.