Enhanced competition, declining revenue and deeply discounted fares pushed Delta Air Lines into a loss of $86.7 million ($2.18 per share) in fiscal 1983 yesterday, the first annual loss the company has reported since 1947.

In other reports yesterday, Xerox Corp. said yesterday earnings from its reprographics and information systems businesses declined in the second quarter, pushing profit for the period down 5 percent.

And Gulf Oil Corp. posted a slight decline in earnings for the period, while Standard Oil Co. (Ohio) said its profits inched up.

The Atlanta-based airline, which reports on a fiscal year basis, ending June 30, also reported fourth-quarter loss of $25.5 million.

Both results exceeded analysts' most pessimistic predictions and left company officials a bit stunned.

"This is a little unusual for us," Bill Jackson, a company spokesman, said.

Jackson said that 82 percent of Delta's traffic during the fiscal year traveled on discount fares compared with 69 percent last year.

For the fiscal year, Delta's operating revenues were $3.6 billion, a slight decline against operating expenses of $3.8 billion.

Operating revenues for the quarter ended June 30 were down 2 percent to $936 million.

Xerox reported a profit of $153 million ($1.48 a share) for the April-June period, compared with earnings of $161 million ($1.91) for the same period last year.

Revenue rose 2.5 percent to $2.164 billion from $2.111 billion.

For the first six months, Xerox earned $282 million ($2.73), up 4 percent from $271 million ($3.20). Revenue totaled $4.174 billion, up 1.4 percent from $4.114 billion last year.

In Pittsburgh, fifth-ranked Gulf Oil Corp. said that, even though it is selling gasoline at a profit in the United States once again, its second-quarter earnings slipped 2 percent from a year earlier.

Gulf said its earnings slipped to $223 million ($1.32), from $227 million ($1.24) a year earlier. Revenue was off 6 percent to $6.99 billion from $7.42 billion.

For the first half of 1983, Gulf earned $405 million ($2.38), down 21 percent from $515 million ($2.80) in the comparable 1982 period. Six-month revenue fell to $13.89 billion from $14.95 billion.

In Cleveland, No. 13 Sohio said second-quarter profit edged up 1 percent to $476 million ($1.94), from $473 million ($1.92) a year earlier. Revenue fell to $3.1 billion from $3.4 billion.

In the first half, Sohio's earnings tumbled 19 percent to $753 million ($3.06), from $928 million ($3.77). Six-month revenue fell to $5.9 billion from $7 billion.