Lithology identification and shale content calculation for high-gamma tight sandstones in the Taiyuan Formation of Daniudi gas field

Abstract

Lithology identification is pivotal for the refined evaluation and efficient development of tight sandstone reservoirs. The second Member of the Taiyuan Formation in the Daniudi gas field is a typical tight sandstone reservoir characterised by complex lithology, primarily comprising quartz sandstone, lithic quartz sandstone, and lithic sandstone. These lithologies exhibit significant differences in petrophysical and gas-bearing properties. A major interpretation challenge is the presence of widespread high-gamma sandstones. These sandstones are often misidentified as mudstones in conventional log analysis, leading to underestimation of net sandstone and gas pay thicknesses. The primary objectives of this study are to accurately identify lithology and to precisely calculate the shale content for these high-gamma sandstones. The methodology integrates thin-section analysis, petrophysical laboratory data, and conventional well logs. The study systematically established the correspondence between mineral composition and log responses for different sandstone types. This led to the development of a sensitive log-curve identification method specifically for high-gamma sandstones. Furthermore, a multivariate fitting model was constructed to establish an improved shale content calculation method, effectively overcoming the limitations of traditional models. Software for automatic lithology identification and shale content calculation was developed and successfully applied to log data from over 30 wells in the field. The results demonstrate that the proposed method achieves high accuracy, with lithology identification conformity exceeding 90% and shale content calculation conformity exceeding 95%. This study provides crucial technical support for reservoir delineation and gas–water layer identification in the Daniudi gas field. It significantly enhances the precision and efficiency of log interpretation and offers a valuable reference for the evaluation of analogous complex tight sandstone reservoirs.