* Shale-rich Neuquen province pulls two YPF concessions
* Follows similar moves last week by Chubut, Santa Cruz
* Officials say YPF not investing to boost output
BUENOS AIRES, March 19 (Reuters) - Argentina's Neuquen province, home to huge untapped reserves of shale oil and natural gas, said on Monday it was stripping the country's biggest energy company YPF of two concessions due to insufficient investment.
The move by Neuquen, which affects two unproductive areas, comes days after fellow Patagonian oil provinces Chubut and Santa Cruz reverted YPF concessions.
YPF, controlled by Spain's Repsol, is under government pressure to increase oil and natural gas output in order to reduce a mounting fuel bill that is eroding the trade surplus.
YPF's Chihuido de la Salina and Portezuelo Minas concessions would be taken over by the state-run company, Gas and Oil of Neuquen, the provincial government said in a statement.
YPF has rejected the charges of unfulfilled investment and commitments, vowing to take legal action against the "unilateral" revocation of its concessions in Chubut.
Neuquen is home to YPF's Vaca Muerta property, which could potentially double Argentina's energy output within a decade.
An industry source, who asked not to be named, said YPF would hold a board meeting on Wednesday to discuss the issue of dividend payments. (Reporting By Jorge Otaola; Writing by Helen Popper; Editing by Ramya Venugopal)