Abstract: The conglomerate sandstone oil reservoir from the Paleogene Liushagang Formation in the Wushi Sag of the Beibu Gulf Basin exhibits significant resistivity differences. High resistivity and low resistivity oil layers co-exist; they can be hardly distinguished based on their fluid properties, leading to difficulties in oil field exploitation. Thus, we conducted in-depth investigation of the formation mechanism of low resistivity oil layers using data from well logging, mud logging, well testing, and core analyses. The results show that mud intrusion, conductive minerals, and mineralization of formation water slightly influence resistivity, while irreducible water saturation is the major factor leading to the formation of oil layers with relatively low electrical resistivity. Based on this knowledge and detailed assessments of the microscopic pore structures, we found that the enrichment and accumulation of low resistivity oil layers results from a combination of primary intergranular pores and secondary mold pores. The development of planar and curved throats, large displacement pressures, small pore throat radii, and complex pore structures are the fundamental causes of irreducible water saturation in the reservoir layers.

Key words: Wushi Sag, conglomerate sandstone, low resistivity, irreducible water saturation, pore structure