Abstract:  A typical low resistivity reservoir is developed in the NgⅢ Formation (3rd Member of Neogene Guantao Formation) of C oilfield， Bohai Bay basin. Analyzing the influential factors of reservoir resistivity is of great significance to the efficient development. In this research， X-ray diffraction (XRD)， scanning electron microscopy （SEM）， blue-dye thin sections， and cation exchange capacity test (CEC) were used to quantitatively characterize the micro-features, including the clay mineral content and morphology， rock framework grain contact， and reservoir conductivity. The results showed that the content of smectite in clay minerals of the normal resistance reservoir is 23%-44%， while that of the low resistance reservoir is 39%-64%. According to the CEC experiments of reservoirs in different regions， the content of illite-smectite mixed layers is positively related to the cation exchange capacity. The smectite in the study area mainly exists in the form of illite-smectite mixed layers, and the illite-smectite mixed layers are distributed in two types. The first type is thin film， which wraps rock framework grains， leading to the formation of a conductive film on the surface of rock framework grains， this type accounts for about 80%; The second type exists between rock grains in flocculent and dispersed form， and accounts for about 20%， this type is relatively dispersed and fails to form a continuous conductive structure. Moreover， the low resistivity reservoir in the study area is mainly in Guantao Formation， according to the observation of blue-dye thin section, the rock framework grains are in point contact to line contact due to compaction, which leads to the contact of illite-smectite mixed layers conductive film on the surface of rock framework grains and the formation of continuous conductive structure. When illite-smectite mixed layers are in film shape and rock framework grains are in contact with each other, illite-smectite mixed layers can easily form a continuous conductive structure.

Key words: Bohai Bay oilfield, Guantao Formation, conductivity of clay minerals, clay mineral morphology, low resistivity reservoir