Abstract: Abstract：The study area belongs to an oilfield in Northeast China and the development of the oilfield has a history of nearly 50 years. With increasing demand for oil, vast amount of oil has been exploited. However, during exploitation, groundwater was polluted because of spilled oil and wastewater in this oilfield. Therefore, protecting groundwater from further pollution is essential to the health and safety of citizens in the oilfield. Based on an analysis of water samples, the authors have compiled a contour diagram of oil pollutant concentrations in groundwater in the study area and found that, except in a small scope where oil pollutants are distributed areally, most oil pollutants are distributed as patches in the study area. The contour diagram of oil pollutant concentrations, combined with substantial investigation and research on the geologic environment, structural setting, surface water flow and possible surface polluting source of the study, suggests that the oil pollutants mainly moved into groundwater by accidental pollution and ground cracks produced by over-development of groundwater and neotectonic fracturing. Due to long-continued over-pumping of groundwater near hole ZK6 in the study area, the direction of groundwater changed, and ground subsidence occurred and many ground cracks appeared at the surface; so groundwater pollution by oil is very serious. In addition, neotectonic faults inherited the NW, NNE and E-W sets of crustal faults, thus causing oil/gas in the underlying oil layers to move up along the fractures and pollute the overlying aquifers. Furthermore, large numbers of ground channels and ground cracks formed at the surface, and spilled oil and other pollutants polluted the aquifers through them.