Main Controlling Factors and Identification Methods of Low-Resistivity Oil Layers of Neogene Guantao Formation in the Shijiutuo Uplift, Bohai Bay Basin

In recent years, a large number of low-resistivity oil layers have been found in the glutenite of Shijiutuo Uplift in the central part of the Bohai Sea, and the oil and gas exploration potential is greatly restricted by the lack of clear causes, the small difference between the oil and water layers, and the difficulty of distinguishing conventional data. To clarify the main controlling factors for the low resistivity of the oil layer in this area and to distinguish them effectively and quickly, the characteristics of the low-resistivity oil layer are systematically analyzed using core, mercury injection, thin-section, and logging data. The low-resistivity control factors of the oil and water layers are quantitatively analyzed from the perspectives of rock and fluid using statistical methods, and a mathematical function is established to effectively distinguish the fluid based on the relationship between the resistivity of the oil and water layers and the main controlling factors. The results show that the low-resistivity layer in this area is characterized by braided-river channel and channel-bar deposits, and the lithology is mainly pebbly sandstone, medium-fine sandstone, and siltstone. The rock composition maturity is low (average 0.37), the pore throat heterogeneity is strong (homogeneity coefficient 0.3), and the clay mineral content is high (average 16 %). High clay mineral content and strong pore-throat heterogeneity led to a high irreducible water saturation in the rock (average 45 %). High irreducible water saturation is the main controlling factor of the low resistivity of the oil layer. Based on the exponential negative correlation between irreducible water saturation and rock resistivity, an exponential function is established to increase the resistivity ratio of the low-resistivity oil-water layer to more than 1.6, thereby enabling rapid and effective discrimination. The practical verification method is scientific and effective. The findings of this study enable the identification of low-resistivity oil zones using well-logging resistivity data, providing valuable references for assessing hydrocarbon exploration potential in the Bohai Sea and similar blocks worldwide.