The company plans to sign a deal with Japan-based Marubeni and the new partner in the second half this year.
Reuters cited PTTGC finance executive vice-president Patiparn Sukorndhaman as saying that the company will hold more than 51% interest in the project.
The remaining stake to be owned by Marubeni and third partner are due to be finalized.
PTTGC and Marubeni are considering Belmont County in Ohio, US, as possible location for the olefin cracker, which will use ethane extracted from the Marcellus and Utica shale formations as feedstock.
The companies will soon conduct detailed engineering design and permitting process at the Ohio site. The final investment decision on the project is expected in 2016.
PTTGC and Pertamina are working on a $5bn olefin cracker with a capacity of 1.5 million ton.
PTT Global owns a refinery with a crude and condensate refining capacity of 280,000 barrels per day.
