AMSTERDAM — Royal Dutch Shell PLC has reported a 57 percent fall in second-quarter net profits as it suffered from attacks on its operations in Nigeria and it significantly wrote down the value of its shale oil fields in North America.

In addition, Europe's largest oil company abandoned long-term targets to increase production to 3.7 million barrels a day by 2014 and 4 million per day by 2018. The company has been investing heavily in new production facilities since an accounting scandal forced it to write down its proven reserves in the early 2000s.

Production of oil and gas equivalents fell 1.3 percent from a year earlier to 3.06 million barrels per day. Chief Executive Officer Peter Voser, who is due to retire next year, insisted the company's real long-term focus had always been on increasing earnings, and it will meet long-term financial goals.

Shell repeated it plans to generate $200 billion in cash flow from 2012-2015, assuming oil stays around $100 a barrel.

"We always said production targets were kind of a proxy for financial goals," Voser said.

Net profit in the quarter was $1.74 billion, down from $4.08 billion in the same period a year ago, in part because of a $2.2 billion impairment charge on its shale oil assets in North America.

The company said it had reassessed how profitable they are likely to be.

Shell's shares fell 3.9 percent to 24.595 euros in early trading in Amsterdam.

The company's earnings on the "clean current cost of supplies" measure preferred by many analysts — which strips out one-time charges and the impact of changes in oil price in the pipeline — would have been 20 percent lower at $4.6 billion, Shell said.

Analysts Neill Morton and Brian Gallagher of Investec said in a note on the earnings that the company's abandoning of production targets didn't come as a surprise. "We think no-one externally, at least believed it anyway."

They repeated a Buy recommendation on shares, saying that the key lesson of weaker-than-expected results from both BP and Shell is "not to get too carried away with single, blowout quarters" such as both companies had in the first quarter of 2013.

"Despite the miss, we remain positive on Shell," they wrote. Strengths include a strong balance sheet, a good dividend yield and a cheaper valuation than most European competitors.

Shell, the largest foreign oil producer in Nigeria, has suffered several attacks on its pipelines in the country's eastern delta region in recent months. Shutdowns cost it about $250 million in the second quarter and are continuing. Shell blames the attacks on thieves, though environmental groups have criticized Shell for operating wells in a region where it cannot oversee the consequences.

Voser said the company has been selling some Nigerian assets and stakes to investors in the region in order to build local support for projects.

"I think the key there is to really solve sabotage and it has to be a multi-stakeholder approach, led by the Nigerian government," he said. "In the long term, Nigeria is an important resource and we hope that we can sort some of the short to medium-term sabotage and environmental issues out."

Voser is to be replaced by Ben van Beurden, who currently oversees the company's refining and chemicals marketing businesses.