Conventional oilfield operations recover, on average, about a third of the crude oil originally present in a reservoir. Improving the yield depends both on overcoming macro- and micro-geological problems and on progressively compensating for the pressure drop in the reservoir as it is depleted of oil. In some cases this may be achieved by treating the oil-bearing matrix with certain chemicals, most commonly surfactants and viscous or insoluble polymers. One effective way of introducing the chemicals is by employing a population of bacteria in the reservoir as synthetic agents. Some of these processes are shown in the Centrespread illustration.