This country's previously buoyant economy has been plunged into financial straits similar to those affecting other Third World petroleum exporters as the global oil glut dramatically cuts back Nigeria's share in the world market.

Staggered by a drop of more than 50 percent in oil revenues during the past two years, for example, Nigeria -- which once had abundant cash supplies -- today is faced with a sharply contracting economy as evidenced by a $5 billion arears debt on imports and other short-term financial obligations such as foreign profit remittances.

The growing backlog, according to Western economists, is caused by a document-processing slowdown at the Nigerian Central Bank. Previously there was a one-month average for payments, but that has been prolonged currently to as much as a three-month delay because foreign cash reserves have fallen from the $10 billion surplus of 1980 to less than $1 billion at present -- not enough to finance one month's imports at current volumes.

An ominous development for Nigeria's future financial standing is the refusal of foreign banks in Lagos to accept or issue letters of credit while the payment backlog persists, the economists said.

As in other Third World countries whose economies are heavily dependent on crude oil, such as Mexico and Indonesia, Nigeria's economic crisis illustrates the fragility of singularly oil-based economies and the dislocation that occurs when demand and prices drop. The government will have to finance an estimated $4.3 billion austerity budget deficit next year with expensive overseas borrowing.

Economists here said Nigeria will be facing a critical economic crunch in the immediate future. But they are optimistic that Africa's most populous nation, with an estimated 100 million people, will weather the current crisis in the long run by reducing fiscal expenditures and scaling down ambitious development projects, that Nigerian leaders have looked to as a means of diversifying the economy.

One $2.5 billion project, a 250-mile railroad linking the coastal city of Port Harcourt with the steel mill under construction in the city of Ajaokuta, has been temporarily abandoned.

Nigeria's external long-term debt is a relatively small $12.3 billion, while its debt-service ratio, compared to other recession-plagued African countries, is an easily managed 15 percent.

Even with the drop in total government revenues from $26 billion in 1980 to a hoped for $14 billion this year, Nigeria remains underborrowed, according to economists.

In a budget speech early this month, Nigerian President Shehu Shagari -- using Mexico's plight as an example -- made the same point.

Mexico "is an oil producing country which today has a foreign debt figure of over $80 billion," said Shagari. What Nigeria is "now experiencing is by comparison not as alarming, though this is not to say there is no cause for concern," he added.

"The position in which we have found ourselves is almost inescapable because, being part and parcel of the world economy, there is no way in which we can fully be insulated from the global economic blizzard," Shagari said.

Yet, while Nigeria, unlike Mexico, is in a good credit position for long-term commercial loans, the country's economic planners may run into difficulties in these sombar times borrowing billions of dollars overseas to finance budget deficits.

"Nigeria is not overborrowed," said one economist, "but its reputation of timely [payment] of its debts is bad, and banks have become much more cautious in these times than they have been."

And there "is not a lot of commercial loan money available these days," he added.

He and another economist said the Nigerian government in 1979 had projected oil production of 2.2 million barrels a day and a continual increase in oil prices up to 1985. "It is a classic example of government misplanning," said one. Budgetary growth and development plans were based on unrealistically high income expectations.

In March, Nigeria's Central Bank startled local businessmen and bankers when it ordered a sudden halt to issuing letters of credit, the means to finance imports. At the time, oil production plummeted to a low of 700,000 barrels a day. When import financing facilities were resumed in April, Shagari simultaneously introduced stringent austerity measures -- a 250 percent preorder deposit duty on cars, for example -- that dropped the monthly import bill from $1.8 billion to its present $1.2 billion.

Oil prices have fallen from a high of $40 a barrel to $33.50 a barrel. In 1980, oil exports produced 90 percent of government revenues, but today they account for 60 percent. The rest is made up by cocoa and other nonpetroleum exports.

A loyal member of the Organization of Petroleum Exporting Countries, Nigeria's pricing, production and marketing strategies are limited by its adherence to the organization's production quotas and price agreements as largely determined by Saudi Arabia, OPEC's most powerful member in establishing world petroleum supply levels.

Nigeria's production gradually climbed back to 1.3 million barrels a day by May and averaged a high of 1.5 million barrels in October, but the heady high-income days of more than 2 million barrels-a-day seem to be gone for the near future.

OPEC producers are facing increasing price competition from non-OPEC oil exporting countries, such as Great Britain's North Sea exports.

Oil companies have retooled their refineries to use the harder to process, but cheaper heavy crudes substantially lowering the demand for the higher-priced light sulfur "sweet crudes" like Nigeria produces.

Energy conservation measures and the world recession also have cut demand.

All of these factors have recently pushed Nigeria from second place after Saudi Arabia as a supplier of imported oil to the United States to fourth place behind Mexico and Britain, which hold second and third places, respectively.

News reports here quote one of Shagari's economic advisers, John Jideonwo, as saying that Nigeria's oil production can be expected "to stagnate" at 1.3 million barrels a day until 1985. Jideonwo said he based his calculations on OPEC's declining share of the world oil market.