Oil services stocks and the corresponding exchange traded funds have been leaders in the energy sector’s rally this year, but that leadership is in danger as sanctions against Russia restrict activity there by Western energy firms.

Shares of Schlumberger (NYSE: SLB), the world’s largest provider of oilfield services, are off 1.1% Tuesday after the company said third-quarter earnings will be dinged by the sanctions against Russia.

“The sanctions are placing some restrictions on the engagement of certain people and equipment in our Russian operations which in the short term will have an impact on operational efficiency and costs in Russia. The financial impact of the sanctions in the third quarter is limited, and is currently estimated to be up to $0.03 of earnings per share,” said Schlumberger in a statement.

Lower earnings for Schlumberger could prove problematic for oil services ETFs, particularly the Market Vectors Oil Services (NYSEArca: OIH) and the iShares Dow Jones U.S. Oil Equipment Index ETF (NYSEArca: IEZ).

Home to almost $2 billion in combined assets under management, OIH and IEZ are cap-weighted ETFs, each of which devotes an arguably excessive portion of its weight to Schlumberger. OIH allocates 20.6% of its weight to Schlumberger while IEZ has an almost 22% weigh to the stock. [Oil Services ETFs Lead Energy Rally]

OIH and IEZ are two of the best-performing energy ETFs over the past month, but those funds’ Russia exposure does not end with Schlumberger. Halliburton (NYSE: HAL), Baker Hughes (NYSE: BHI) and Weatherford International (NYSE: WFT) “each generate 4 percent to 5 percent of their global sales from Russia, while Schlumberger gets 5 percent to 6 percent,” David Wethe reports for Bloomberg, citing RBC Capital Markets.

Halliburton, Baker Hughes and Weatherford combine for 22.4% of OIH’s weight, according to Market Vectors data.