London — Good New Year news for European and North American homeowners and motorists, and for much of the third world as well: World oil prices seem likely to keep falling, in what one London oil analyst calls ''small leaps,'' during 1985.
The news looks grim, however, for the 13 members of the Organization of Petroleum Exporting Countries, whose power as a cartel seems to be weakening, and for its third-world members who rely on oil revenues to pay off debts and buy imports.
Not everyone in the oil industry agrees with the scenario, but the bulk of opinion is that:
* The latest OPEC ministers' meeting in Geneva, which ended Saturday, took only small and largely ineffective steps toward propping up oil prices.
It reaffirmed existing reference price and production ceilings, narrowed the price range between light and heavy crude oils, and tried to set up a stricter auditing system to check on price cheating.
''But the market takes a cynical view of this,'' according to a veteran oil analyst in London. ''OPEC production is already 16.7 million barrels per day higher than its formal limit of 16 million barrels per day. The price narrowing is too little, and the very fact that OPEC is discussing new price monitoring at midwinter shows OPEC to be in a position of weakness.''
* World demand is seen rising only slowly, for a number of reasons, while OPEC and especially non-OPEC oil production will continue to be high.
Individuals and industry are conserving energy: according to published estimates here, oil demand in Britain has fallen by 37 percent in the last 10 years.
Oil companies continue to draw on reserve stocks while waiting for market prices to fall. Figures from the 21-nation International Energy Agency indicate oil stocks may be down to as few as 70 days of supply, the lowest for 11 years.
Unemployment stays at record levels in Europe (up from 19 to 32 million in major industrial nations since 1979), and non-OPEC producers such as Britain, Norway, the United States, and the Soviet Union are likely to fill much of whatever extra demand is generated by economic growth in the US and Japan.
In 1979, OPEC produced 63 percent of world crude oil. By late 1984 its share was down to 39 percent.
Royal Bank of Scotland figures show non-OPEC Britain, with a record 2.68 million barrels per day in November, now is the world's fifth-largest oil producer. The first, second, and fourth producers are all non-OPEC (the USSR, the US, and Mex-ico), leaving Saudi Arabia, in third place, as the only OPEC member in the top five.
''Remember two other factors,'' advises the London oil analyst. ''If, as rumored, the US won't be adding to its strategic oil reserves in 1985, that could remove 200,000 barrels per day from market demand. Also, if the coal miners' strike in Britain is eventually solved, the British won't have to import extra oil to burn in its electricity generating plants.''
OPEC ministers paint a different scenario. They hope the mild winter weather in Europe and North America will soon revert to cold, thus sending demand for oil up. They also hope their two sessions in Geneva this month reassured the oil market that OPEC has found new unity.
The OPEC marker price for light crude oil remains at $29 a barrel and its formal production ceiling at 16 million barrels per day. Heavier crudes are to be more expensive and lighter crudes cheaper.
But ministers concede the new monitoring system is interim and incomplete.
Sceptics note that any oil monitoring system takes time to put in place. They point out that OPEC is expected to meet again Jan. 18 in Geneva to review the system and that members of the monitoring committee were split.
Nigeria, Algeria, and the United Arab Emirates, producers of light crude, want the prices of heavier crudes raised by much more than the 50 cents a barrel tentatively agreed in Geneva. Both Algeria and Nigeria have said they will ignorethe price fixing rules.
The next trigger for a price drop could be a decision by non-OPEC Britain to reduce its North Sea oil - not immediately but within the next few months. This could cause debt-ridden Nigeria, a member of OPEC, to lower the prices of its crude oil, which is similar in density and weight to British oil and competes directly with it in world markets.
Other OPEC producers of such light oil would then find themselves under heavy pressure to match price cuts.