U.S. petroleum company Shell Oil says Nigeria loses an estimated 150,000 barrels of crude oil each day because of theft in the Niger Delta region.

Shell Sub-Saharan Africa Shell Exploration and Production Africa Limited Executive Vice President Ian Craig told the Nigerian Oil and Gas Conference the loss of oil is only one of the "major challenges" of working in the Niger Delta.

"The volume of oil which is stolen is difficult to estimate but is probably in the region of 150,000 barrels per day," Craig said.

"(There is) chronic underfunding of the onshore joint ventures where the Nigerian National Petroleum Corp. is the majority shareholder."

The biggest issue isn't with infrastructure or production.

"The greatest challenge, however, is the massive organized oil theft business and the criminality and corruption which it fosters," Craig said. "This drives away talent ... increases costs, reduces revenues both for investors and the government and results in major environmental impact."

He said investigations have uncovered more than 50 illegal bunkering points along the line and "associated industrial scale illegal refining with major environmental impacts."

"These are, of course, now being removed," Craig said.

He said the hurdles, on top of the expected costs, can make any potential investor think twice.

"When you are lucky enough to have a commercial discovery, the development costs and timescales are daunting; perhaps $10 billion for a large deep-water development or $20 billion for a large integrated oil and gas project," Craig said. "The execution of the project may take five years or so and many years beyond that before the initial outlay can be recouped."

Nigeria produces roughly 2.5 million bpd, making it one of Africa's largest oil exporters and the world's 14th largest oil producer.

The Nigerian government in 2009 signed an agreement granting amnesty for members of the Movement for the Emancipation of the Niger Delta. That caused the attacks to diminish sharply but illegal bunkering operations siphoning off Niger Delta crude remain a costly headache for Shell.

Nigerian Petroleum Minister Diezani Allison-Madueke said, while the country produces around 2.5 million bpd of combined crude oil and condensate, the government intends shortly to increase national oil output by an additional 180,000 bpd.

Corruption in Nigeria's oil sector is a longstanding problem. The U.N. Office on Drugs and Crime said of the roughly Nigeria $1 trillion the country's energy sector has earned since independence in 1960 until 1999, some $400 billion was stolen.

Nigeria's Parliament has begun looking into business practices at the NNPC as it currently receives all oil revenues into its foreign bank accounts, leaving the Federal Ministry of Finance unaware of the exact amount of revenue that the nation is earning from oil exports at any given time.