An Indian consortium said it was no longer considering making an offer for Britain's Cove Energy
The Oil and Natural Gas Corp
The withdrawal of the consortium leaves Shell and PTT vying for Cove, with the latter currently in pole position.
PTT's $1.9 billion, 240 pence per share offer for Cove is higher than Shell's, although a number of analysts expect the oil major to return with a higher bid, possibly before the first closing of PTT's offer on Friday.
At stake is access to East Africa's huge gas resources through Cove's 8.5 percent ownership of a block off the coast of Mozambique.
East Africa is set to become one of the world's largest gas exporters supplying energy-hungry Asia, after a string of major discoveries across Mozambique and Tanzania.
Shares in Cove, which continue to trade above the offer price on investor hopes of a higher bid, were up 0.9 percent to 266.25 pence at 1152 GMT.