BANGKOK, March 18 (Reuters) - Thai Oil Pcl , the country's top oil refiner, said on Friday it was interested in acquiring Esso (Thailand) Pcl , a unit of Exxon Mobil , as part of its expansion drive. "I admit that we are studying the possibility of buying Esso," Chief Executive Surong Bulakul told Reuters. "There's no specific timeframe when the study will be done because we have to look at several details."