The Niobrara and Codell in the Wattenberg Field of the Denver-Julesburg Basin (DJ Basin)have been in the centerstage of horizontal drilling and multi-stage hydraulic fracturing ever since 2007. Based on the current well completion strategy, oil rates drop to 20 bbl/day/well in five years of primary production. The cumulative primary production in the first five years amounts to 3%. Nonetheless, a substantial amount of producible hydrocarbon still remains. In this paper, we propose a most feasible enhanced oil recovery (EOR) technique for the Niobrara and Codell and other similar unconventional oil reservoirs. Realizing the unavailability of CO2 in the area while having easy access to methane, ethane, propane and butane, we designed an injecting gas consisting of ethane enriched with methane, propane and butane for EOR. A dual-porosity compositional model was constructed using data from seismic, well logs, core analysis, and production performance. After successful history matching, as well as verification with seismic and microseismic interpretations, a producer with five years of production history was converted to an EOR-gas injector in the numerical model. We used the model to determine the optimal injection gas composition for producing the largest amount of oil. We also studied the contribution of molecular diffusion at the fracture-matrix interface for the incremental oil recovery from gas injection. Model results indicate that converting three producers to injector wells, and producing from the remaining eight producers, yielded total oil recovery of 4.68% in fifteen years of production with 13% of which attributed to gas injection EOR.