Saudi Arabia and its oil-producing allies are considering a "small and gradual" decrease in the base price for oil, an oil industry newsletter said yesterday.

Quoting sources in the Middle East, Petroleum Information International said a price cut announcement "is likely to come out of the meeting of oil ministers of the Gulf Cooperation Council scheduled to take place around the second week in January." The Gulf Cooperation Council is comprised of the oil-producing nations along the Persian Gulf.

The cut, if it comes, will probably be in the range of $1.50 a barrel for the light crude oil now priced at $34 a barrel, the Houston-based newsletter said.

The Organization of Petroleum Exporting Countries officially reaffirmed its support of the $34 price in December. But some OPEC members, notably Iran, have been selling their oil at well below that level in an effort to maintain production during a time of weak demand.

In other industry developments yesterday, published reports indicated that Indonesia will increase crude oil production by 200,000 barrels to 1.5 million barrels daily this year, and that Britain's output of North Sea oil will drop by almost half between 1985 and 1990.

Indonesia is seeking the increase to bring in more foreign exchange in the face of declining non-oil exports. More than 60 percent of Indonesia's foreign exchange earnings comes from oil.

The government is expected to increase its oil exports from 800,000 barrels to one million barrels daily, limiting domestic consumption to 500,000 barrels daily, and to seek other sources of energy to replace oil.

London stockbrokers Phillips and Drew said the projected decline of North Sea oil output would pose major problems for the government after the next general election--due by May, 1984--because of lower oil tax revenues and a weakening balance of payments.

The brokers stated that in constant prices, the government's North Sea revenue would drop from a peak of $15.60 billion in 1983-84 to $10.50 billion in 1990-91.