The only black African country to strike it rich in oil, Nigeria is suddenly discovering there is such a thing as false black gold.

Out-priced on the sluggish world market and facing sharp competition now from the North Sea and Alaska, Nigeria's once highly prized low-sulphur oil is no longer in such demand and production has dropped from more than 2 million barrels a day last year to 1.6 million in February.

Initial estimates of the drop in government revenue as a result of the oil slump vary between 20 and 40 percent. Last year's earning of over $9 billion made Nigeria by far the richest black African nation.

The United States alone took $6.2 billion of Nigerian oil last year, running up a trade deficit of more than $5 billion with this country.

Heavy dependence on the American market has further weakenend Nigeria's financial situation through the decline in the dollar's value. Nigeria lost around $140 million last year and is presently losing $200,000 to $300,000 a day because of the ever-weakening dollar, according to western economists here.

Nigerians, who had become accustomed to oil money flowing in the streets, are now being lectured on "austerity."

As President Carter was flying into Lagos for his state visit, Nigerian chief of state Lt. Gen. Olusegun Obasanjo was delivering over radio and television a tough budget speech annoucing a 10 percent cut in government expenditures for the coming year and a host of new hardships for individual Nigerians.

Banned from further importation were such daily food items as fresh meat, sugar, spaghetti, beer and breakfast cereals. Slapped with up to a 100 percent increase in customs duties were cars, cosmetics, and other luxury goods.

Gen. Obasanjo had some frank words for free-spending, high-living Nigerians who had been making small fortunes off the oil boom as middlemen and traders. "It is time that we in Nigeria, like most developing countries, learn to realize that the mode of living we have adopted for ourselves has no foundation or relevance to our culture and material background," he said.

"Nigerians must stop to believe and behave as if we are a member of the club of developed nations," he admonished, pointing out that Nigeria, with a population of at least 80 million, is still poor in per capita income ($325 in 1977).

The thrust of his budget message was that development in both the capitalist West and socialist East had required hard work and sacrifice and the same had to be true for Nigeria if the country was going to make it: "We must get out of the illusion of wanting to be like Europe or America without making the necessary sacrifice."

The general then announced a new drive to lessen dependence on oil, now providing 80 per cent of the government's revenues. He said heavy emphasis would be placed on the development of agriculture and industry, both badly neglected in the heat of the oil rush. The lion's share of the $8.6 billion already approved for new capital expenditures in the coming fiscal year will go to these neglected sectors, he said.

The idea that Nigeria might be running out of oil money must have shocked most Nigerians after watching skyscrapers rise like mushrooms in Lagos and super highways open across the country.

But a sobering shock is precisely what Nigeria needs at this point in its helter-skelter, sky's-the-limit approach to development, according to Western economists. They calculate that the 1975-80 national development plan will probably cost twice the original government pricetag of $53 billion due to inflation and faulty first estimates for many projects.

The shock began to set in, they say, when the federal government's economists suddenly realized that commitments for capital projects were more than three times the money available. This has led Nigeria to arrange a $1 billion Eurodollar loan, the largest ever floated for a black African country, and to plan for several more of the same dimension by 1981.

Among the benefits Western economists see coming out of this financial squeeze is an end to the "edifice complex," as one wag called the love here for the prestige public monuments. Further, they expect establishment of clear priorities, collection of better data about the economy, build-up of a domestic manufacturing base and perhaps even a revival of agriculture.

Until rceently, the federal military government was trying to do everything for everybody, all at once - from building roads, universities, schools and airports to putting in a new telephone system, port facilities, prison system, refineries and other basic industries.

Despite the scare over slumping oil revenues, Nigeria is hardly on the verge of bankruptcy. Its foreign exchange holdings are still over $3 billion, a huge amount by black Africa standards, and its credit rating with Western banks is excellent. Known oil reserves should last 20 years or more and there is thought to be a lot more to be discovered.

Thus Western diplomats and businessmen are generally upbeat about Nigeria's prospects for eventually becoming a real regional and even international economic power. "Nigeria is the biggest thing in Africa," remarked one of them referring to the sheer size, population and oil wealth of the country."Politically and economically, its a powerhouse."