Application of high-resolution imaging logging technology in geological evaluation of ultra-shallow gas reservoirs in deep-water areas

To finely characterize the lithological and lithofacies features, sedimentary environment characteristics, and evaluate the seal capacity of shallow gas reservoirs in the ultra-shallow layers of the Quaternary Ledong Formation in the Lingnan Low Uplift area of the Qiongdongnan Basin, South China Sea, high-resolution resistivity imaging logging technology was introduced for the comprehensive geological evaluation studies of ultra-shallow reservoirs for the first time. Results indicate that the lithology of the ultra-shallow reservoirs in the Ledong Formation consists of mainly fine sandstone, siltstone, and muddy siltstone, having four types of sandstone lithofacies, two types of mudstone lithofacies, and mixed accumulation lithofacies developed. Based on the comprehensive lithofacies and regional geological background, the ultra-shallow reservoirs in the study area are characterized by depositional microfacies including mid-fan distributary channels and sheet sands within submarine fans and overlain by thick deposits of deep-sea mudstone, indicating a favorable reservoir-caprock configuration. The degree of natural fracture development in the study area is relatively weak, and horizontal bedding was observed in the mudstone, suggesting a stable post-depositional tectonic environment conducive to the preservation of ultra-shallow gas reservoirs. Additionally, the ultra-shallow reservoirs have undergone significant deformation due to massive deposition transport (MDT) modification, resulting in poorer reservoir properties and the formation of local caprocks for the ultra-shallow gas reservoirs. The application of high-resolution resistivity imaging logging technology in the evaluation of the geological characteristics and seal capacity of ultra-shallow reservoirs compensates for the lack of core data due to the shallow burial and loose structure of the reservoirs and provides strong geological support to the subsequent well deployment.