The stability of gas injection in a layered reservoir drilled with lateral wells, is studied using a generalized pressure distribution-dependent mobility ratio expression. Stable injection guarantees clean oil production. The mobility ratio compared layers’ fluid velocities across a common permeable interface. Studies were based on injected gas compressibilities and viscosities only. Results show that injection stability is affected by (1) injected gas properties, and (2) injection layer; i.e., whether gas cycling (bottom layer injection) or gas injection (top layer injection). Gas cycling tends to exhibit more instability than gas injection operation.