This paper presents a quantitative assessment of the impact of two gas pipeline projects, Nabucco and South Stream, on Turkey's energy security. The incidence of the impact is based on three dimensions of energy security: supply-demand balance, production source diversity,and transit route diversity. This paper relies on the Herfindahl Hirschman Index (HHI) and the adjusted Shannon Weiner Index (SWI) to evaluate and compare the impact of the various project implementation scenarios. The main findings are that both projects enhance Turkey's energy security and provide valuable and timely energy supply in the medium-term but their contribution is inadequate and marginal in the long-run. More specifically, the implementation of Nabucco significantly reduces the market concentration of producers whereas the South Stream project improves transit diversity by including Bulgaria as a major transit player. Surprisingly, implementation of Nabucco reduces transit diversity security because it includes politically volatile regions like Iraq and Georgia