Objective The Carboniferous and Permian coal-bearing strata in the southern Wuxiang block, Qinshui Basin, Shanxi Province, China contain abundant coalbed methane (CBM) resources. To develop intelligent discrimination technology for CBM exploration in this block, it is necessary to establish lithology discrimination criteria based on log data, reveal the response relationships between logging and sedimentary facies, and determine sedimentary patterns.
Methods Using a comprehensive analysis of lithologies, sedimentary facies, and logging facies, this study systematically investigated the distributions of logging and sedimentary facies in the Shanxi and Taiyuan formations—the primary coal-bearing strata in the southern Wuxiang block. By comprehensively analyzing the core and log data, this study proposed a quantitative lithology identification method based on multivariate log data. Furthermore, this study established a lithology identification model using three-dimensional cross plots and determined the sedimentary patterns of the block by combining log curve morphologies and lithologic assemblage characteristics.
Results and Conclusions The identification model, which correlated lithologies, sedimentary facies, and logging facies, was constructed in this study. Using this model, 12 common lithologies in the primary coal-bearing strata in the southern Wuxiang block were divided into four categories. The log curves of the Shanxi Formation predominantly exhibited four morphologies, i.e., box, bell, finger, and funnel shapes, while those of the Taiyuan Formation exhibit four typical lithologic morphologic combinations, i.e., sawtooth, box + sawtooth, box + bell, and bell + finger combinations. The sedimentary characteristics of the primary coal-bearing strata in the southern Wuxiang block were determined based on the compositions and distributions of sedimentary facies and microfacies. Specifically, the Shanxi Formation exhibits a deltaic plain sedimentary system, which consists primarily of three sedimentary microfacies: distributary channels, interdistributary bays, and peat swamps. In contrast, the Taiyuan Formation is dominated by deposited marine carbonate platforms, with principal sedimentary microfacies including neritic facies, barrier islands, lagoons, and tidal flats. The tidal flat facies can be further divided into sand, peat, and mixed flats. Finally, the sedimentary patterns and sedimentary sequence types of the Shanxi and Taiyuan formations in the southern Wuxiang block were established, providing a basis for determining the CBM resource endowment in the block.
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