Abstract: In the Shengli oil field, there are a number of low-resistivity pay sands whose resistivity approximates to that of the aquifers, and their discovery has added to the wealth of our knowledge about the pay sands. Drawing on the data from the scanning electron microscope and gamma diffraction, the paper attempts to expound the lithology of the low-resistivity pay sands, the nature and distribution of clay contained in the sands and their pore structure and wettability. In fact, low-resistivity pay sands are caused by the high water saturation of the sands that contain primarily bound water. This knowledge which constitutes an important theoretical ground for analysing log data and identifying pay sands is in some way significant in that it helps increase the chances of success in the interpretation of log data and discovery of more oil reserve.