SAO PAULO--Brazilian stocks posted gains Tuesday helped by news Brazil's government will lower taxes on sugarcane and petrochemical industries to boost the competitiveness of its exports.

Brazil's benchmark Ibovespa stock index closed 1.1% up, at 54,884.

The country will "neutralize" the so-called PIS/Cofins social security tax on ethanol producers and petrochemical companies as the U.S. is expanding its shale gas production, undercutting global prices, Brazil's Finance Minister Guido Mantega said Tuesday.

Shares of Braskem SA, Brazil's largest chemical company, soared after the news, leading the Ibovespa index gains Tuesday. Braskem ended up 8.3%, at 16.18 Brazilian reais ($8.09). Shares of Cosan S.A. Industria e Comercio, a large producer of bioethanol and sugar, finished up 3.2%, at BRL46.07.

Tuesday's session was again marked by intense volatility of shares of oil company OGX Petroleo e Gas Participacoes S.A, which is controlled by Brazilian billionaire Eike Batista. After soaring more than 18% Monday and posting gains for most of the session Tuesday, OGX finished the session down 4.3%, at BRL1.54.

Lukoil's president Vagit Alekperov has denied the company is involved in negotiations to buy the Brazilian firm, according to Russian newswires.

Folha de S.Paulo reported Sunday that billionaire Eike Batista is in talks to sell a 40% stake in the oil producer to Russia's Lukoil. OGX is also in talks to sell a 40% stake in its Tubarao Martelo field to Malaysia's Petroliam Nasional, or Petronas, the newspaper said. OGX denied it.

"OGX is not longer trading on fundamentals," said Bruno Goncalves, a stock analyst at Brazil's brokerage Alpes. "The market now expects that OGX will, at least, announce a farm-out, likely in the first half of the year," Mr. Goncalves added.

Investors are trying to recalculate what would be right price for OGX if it were to sell a stake to a competitor, said Joao Pedro Brugger, a stock analyst at Brazil's brokerage Leme Investimentos.

OGX, the flagship company of Mr. Batista's EBX holding, triggered a confidence crisis in the group's ability to fund its massive infrastructure investments after reporting production levels well below expectations.

Other shares in the "X" empire also finished lower on Tuesday. Shares of logistic firm LLX Logistica SA were down 3.6%, at BRL1.90. The group's mining firm MMX Mineracao e Metalicos lost 4.8% to BRL1.98.

UnitedHealth Group Inc., the biggest U.S. health insurer, and Brazil's Amil Participacoes, the country's largest health insurer as well as an operator of hospitals and clinics, finalized Tuesday the tender offer for the acquisition of the remaining Amil's shares. The offer amounted to BRL2.9 billion, according to Brazil's stock exchange BM&FBovespa. United Health bought 90% of Amil in 2012 for $4.3 billion.

Write to Luciana Magalhaes at luciana.magalhaes@dowjones.com

Subscribe to WSJ: http://online.wsj.com?mod=djnwires