Nigerian President Umaru Yar'Adua has replaced six of the top officers at the country's state-run petroleum company in an effort to prepare the Nigerian National Petroleum Corp. for proposed reforms to the sector.

"The ongoing changes are in response to the enormous challenges of survival and growth of the corporation," Mohammed Sanusi Barkindo, managing director of the NNPC, said in a statement following Wednesday's announcement.

The reorganization of the NNPC started earlier this year with the appointment of Barkindo, who had previously served as head of OPEC.

Another former OPEC president, Rilwanu Lukman, was named Nigeria's oil minister, the first of Yar'Adua's moves in a promised shake-up of the sector ahead of the proposed changes.

Additional changes to the governing body for Nigerian oil and gas are expected in the coming weeks and months, according to Nigeria's The Guardian newspaper.

The across-the-board changes to the sector come ahead of the new proposed petroleum bill that has been sent to the Nigerian National Assembly.

The call for reform of the Nigerian oil sector comes at a time when the industry is faced with production far below 2 million barrels per day, placing the industry second among African oil exporters behind Angola. Heading a long list of proposed changes to the sector is the ability for the NNPC to solicit private funds for investments in joint ventures with foreign energy firms, ending the longstanding policy that required the NNPC to ask the federal government directly for capital.

Foreign oil executives at Royal Dutch Shell in Nigeria, the largest foreign petroleum operation in the country, said their output has fallen since the onset of violence by militant groups such as the Movement for the Emancipation of the Niger Delta at the end of 2005.

Shell in February announced a force majeure, cutting oil exports from Nigeria because of increasing militant attacks on the country's oil installations and pipelines, just the latest of several work stoppages that have halted tens of thousands of barrels of oil production for the company over the last 12 months.

The Dutch firm has since February restarted production at some facilities, though the threat of violence against its operations by MEND and other militant groups continues.

Militants operating in the oil-rich Niger Delta say they are fighting for a more equitable distribution of the country's oil wealth, though critics of the armed groups contend that their so-called advocacy is merely an excuse to illegally siphon oil from pipelines and sell it on the international black market.

The ever-present violence and corruption in the delta have prompted some foreign oil operations to pull up stakes entirely and have made others question the viability of the operations there in the years to come.

Amid the violence hampering oil production, concerns are reportedly increasing among potential foreign investors regarding the sector, particularly the Nigerian government's recent decision to cancel oil prospecting licenses that were granted to a South Korean consortium that paid up front for the right to explore on- and offshore in the delta.

Not all foreign firms have been cowered by the violence and uncertainty in the Nigerian petroleum sector, however.

Last week, Russian energy firm Gazprom signed an agreement with the NNPC for exploration of three oil blocks in the country's north, officials from both countries said.