Industrialized countries' demand for imported oil from the international oil cartel will remain the same next year or decline slightly, according to an unpublished assessment by the International Energy Agency.

This slackening in demand, which U.S. government and oil industry officials also foresee, is a function of new production from the North Sea and Alaska and lagging economic recovery in some industrialized countries, oil analysts say.

Industry and government analysts are anxiously weighing projections of world oil demand in anticipation of the meeting later this month of the Organization of Petroleum Exporting Countries. The meeting is scheduled to open Dec. 20 in Caracas.

There is wide speculation that there will be a continued price freeze. This speculation is fueled, in part, by the soft condition of the world oil market. The shah of Iran and Saudi Arabia's oil minister, Zaki Yamani, have said they favor a price freeze.

"There will be a flat demand for imported oil next year," said James W. Reddington, a senior economist with the Paris-based IEA, in a telephone interview.

The 19-nation IEA is made up of the industrialized countries, with the expection of France. Nearly all of the members are oil importers.

Reddington describes the IEA's analysis as "a current assessment" of demand for oil imports. Free-world demand for OPEC oil rose slightly in 1977 to 28.8 million barrels a day, up slightly from 28.3 million barrels a day in 1976, and "imports will remain essentially unchanged for 1978," he said.

In Washington, Deputy Assistant Secretary of State Stephen W. Bosworth said the State and Energy departments agree with the IEA forecast. "Demand for OPEC oil is going to remain relatively flat because of North Slope and North Sea oil. Another major factor is the final shape of U.S. energy policy."

The lack of significant growth in industrialized countries' oil imports and the so-called oil glut are having an effect on OPEC production. Saudi Arabia, the cartel's leading producer and Abu Dhabi, a member of the United Arab Emirates, have announced cuts in oil production over the last two months that will reduce OPEC production by 1 million barrels of oil a day.

OPEC is now producing about 31.5 million barrels of oil a day, and could produce 7 million barrels more.

Slackening demand for imports could make it more difficult for the cartel to make a price increase stick, unless porduction is reduced.

Reddington said the IEA projections, which do not include 1977 year-end consumption data from member countries, take into consideration the growth of imports to the United States and Japan to fill strategic oil reserves. Together, both countries will be importing 500,000 barrels of oil a day or more next year for storage. Japan recently announced an ambitious program to have 90 days' oil supply in storage by 1980. President Carter has set of goal of 1 billion barrels of oil in storage by the early 1980s.

John Lichtblau, an international oil consultant, said his analysis shows a slight, but not significant, increase in demand for OPEC crude oil."Under our assumptions we see a very small growth, but one could also come up with no increase in demand for OPEC oil because of new supplies," says Lichtblau of the Petroleum Industry Research Foundation in New York.

The IEA projections also forecast a reduction in U.S. imports for 1978.

Recently, Energy Secretary James R. Schlesinger Jr. said U.S. imports would decline in 1978, "though not as a result of a reduction in demand." Due to increased production from Alaska, U.S. imports, including oil for the strategic reserves, will drop from about 8.7 millions barrels of oil a day to 8.3 million barrels or fewer, he said.

Reddington emphasized the need to press for aggressive conservation programs and efforts to increase alternatives to OPEC oil in the industrialized countries. "We just happen to have a happy confluence of increases in supply, while we have a pause in energy requirements," he said.