For a while today, it looked like the stock market would be able to dismiss Europe's financial woes and rebound from its initial decline this morning. But with a brewing banking crisis in Cyprus and investors reacting negatively to news that holders of bank accounts would lose as much as 10% of their deposits, the S&P 500 (SNPINDEX:^GSPC) finished down more than half a percent, falling 8.6 points to close at 1,552.

Within the S&P 500, the worst-performing stocks were all related to the oil and gas industry. Helmerich & Payne (NYSE:HP) fell 5.5% after an SEC filing released information about the company. Even though Helmerich left its outlook for the current quarter unchanged,its active rig count remains flat and it said that its competitors are putting pressure on pricing for land-based rigs. Still, H&P has become a much bigger player in the industry than it was a decade ago, and with some of the highest returns on equity in the business, H&P has a competitive advantage over many of its weaker peers.

Oil-services giant Schlumberger (NYSE:SLB) also finished down sharply, falling nearly 4% as the company said that activity in North America was weaker than it had expected, with fewer rigs under operation. Echoing Helmerich's comments, Schlumberger noted pricing pressure for many of its product lines as competition to serve the energy industry ramps up. Halliburton (NYSE: HAL) was also among the top losers on the day, falling 2.8% due largely to the same trend.

Finally, Nabors Industries (NYSE:NBR) rounded out the three biggest losers with a 3.5% loss. Helmerich & Payne actually cited Nabors as one of the companies H&P believes it outclasses, with Nabors' U.S. land-based market share shrinking from nearly 15% in 2003 to just 9% currently. Moreover, Nabors has seen a huge drop of nearly 40% in active land rigs since the industry's peak in 2008. An improving picture in drilling activity would likely lift all of the industry's players, but Nabors needs to work harder to maintain its position in the drilling services field.