WASHINGTON (Reuters) - More oil than previously thought may have leaked into Lake Michigan this week from BP Plc's Indiana refinery, the company said on Thursday, after two U.S. Senators requested a meeting with the British oil major.

The request from Senators Mark Kirk, a Republican and Dick Durbin, a Democrat, both from Illinois, came before BP issued its estimate that between 15 and 39 barrels of oil had spilled - more than an earlier assessment that nine to 18 barrels leaked on Monday.

The Senators asked for details on the spill's cause, an analysis of the impact of the 405,000 barrels-per-day (bpd) Whiting refinery's production increase, and information on what is being done to prevent future spills.

"Given the Whiting refinery's recent expansion of its operations to double the amount of heavy oil sands being processed, this spill raises questions about the long-term safety and reliability of BP's new, expanded production," the senators wrote to John Minge, chief executive officer, BP America Inc.

The U.S. Coast Guard said on Wednesday that between nine and 18 barrels had spilled. BP said on Thursday that the earlier estimate was based on the size of the sheen on the lake's surface when the leak happened.

The company said the updated figure, which BP shared with the Coast Guard and the U.S. Environmental Protection Agency, includes oil collected by vacuum trucks and an absorbent boom.

"BP is conducting further analysis to confirm this estimate," the company said.

(Reporting by Ros Krasny in Washington and Kristen Hays and Erwin Seba in Houston; Editing by Sandra Maler and Chris Reese)