JOHANNESBURG – After half a century of oil spills, Nigeria’s troubled Niger Delta is one of the most polluted places on Earth, and it could take $1 billion and 30 years to clean up, according to a U.N. report released Thursday.

A 14-month study by the United Nations Environment Program that was commissioned by the Nigerian government examined 200 locations and 75 miles of pipeline, more than 4,000 soil and water samples, and the medical reports of 5,000 people.

“Pollution from over 50 years of oil operations in the region has penetrated further and deeper than many may have supposed,” the report said. Some areas that seemed unaffected on the surface are severely contaminated underground and need urgent action to protect the health of fishing and farming communities, it said.

The Nigerian government says there were more than 7,000 oil spills between 1970 and 2000. The U.N. agency handed its report Thursday to Nigerian President Goodluck Jonathan.

The drinking water in at least 10 communities had high levels of dangerous hydrocarbons, and in one village, about half an inch of refined oil was floating on groundwater used by villagers for drinking. The level of carcinogenic benzene in the drinking water at the village, Nisisioken Ogale, was 900 times World Health Organization standards.

The Ogoni people have struggled to get compensation for the damage caused by oil production since the early 1990s, a movement spearheaded by Nigerian environmental activist Ken Saro-Wiwa, who was executed in 1995 by the military government.

The report puts pressure on Shell Petroleum Development Co., the major operator over the period, which has had a bitter relationship with local communities. It produces about 40 percent of Nigeria’s oil in a joint venture with the Nigerian National Petroleum Corp. Shell paid for the report at the request of the government.

The company was forced to leave the Ogoniland region of the Niger Delta in 1993 because of fierce anger from the community, but still operates in other parts of the Delta and maintains a network of pipelines.

The company, part of the Royal Dutch Shell group, has often blamed theft and sabotage for oil spills, and stood by that position in a statement Thursday.

“Although we haven’t produced oil in Ogoniland since 1993, we clean up all spills from our facilities, whatever the cause, and restore the land to its original state,” Shell said in a statement. “The majority of oil spills in Nigeria are caused by sabotage, theft and illegal refining.”

However the report found that control and maintenance of oil facilities in the region was inadequate.