Members of the Organization of Petroleum Exporting Countries will have about $205 billion in investments and assets abroad by the end of 1977, the Morgan Guaranty Trust Co. estimates in the latest issue of its World Financial Markets publication released today.

After adjusting for some $50 billion in external debt for the OPEC nations, net external assets will total $155 billion by year end. Four years ago, prior to the sharp increase in world oil prices dictated by the international cartel. OPEC net external assets amounted to only $3 billion.

Of the projected $155 billion, 95 per cent is accounted for by five of the cartel's 13 members: Saudi Arabia with $68 billion. Kuwait with $31 billion, Iran with $22 billion, the United Arab Emirates with $16 billion, and Venezuela with $10 billion. Four countries - Algeria, Equador, Gabon and Indonesia - will actually be in the red at the end of this year.

The good news, if it can be called that, is that the OPEC surplus in 1977 will grow less rapidly than it did in 1976. This moderating trend is expected to continue because OPEC imports of goods and services are rising more rapidly than are the oil revenues together with their foreign investment earnings.

Morgan Guaranty assumes in its analysis that the moderating trend in the growth of the OPEC surplus next year will be based on a 5 per cent OPEC oil price increase at the end of this year, and 4 per cent growth in oil consumption by OPEC customers in 1978, leading to a 6.2 per cent rise in OPEC oil income while import spending gains by 17 per cent.