BOGOTA, Feb 23 (Reuters) - Parex Resources said on Monday it has submitted an all-cash proposal to acquire the Colombian upstream operations of Frontera Energy for $500 million.

The offer includes the assumption of debt and a contingent payment of $25 million. Parex said the bid represents a $125 million premium over a previously announced acquisition agreement involving Frontera.

GeoPark in January announced a definitive agreement to acquire all of Frontera Energy's oil and gas exploration and production assets in Colombia for $375 million.

GeoPark confirmed on Monday that the Parex proposal constitutes a counter-proposal but said it had no further comment for now.

Calgary-based Parex said the combination of Parex and Frontera's portfolios would immediately create the largest independent Colombian-focused energy company. The two companies are already partners in Colombia's VIM-1 block.

"Our all-cash offer... provides immediate and greater value for Frontera and its shareholders," Parex Chief Executive Imad Mohsen said in a statement, adding that he expects the board to categorize the bid as a "superior proposal."

Frontera's Colombian portfolio comprises 17 exploration and production blocks, including the Quifa and Cubiro fields. The company reported average annual production of 38,934 barrels of oil equivalent per day at the end of the third quarter last year.