Exxon Corp. yesterday forecast that conservation efforts are expected to reduce demand for energy resources in 1990 in the United States by some 25 percent - the equivalent of 17 million barrels of oil a day - below trends evident before the 1973 oil embargo.

Despite this "impressive conservation," total energy demand will increase by more than one-third between 1978 and 1990, Exxon officials said here yesterday.

Coal and nuclear power will grow in importance, accounting for some 80 percent of the increased demand, but oil and natural gas still in account for 60 percent of demand in 1990 Exxon said in a semiannual energy outlook assessment released yesterday.

Exxon, the world's largest petroleum firm, yesterday also reported a modest increase in first-quarter profits. Several other oil giants - Standard Oil of Indiana, Atlantic Richfield Co. and Union Oil of California - also reported slightly higher earnings, while Ashland Oil Inc. posted a sharp decline that was related to the firm's coal operations.

First quarter profits for Exxon were up 5.4 percent to $680 million ($1.52 a share) from $645 million ($1.44). for the same period a year ago. Revenues increased to $15.3 billion from $14.3 billion.

Exxon's earnings from actual operations, however, rose at a more rapid pace. Not counting foreign currency exchange accounting effects and other non-operating items, Exxon operating profits rose 13.4 percent to $840 million.

Because of a weakening of the U.S. dollar against the German mark, Japanese yen, Dutch guilder and French franc in the recent quarter, Exxon suffered a $72 million quarterly loss on its books.

Chairman Clifton Garvin Jr. said profits from exploration and production in the U.S. jumped more than 21 percent in the recent quarter, mainly because of Alaska North Slope operations and higher natural gas income.

Exxon's energy forecast yesterday is based on a number of broad assumptions - including the goal of a full-employment economy that will result in an estimated average annual growth rate of the nation's gross national product of about 3.9 percent through 1980 and 3.4 percent from 1980 through 1990.

Among Exxon's conclusions:

Synthetic products are expected to account for about 2 percent of 1990 supplies, mostly from oil shale.

To meet energy requirements, the United States will continue to rely on oil imports, which are expected to account for 51 percent of oil demand in 1990 compared with 45 percent last year.

Imports are projected to peak at 12.2 million barrels a day in 1983 and then decline gradually to 11.4 million barrels a day by 1990.

Standard Oil Co. of Indiana, meantime, cited record worldwide production of crude oil and higher profit margins for domestic products as factors that contributed to a 3 percent increase in first-quarter earnings.

Net income was $252.2 million ($1.72 a share) compared with $244.2 million ($1.67) in the same period last year, as revenues increased 11 percent to $3.8 billion.

Standard Oil Co. of Ohio said Alaska crude oil operations contributed to a sharp increase in first-quarter profits to $36.3 million (75 cents a share) from $18.7 million (48 cents) a year earlier. Revenues rose to $1.03 billion from $823.5 million.

Other petroleum company reports for the quarter ended March 31, compiled from news services, included:

Atlanta Richfield Co., $150.3 million ($1.23 a share) vs. $145.3 million (1.20); sales of $2.93 billion vs. $2.77 billion.

Union Oil, $79.3 million ($1.84) vs. $77 million ($1.83); sales of $1.45 billion vs. $1.46 billion.

Ashland Oil, $15.5 million (43 cents) vs. $28.5 million (92 cents); sales of $1.21 billion vs. $1.15 billion.