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The Buenos Aires Stock Exchange has shaken up its benchmark Merval index, with energy conglomerate Pampa Energia (PAM, PAMP.BA) emerging as the biggest winner.

Although all 14 companies in the index remain the same, the two traditional heavyweights--steel tube manufacturer Tenaris (TS, TEN.MI) and Brazilian oil company Petroleo Brasileiro (PBR, PETR4.BR), or Petrobras--saw their share of trading decline in the fourth quarter.

The changes, which are part of a normal quarterly revision, took effect Monday.

Tenaris's weighting fell to 26%, down from 33.5% last quarter, although that still kept the firm as the single largest share, and by a considerable margin.

Pampa Energia climbed into second place, from third, with a weighting of 14.1%, from 10.2% in the fourth quarter. During the third quarter, the firm's American Depositary Receipts started trading on the New York Stock Exchange.

Financial conglomerate Grupo Financiero Galicia (GGAL, GGAL.BA) moved up to third, at 8.9%, fractionally higher than 8.8% last quarter.

The locally traded shares of Petrobras fell to fourth place, with a weighting of 8.3%, down from 10.4%.

Merval officials look at the stocks whose trading makes up 80% of the total volume traded over the three previous months. They then pick the stocks that were traded during 80% of the days the exchange was open during the quarter.

In total, energy companies account for 63% of the Merval's weighting, down from 68% last quarter, so the sector retains a great deal of influence over the index's overall performance. As in previous quarters, that means the Merval's performance will often be determined by global oil prices rather than local considerations.

Other components include: Banco Macro SA (BMA, BMA.BA) 5.9%, Edenor (EDN, EDN.BA) 5.7%, Petrobras Argentina (PZE, PESA.BA) 5.1%, Banco Patagonia (BPATL, BPAT.BA) 4.7%, Telecom Argentina (TEO, TECO2.BA) 4.5%, Aluar (AAARF, ALUA.BA) 4.4%, Siderar (SDDFF, ERAR.BA) 4.3%, Transener 3.6%, BBV Banco Frances (BFR, FRAN.BA) 2.9%, and Banco Hipotecario (BHIP.BA) 1.6%.

-By Matthew Cowley, Dow Jones Newswires; +54 11 4103 6740; matthew.cowley@dowjones.com