Energy companies that suffered as oil prices plunged. Several oil producers and service companies have announced layoffs and reductions in spending on new drilling projects. Lower oil prices are good for the economy and most businesses, but they are bad for the stock market in the short term. Instead of giving equal weight to each of the companies, the S&P 500 ranks them according to their market value. When all the results are tallied, the plunge in energy company earnings is expected to be by far the worst among the 10 sectors in the S&P 500, according to FactSet, a financial data provider. Companies such as Exxon, Chevron, Shell and BP don't just produce oil, they also buy and refine it into fuels and chemicals. The plunge in crude prices meant Shell's refining operations paid less for the oil it buys on the open market. When oil prices drop and drilling activity slows, rig operators and other companies that work for big oil producers charge less.