Modi was President and Chief Operating Officer of the company which produces natural gas from coal seams (coal bed methane or CBM) in West Bengal.
He takes over the new role from his father, Yogendra Kumar Modi, who was the Chairman and CEO of the company.
Senior Modi "will now be the Executive Chairman of the company," GEECL said in a statement.
Prashant Modi was appointed to "the Board of GEECL as the Managing Director and Chief Executive Officer of the company with effect from September 22, 2015," it said.
GEECL produces CBM from Raniganj (South) licence area, which covers 210 square kilometers, with 2.62 trillion cubic feet of inplace gas reserves.
Its second asset is the Mannargudi licence situated in Tamil Nadu, which covers an area of 667 sq km and 0.98 Tcf gas-in-Place.