Abstract: This paper analysed geological conditions and production materials of low permeability oilfields (a domestic oilfield Y block as an example) . determined the oil and water distribution in the middle and high water content stage. Through the production split, calculated monthly for each oil layer, total oil production and the monthly fluid volume, total liquid production, and gained water drive curve of each small layer, and then get the remaining oil in a small layer , and the average saturation. According to the residual oil saturation contrast between the calculated geological reserves and given geological reserves, the actual geological reserves of each small layer can be determined.