Identification of complex lithofacies based on compositional indicators：a case study of deep-water low-permeability gas reservoir in Baodao Sag, South China Sea

Aiming at the problem of complex lithology of deep offshore low-permeability sandstone reservoirs and difficulty in identifying favorable reservoirs, the complex lithology in the area was classified based on lithological laser particle size analysis and thin section identification. Through the analysis of the relationship between lithofacies and well logging curves, the sensitive well logging curves that can identify the lithofacies of the low-permeability reservoir were selected. By utilizing the principle of principal component analysis, component factor curves that can accurately identify three types of lithofacies were constructed. Results show that the lithology of the low-permeability reservoir is complex and variable, ranging from siltstone to gravelly medium and coarse sandstone. In the median particle size, the lithology in the lithofacies of this area could be divided into three types, i.e., coarse-grained lithofacies (median particle size (Ms) ＞0.1 mm), fine-grained lithofacies (Ms 0.1–0.0625 mm), and very fine-mudstone lithofacies (Ms ＜0.0625 mm). The coarse-grained lithofacies is the relatively high-quality lithofacies zone. In addition, lithological component indicator factors were constructed with which the accuracy of the lithofacies identification was improved significantly compared to the conventional methods using single well logging curve. The lithofacies identification method we established could better guide the sand entry position of horizontal well in the study area, which has important significance for the development of the low-permeability reservoir in the study area.