Reservoir characteristics and cutoffs of low resistivity reservoir in Songtao Uplift, Qiongdongnan Basin

To finely characterize reservoir property, understand the genesis and main controlling factors, and quantitatively determine the cutoffs of low-resistivity reservoir in Songtao Uplift of the Qiongdongnan Basin in South China Sea, conventional logging, nuclear magnetic resonance logging, resistivity imaging logging, and formation testing data, as well as the core data were analyzed. Results show that the lithology of the low-resistivity reservoir is mainly fine-grained sandstone, including siltstone, argillaceous siltstone, and bioclastic sandstone. The porosity and permeability vary greatly, and the resistivity increase factor is less than 1.5. The sedimentary environment was sea shore, featuring fine-grained materials that have poor pore structure and pore connectivity. This feature resulted in the high content of pore irreducible water and capillary irreducible water in the sedimentary rock, which is the key reason for the low-resistivity oil reservoir, while mud invasion, conductive minerals, and additional conductivity of clay minerals had little effect on the reservoir resistivity. Therefore, the reservoir cutoffs were analyzed and determined quantitatively, including pore radius, porosity, permeability, resistivity, oil saturation, and clay content. This study paved road for identification and evaluation of low-contrast reservoir and will accelerate the development of low-resistance reservoirs in the study area.