Schlumberger Posts Profit But Shares Fall  HOUSTON (Reuters) - Schlumberger Ltd. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=SLB.N target=/stocks/quickinfo/fullquote"&gt;SLB.N&lt;/A&gt;, the world's  largest oilfield services company, on Friday posted a  third-quarter profit but weakness in the U.S. Gulf of Mexico  and a pullback in its Russian operations weighed on shares.