Shale lithofacies are of vital importance to the assessment of shale gas resources and development potential. The lithofacies classification is generally based on petrological and geochemical tests. However, due to the limited samples and economic costs, lithofacies prediction based on logging data has become a trend. In this study, lithofacies were classified using TOC, XRD, and thin-section identification (TSI) analyses of marine-continental transitional shale samples from the Benxi Formation in the Hengshan-Wubu area, Ordos Basin. A hybrid predictive model integrating the back-propagation (BP) neural network and conventional log-interpretation methods was subsequently developed. Based on the predicted lithofacies from logging data, both vertical and planar distribution patterns were characterized, and favorable shale gas intervals (sweet spots) were identified. The TOC tests show that TOC values of the shale in the Jinci Member range from 0.15% to 19.1%, with an average of 3.50%. The XRD tests show that the Jinci Member shale is rich in clay minerals, ranging from 5.1% to 95.7% (avg. 54.2%), followed by quartz, which varies from 1.1% to 65.6% (avg. 30.5%). The test results reveal that shale lithofacies in the study area are predominantly composed of argillaceous shale, with organic-rich shales mainly occurring within the argillaceous and mixed shale categories. Among all samples, organic-rich siliceous argillaceous shale is the most abundant, accounting for 20.0%, followed by organic-rich mixed shale (18.6%) and organic-poor siliceous argillaceous shale (15.7%). Characterized by high TOC content and abundant brittle minerals, the organic-rich siliceous argillaceous and mixed shales are favorable types of lithofacies. Prediction results show that organic-rich lithofacies are vertically concentrated in intervals between Coal Seams No. 8 and No. 9 and in the lower part of Coal Seam No. 9 within the Jinci Member, defining the vertical sweet spots. Laterally, favorable areas are mainly distributed in the northwestern and western parts of the study area. These findings provide a valuable reference for target selection in shale gas exploration within the Carboniferous-Permian marine-continental transitional shales of the Ordos Basin.
Meng et al. (Fri,) studied this question.