Schlumberger Ltd, the world's largest oilfield services company, is exploring the sale of its oilfield tools rental unit Thomas Tools, which could be valued at more than $600 million, according to people familiar with the matter. An auction is already underway for Thomas Tools, which has annual earnings before interest, tax, depreciation and amortization of around $80 million, the people said this week. Private equity firms keen to buy the unloved divisions of big conglomerates are among those that have shown interest in Thomas Tools, the people said. Boutique investment firm PPHB is advising Schlumberger on the sale process, one of the people added. The sources asked not to be identified because the process is confidential.