Standard & Poor's, the financial information service firm, announced a shakeup in one of its indices on Tuesday in the wake of National Oilwell Varco Inc's acquisition of Grant Prideco Inc.

National Oilwell Varco Inc, which sells oil and gas drilling equipment, will replace industrial automation provider Rockwell Automation Inc in the S&P 100, Standard & Poor's announced late Tuesday.

Affiliated Managers Group Inc., an asset management firm, will replace drill tool maker Grant Prideco Inc in the S&P MidCap 400 after the market closes on April 21.

Rockwell Automation will remain in the S&P 500. S&P said additions or deletions to the index to not reflect an opinion about the merits of the companies concerned.