Abstract: M₁ oil measure of A reservoir in Pearl River Mouth basin is characterized by high porosity, low permeability and low resistivity. In order to examine its main control factor of low resistivity, this paper conducts the analyses of the grain size distribution, shale content, clay mineral type and distributive form, pore structural feature, conductive mineral content, thin sand-shale interlayer development status, with comparison with M₂ oil measure of high resistivity. It is concluded that there are three factors for it: the reservoir is of high fine-particle content, high shale content, high percentage of fine pore throat volume, which result in its high irreducible water saturation and enhance its conductive property; its clay mineral is dominated by illite-montmorillonite mixed layer and kaolinite, showing film-like, flocculation-like and dispersion-like distribution, enhancing its additional conductivity; the development of thin sand-shale interlayer allows the low-resistivity formation of its own to become much lower by the effect of surrounding rocks with low resistivity. Also, the conductive minerals and the salinity of formation water have some effects on reducing resistivity of the reservoir, but they are not main factors for it.

Key words: low-resistivity reservoir, irreducible water saturation, pore structure, clay mineral, additional conductivity, thin interlayer, origin, genesis