The objective of this paper is to quantify the incentive effects of petroleum taxes. This enables us to estimate individual petroleum reservoirs' supply functions before and after tax. Of central importance are the marginal reserves within the petroleum reservoir itself. The recovery factor of oil and gas reservoirs is influenced by economic considerations. Both the rate of return requirement and petroleum taxes play crucial roles. The conclusion is that the distorting effects observed in many petroleum taxation systems have serious resource recovery consequences and reduce the recovery factor for individual oil and gas reservoirs.