* Returned drill-rig cost OGX $260,000 a day
* OGX still leasing three Diamond Offshore rigs
* Remaining rigs rent for $261,000 to $301,000/day
RIO DE JANEIRO, Oct 18 (Reuters) - OGX Petroleo e Gas SA returned one of four rigs leased from U.S. drilling rig operator Diamond Offshore after its contract ended, OGX said on Thursday.
A lease for the Ocean Ambassador semi-submersible rig ended at the end of September and was not renewed, OGX's press office said. The rig will be moved to U.S. waters in the Gulf of Mexico, according to Houston-based Diamond Offshore.
OGX shares fell 5.9 percent in Sao Paulo trading on Thursday to 5.24 reais, its lowest level since July 27.
A semisubmersible rig floats on the surface of the ocean and extends a pipe to the ocean floor where it begins drilling. The Ocean Ambassador could drill in water as deep as 1,100 feet (305 meters).
OGX had leased the rig for $260,000 a day, according to the Diamond Offshore's Rig Status Report, updated Thursday. At that rate, Diamond Offshore received nearly $285 million in rental fees from OGX during the three-year contract.
OGX is in the process of reducing expenses on offshore exploration in the Campos Basin and directing more of its efforts to develop discoveries it has already made. Lower-than-expected flows from its first field, which began production in January, also mean it is earning less cash to finance operations than expected.
OGX expects to spend $1.2 billion in 2013, 40 percent less than the $2 billion budgeted for this year.
OGX continues to lease Diamond Offshore's Ocean Lexington, Ocean Quest and Ocean Star semi-submersible rigs, according to the company's rig status report.
The Ocean Lexington has a base day rate of $271,000 a day, or nearly $100 million a year. The rig is under a three-year contract until "early February 2013", and can drill in water as deep as 2,200 feet, the report said.
The Ocean Quest has a base day-rate of $265,000 a day and the Ocean Star has a base rate of $301,000. The Quest can drill in waters 4,000 feet deep and the Star in waters 5,500 feet deep.
The Ocean Quest's lease is up in late December 2013 and the Ocean Star's lease is up late February 2014.