NEW YORK ( TheStreet) -- I was recently asked how a stock like Schlumberger ( SLB) that trades at twice the price-to-earnings ratio of a dominant company like Exxon Mobil ( XOM) can be considered cheap, especially because Schlumberger's P/E of 18 is already a point higher than the industry average?The energy sector has been extremely volatile, but Schlumberger has always carried this sort of premium, even over formidable rivals such as Halliburton ( HAL) and Baker Hughes ( BHI). But Schlumberger has shown no immunity to sluggish oil prices and weak rig counts -- the same issues that has impacted the industry. Nevertheless, given Schlumberger's market-leading position, investors were always willing to pay more. Given how well Halliburton is now performing in international markets, has Schlumberger's weakness in North America narrowed the gap between the two rivals? And does Schlumberger still deserve its premium?