Abstract: The Termit Basin of Niger, covered with low salinity stratum water (salinity 200×10-61500×10-6 ), developed a highly complex oilgaswater system of complex fluid type, with several oilgaswater systems present including normal and lowresistivity oil zones. Previous studies demonstrated that it is very difficult to identify the complex fluid type by single data analysis, especially in lowresistivity oil zones. It has also been proven by drilling that, in the Termit Basin, low resistivity oil zones were commonly developed in the Sokor 1 and Yogou formations in the Paleogene and Cretaceous, respectively. The lowresistivity oil zones have two main resistivity features: similar resistivity for the reservoir and adjacent water zone within the same sedimentary period, and similar resistivity for the reservoir and adjacent mudstone. Here, we studied the microscopic mechanism and factors influencing low resistivity oil zones, by using thin section, scanning electron microscopy, Xray diffraction and clay mineral analysis techniques, combining with capillary pressure data and knowledges of rock size, pore structure, clay mineral content type and thickness of oil layer. The results showed that high saturation of bound water due to rocks fine grain size, high clay content and development of micro pores, was the main microscopic cause for the low resistivity oil zone in the basin; while thin reservoir and invasion of saltwater mud filtrate were largely macro impact factors. According to their dominant characteristics and cause, the low resistivity oil zones can be effectively identified by cross plotting resistivity vs. natural gamma, resistivity vs. relative value of spontaneous potential, GEOFI vs. TG, and using fluid density calculated by RFT pressure data. Furthermore, our research revealed that the lowresistivity oil zones are laterally distributed in the oil field of Dinga fault terrace, Fana low uplift and Yogou slope, and vertically distributed in the fluvial sediments of E1 and E2 sand in the Sokor1 Formation and delta, and lacustrine sediments of the Cretaceous.

Key words: Niger, Termit Basin, low resistivity oil zone, genetic mechanism, logging comprehensive identification technology