GeoResources Inc. and Prospect Capital Corp. will be added to the S&P SmallCap 600 index, according to Standard & Poor's, sending shares of both companies higher Tuesday.

S&P announced the planned index changes following Monday's close of trading. Stocks that are about to be added to indexes typically rally, because it's expected that mutual funds and other investment vehicles that track the indexes will buy shares.

GeoResources, a Houston-based developer and explorer of oil and gas reserves, will be added to the S&P SmallCap 600 after the close of trading on Wednesday. It will replace Seahawk Drilling Inc., which filed for bankruptcy protection, and is no longer eligible for inclusion in the index, S&P said.

Shares of GeoResources rose $1.07, or about 3.8 percent, to $29.29 in afternoon trading, after reaching a 52-week high of $29.94 earlier.

Prospect Capital, a New York-based lender and investor in privately held middle-market companies, will be added to the index after Friday's close of trading. It will replace Martek Biosciences Corp., which is being acquired by Royal DSM N.V. in a deal expected to be completed soon.

Shares of Prospect Capital rose 18 cents, or about 1.5 percent, to $11.78.

In addition to joining the SmallCap 600, GeoResources will be added to the index's Oil & Gas Exploration and Production Sub-Industry index. Prospect Capital will be part of the Asset Management & Custody Banks Sub-Industry index.