The nation's energy banks have $61 billion in oil and gas loans on their books, with at least $10 billion having been identified as potential or actual problems long before the current plunge in oil prices, according to a survey by the Federal Deposit Insurance Corp.

Federal banking regulators privately predict the number of problem loans will increase because of falling oil prices. Regulators are conducting special examinations of the 73 biggest bank companies with heavy concentrations of energy loans.

There are 563 energy banks in the United States. Energy banks are banks that have oil and gas loans equivalent to 25 percent or more of their total capital.

Robert V. Shumway, the director of bank supervision for the FDIC, has testified that most of the oil and gas loans -- about 92 percent of them -- are held by bigger banks with assets of $1 billion or more.

Presumably most bigger banks are less vulnerable to losses on oil and gas loans because they can offset losses on energy loans with earnings on loans they own in other industries.

Shumway said that 17.5 percent of the oil and gas loans were "criticized" by regulators last April compared with 5.4 percent of loans to nonenergy borrowers. Criticized loans include problem loans -- those not being paid on time -- as well as loans that may be current but questioned by examiners.

The Comptroller of the Currency's Office, which regulates federally chartered banks, found last September that the 245 federally chartered energy banks had $40 billion in oil and gas loans on their books and $3.1 billion of them were not being repaid on time. The FDIC survey included both federal and state chartered banks.

About two-thirds of the problem loans were made to drilling companies, production companies and oil-service companies. These companies are the biggest bank borrowers, accounting for 53 percent of the oil and gas loans made by federally chartered energy banks.

The drilling, production and service companies also are the hardest hit by the plunge in oil prices. Since late December, the spot price of crude oil has fallen from more than $25 a barrel to about $13 a barrel.

As a result, exploration for oil has dropped sharply. The number of drilling rigs actively searching for oil has fallen by more than 40 percent since Jan. 1. When drilling rigs are not at work, they are not paid.

Independent production companies often are higher-cost producers that sprang up in the aftermath of the jump in oil prices that began in late 1973. At current prices, many of these producers cannot cover their costs.

Last September, according to testimony before the Senate Energy and Natural Resources Committee by Jonathan L. Fiechter of the comptroller's office, 7 percent of the loans to indpendent producers and nearly 16 percent of the loans to drilling companies and service companies were problem loans.

Fiechter, director of economic and policy analysis, said the data do not "reflect the effects of the . . . drop in energy prices" that has occurred since September.

The best credit risks in the energy industry -- major oil companies such as Exxon Corp. and Mobil Oil Corp. -- are not major bank borrowers. Although the 20 major oil companies control 80 percent of the oil reserves and 60 percent of the gas reserves, they account for only 8.5 percent of the energy loans. They have vast internal resources and access to cheaper sources of money such as commercial paper.

Fiechter cautioned that, even at national banks with the biggest concentration of energy loans, oil and gas lending accounts on average for about 9 percent of total loans.