A large-scale groundwater-flow model has been developed for the Niger Delta, Nigeria. The Niger Delta covers an area of about 75,000 km² (28,957 mi²) in southern Nigeria, where the Niger River discharges its water into the Atlantic Ocean through a series of distributaries. The delta is underlain by a sequence of three layers of clastic Tertiary sediments, from top to bottom: the Benin, Agbada, and Akata formations. The delta is one of the most hydrocarbon-rich regions in the world. Petroleum has been produced over the last three decades from the Agbada Formation by several national and international oil companies. This oil and gas production and a rapidly growing population have resulted in environmental degradation of the delta. An understanding of groundwater movement will aid in finding some solutions to the environmental problems. The major objectives of this study were to perform an initial simulation of groundwater flow in the delta, analyze groundwater-flow paths, delineate discharge and recharge areas, and determine any possible relationship between the flow pattern and petroleum occurrence. The U.S. Geological Survey three-dimensional finite-difference code, MODFLOW, was used to simulate steady-state flow. The model was reasonably calibrated to reported groundwater recharge. No regional flow system occurred, and flow was concentrated in intermediate and local systems. A major discharge area, trending east-west, receives its water from local systems and from intermediate systems that originate at midtopographic elevations. This discharge area appears to coincide with a prolific oil-rich zone. This finding may have implications for oil exploration in the Niger Delta.