El Gassi is a volatile oil field located in the Algerian desert. The field has produced about 62 MMSTB oil since 1959, through primary depletion. Due to the heterogenity of producing formation, the total recovery during 45 years is only about 11%. The implementation of water-flood project was avoided due to low achievable water injection rate. A miscible gas injection project is examined in this study. The purpose of gas injection is to maintain reservoir pressure and improve the microscopic efficiency to increase the overall recovery in the field. A laboratory study revealed that the miscibility could be achieved with a pressure of about 241.4 bars (i.e. 3500 psia), which is below the reservoir average pressure in El Gassi field. Typical reservoir heterogeneities encountered in El Gassi field were incorporated into the simulation model. The study includes history matching of past reservoir performance, investigation of different injection scenarios, optimization of injection well number, optimization of gas injection and oil production rates, and the benefit of drilling infill wells. Edge line gas injection appeared to be promising.