Current US unconventional resource revolution has more than doubled US oil production and made US the largest oil production country in the world recently. Although the industry is trying all means including screening sweet spots, conducing infill drillings and optimizing the well spacing to improve recovery factor (RF) from primary production, unconventional reservoirs will encounter what the past conventional reservoir production experienced: difficult to explore new giant resource, run out of 'easy oil' (sweet spot), production comes to a plateau and declines. Therefore, it is time to implement enhanced oil recovery for unconventional resources. The goal of this work is to improve oil recovery by implementing and optimizing gas Huff and Puff method for one of the US major tight oil reservoirs-Bakken formation. Pressure dependent permeability and porosity are implemented into the compositional reservoir simulation to represent geomechanics effects. Gas diffusion effects is systematically analyzed to quantify the importance comparing with the natural fracture system in fight oil fractured reservoirs. Alternative gas composition instead of using CO2 in the Huff and Puff process is investigated and the recovered oil components is analyzed. The complex natural fracture structure and natural fracture permeability proved to be important affecting factors to oil production of Bakken reservoirs.