ConocoPhillips said on Thursday that the total volume of oil spilled from the Penglai 19-3 Oilfield in northern China's Bohai Bay is expected to reach about 1,500 barrels.

The amount also includes oil-based drilling fluids.

"We're working with independent experts to validate the total spill quantity," said the US energy company, the operator of the field, in an e-mailed statement.

China's ocean watchdog, the State Oceanic Administration (SOA), on Wednesday ordered ConocoPhillips China to shut down production at two platforms at the Penglai 19-3 Oilfield to reduce the risk of further leaks.

The suspension may lead to a reduction in production of about 17,000 barrels of oil a day for ConocoPhillips, which holds a 49 percent stake in the oilfield.

China National Offshore Oil Corp (CNOOC) Ltd, which owns the remaining 51 percent, said on Wednesday that the net production from the two platforms was about 22,000 barrels a day.

The total production from these two platforms is approximately 47,000 barrels a day, accounting for a third of the production from the entire Penglai 19-3 Oilfield.

The leak in Bohai Bay of 1,500 barrels is "definitely a disaster" to the marine environment, said Ma Jun, director of the Institute of Public and Environmental Affairs.

"The oil will enter the food chain in the marine environment, causing an unpredictable effect," Ma said.

"The environmental effects of the incident are hard to estimate, because this is the first time that I have experienced an oil leak lasting such a long time," Liu Qingzheng, director of the environment department of the National Marine Environment Forecasting Center, told China Daily.

The leak in the bay was first observed on June 4 and had not stopped completely by Wednesday, according to SOA. So far, more than 4,240 sq km of coastal waters have been contaminated by the oil leak.

The administration criticized ConocoPhillips China on Wednesday for not taking effective measures to eliminate spill risks that pose "tremendous threats" to the ecological environment of Bohai Sea.

"Technically speaking, it will still take time to stop the oil leak completely after the platform shutdown," said Chen Jianmin, a professor at the School of Petroleum Engineering at China University of Petroleum.

This temporary shutdown will be in effect until the risks of another spill are eliminated in accordance with orders from SOA, ConocoPhillips said.

"The two companies' losses resulting from the shutdown may be big, given the high oil prices," said Lin Boqiang, director of the China Center for Energy Economics Research at Xiamen University.

The major crude oil contract for August delivery on the New York Mercantile Exchange has been hovering around $97 a barrel recently.

"We will work closely with SOA and CNOOC to minimize the impact to the environment," ConocoPhillips said.

But many Chinese environmental organizations questioned the credibility of the spill volume released by ConocoPhillips.

Zhong Yu, senior action coordinator of Greenpeace, an international environmental organization, said the amount is questionable because, apart from ConocoPhillips China and the State Oceanic Administration, no "third party" attended the assessment.

"How did the company get the number? Is the oil dispersed by dispersing agents included in the amount?" Zhong said, adding that the dispersing agent used in the cleanup will create secondary pollution in the marine environment, and that this should not be ignored.

In addition, 11 environmental organizations sent an open letter to ConocoPhillips China and CNOOC Ltd on Thursday, asking the two companies to assist environmental organizations and other people concerned about the incident to visit the scene of the leak to investigate the incident and its aftermath.

The organizations have not yet received a response.