* TSX up 23.45 points, or 0.19 percent, at 12,431.47 * Seven of the 10 groups higher * Celtic Exploration surges 45 percent By Solarina Ho TORONTO, Oct 17 (Reuters) - Canada's main stock index was higher for the third straight session on Wednesday, boosted by soaring shares in Celtic Exploration Ltd, though a disappointing forecast from Potash Corp offset some gains. Energy shares were up 1.07 percent, bolstered heavily by news that Exxon Mobil Corp will buy Celtic Exploration for C$2.6 billion ($2.64 billion). The news sent the Canadian oil and gas company soaring 45.14 percent, to C$26.30 and was the biggest index mover. "There's some excitement around, again, the idea that a major acquisition is occurring with a beloved Canadian name like Celtic this morning. I think that gets the whole space excited," said Mike Newton, associate director and portfolio manager at Macquarie Private Wealth Inc. Natural gas producer, Encana Corp, was the second biggest index mover, with shares climbing 2.25 percent, to C$22.74. Penn West Petroleum, one of Canada's biggest conventional oil and gas producers, was up 0.95 percent at C$13.79, after the company said it would sell some non-core assets for up to C$1.3 billion ($1.32 billion) to repay a portion of its debt. Offsetting some of the gains was a 0.49 percent decline in Potash shares, which were trading at C$40.90. The world's biggest fertilizer producer said its full-year earnings will be below the low end of its previous forecast range due to delays in new contracts with buyers in China and India. "I know the CEO was publicly stating not more than a month ago that he expected the two to three year outlook to be very good, but notably absent was any short-term commentary. So I think the clues were there," said Newton. The overall materials group, of which Potash is a member, was down 0.24 percent. The Toronto Stock Exchange's S&P/TSX composite index was up 23.45 points, or 0.19 percent, at 12,431.47. Seven of the index's 10 key sectors were in positive territory. Royal Bank of Canada was up 0.45 percent at C$58.12, which helped give the financial group, which make up nearly a third of the index, a 0.19 percent lift. In other corporate news, shares of Ivanplats Ltd, an Africa-focused miner, rose 7.8 percent on their debut on the Toronto Stock Exchange early on Wednesday, after raising slightly over C$300 million ($304.2 million) via an initial public offering. Ivanplats, backed by one of the mining sector's most well-known and colorful financiers - Robert Friedland, announced a long-awaited plan to list in Toronto last month.