HOUSTON—Oil and natural gas producer Anadarko Petroleum Corp. said Monday its second-quarter earnings fell dramatically from a year ago because of non-cash hedging losses related to derivatives.

Anadarko, one of the nation’s largest independent exploration and production companies, said net income amounted to $23 million, or 5 cents a share, for the April-June period, versus a profit of $1.3 billion, or $2.81 a share, a year earlier. Revenue fell to $2.78 billion from $4.58 billion.

The earnings report was issued after the stock market’s close Monday. Anadarko’s shares fell $3.07, or 5.6 percent, to $52.05 in after-hours trading after dropping $3.59, or 6 percent, to $55.12 during the regular session.

Net income in the most-recent quarter was reduced by nearly $1.3 billion before taxes by one-time items, primarily mark-to-market losses on derivative contracts. On an after-tax basis, one-time items lowered net income by about $815 million, or $1.73 a share, Anadarko said.

Under what’s known as fair-value accounting, companies have to periodically update their estimate of what investments are worth based on current market values—a procedure more commonly known as “marking to market.”

On average, analysts polled by Thomson Financial were expecting earnings of $1.74 per share on $3.68 billion in revenue. Those forecasts typically exclude special items.

Year-ago results included a $950 million after-tax gain from asset sales. Following its 2006 purchase of rivals Kerr-McGee Corp. and Western Gas Resources Inc. for more than $22 billion, Anadarko embarked on an aggressive strategy to sell assets, reduce debt and gear its portfolio to natural gas.

The company, based in the Houston suburb of The Woodlands, said adjusted net income from continuing operations in the second quarter was $831 million, or $1.76 a share, versus $417 million, or 89 cents a share, a year earlier.

The company said overall production in the quarter amounted to 50 million barrels of oil equivalent, down slightly from 52 million barrels a year ago. The company said it met its production target for the quarter despite losing more than eight weeks of output from a deep-water project in the Gulf of Mexico. Production at Independence Hub, the world’s deepest production platform operating in 8,000 feet of water, was halted April 8 because of a subsea pipeline leak. Limited production resumed in June after the pipeline owner fixed the leak. “In addition to generating more than $1 billion in free cash flow, we are very pleased with our production volumes for the quarter,” Jim Hackett, Anadarko’s chairman and chief executive, said in a statement.

Hackett noted the company was confident it would achieve its full-year production target.