Ecuador hopes that fresh investments in aging fields that have an estimated 803 million barrels of remaining reserves will help recover output and sustain a steady flow of revenues to the cash-strapped central government.

Ecuador estimates that $322 million is needed to raise output by 43,500 barrels per day in two years, according to estimates contained in Energy Ministry documents.

State oil company Petroecuador has had trouble sustaining output due to a lack of cash for investments and government bureaucracy.

Companies must register by Oct. 25 to provide services in Petroecuador's Shushufindi, Auca, Lago Agrio and Culebra-Yulebra-Anaconda fields, according to a government advertisement published in Guayaquil-based Expreso newspaper.

Ecuador failed this year to award contracts for private companies to assume control of these fields due to problems with the bid and the country's oil legislation.

President Lucio Gutierrez tried to amend existing oil laws to encourage private operation of these fields but faced political opposition.

In the hands of state oil company Petroecuador, the four fields currently produce a combined 82,000 bpd. Ecuador produces 521,000 bpd of oil, its main export and the source of a quarter of the government revenues expected this year.