This paper considers how likely it is that a given number of agents who share a homogeneous oil reservoir will exploit the reservoir for their common benefit. A game-theoretical model is used, examining whether one agent would profit from deviating from the cooperative strategy, given that the remaining agents would follow a subgame-perfect retaliation strategy. The paper also examines the sensitivity of the cooperative solution to the number of agents, the time it takes to discover a deviation, the value of production relative to investment, and the discount rate. It is found that the cooperative solution is very sensitive to the number of agents, with more than three agents the cooperative solution becomes very unlikely.