Some of the world's largest oil companies are reporting pretty ugly earnings. Profits at Exxon Mobil, the biggest US oil company, are down 27% off its worst fourth-quarter earnings in four years. Royal Dutch Shell, Europe's biggest oil major, saw its profits tumble 48%. Chevron reports on Friday, but given some of the issues it has faced maintaining production levels, there's not a lot of optimism out there. ConocoPhillips reported a 74% jump in fourth-quarter net income, mostly from all the "non-core"assets it has unloaded recently. Production from continued operations is well below where it was a year ago. In a way, the world's major oil companies all suffer from some version of the same problem: They're spending more money to produce less oil.