SAN FRANCISCO, May 19 (Reuters) - Schlumberger Ltd

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, the world's largest oilfield services company, said on Wednesday it had a crew on the Deepwater Horizon that departed only hours before the explosion and fire that engulfed the rig.

The company, which had not previously revealed its work on the Horizon, said in an emailed statement that it performed wireline services for BP Plc

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on the rig in March and April, completing the last services on April 15 and leaving a crew on standby in case any more were needed.

"On the morning of April 20, 2010, BP notified the Schlumberger crew that it could return to its home base in Louisiana," Schlumberger said in a statement, which a spokesman for the company confirmed by phone.

The crew departed the rig at about 11:00 a.m. on April 20 on one of BP's regularly scheduled helicopter flights, Schlumberger said. The explosion occurred at about 10:00 p.m. that night, and the rig sank two days later, which led to a massive oil spill off the Gulf Coast. [ID:nN19188461] (Reporting by Braden Reddall, editing by Bernard Orr)