Abstract:

In the eastern Ordos Basin, the coal seam No. 8 in the Carboniferous Benxi formation has been extensively developed for coal-rock gas. In the Ningqing block of the western Ordos Basin, however, the coal seams are typically thin, compositionally complex, and sedimentologically intricate, which has hindered their exploration and development. Using microscopic experiments such as proximate analysis, maceral analysis, and elemental analysis in combination with elemental logging and conventional logging data, this study analyzed the coupling relationships between measured data and sensitive logs. Subsequently, the macrolithotype and log-based identification criteria for the coal seam No. 8 in the Benxi formation in the Ningqing block were established. On this basis, sensitive elemental analysis was performed to reconstruct the coal-forming paleoenvironment, and maceral analysis was conducted to define a coal facies calibration index. The sedimentary facies distribution of the coal seam No. 8 was finely characterized, and the corresponding sedimentary facies model was constructed. Proximate analysis results indicate that, for the coal seam No. 8 in the Benxi formation of the Ningqing block, the fixed carbon and ash contents exhibit the strongest coupling with the density (DEN) log. In terms of macrolithotype, semi-dull coal is dominant, followed by semi-bright and dull coal, with bright coal being underdeveloped. Both total sulfur and inorganic sulfur contents exhibit a decrease-increase-decrease-increase cyclic pattern, indicating four sea-level fluctuations during the depositional period. Paleoenvironmental analysis based on elemental logging data indicates that the target interval was deposited in an environment alternating between weakly oxidizing and weakly reducing conditions, with primarily freshwater to brackish water and in a warm, humid paleoclimate, and recorded typical marine-continental transitional facies. Coal facies classification criteria were established based on macrolithotype, industrial components, and lithofacies associations. Accordingly, the facies of the coal seam No. 8 in the study area are categorized into three types: shallowly inundated herbaceous marsh and wetland herbaceous marsh dominantly under continental conditions in the central-northern region, and open-water swamp dominantly under marine conditions in the southern region. Moreover, a sedimentary facies model for the coal seam No. 8 in the Ningqing block was established. The research findings provide a basis for the exploration of coal-rock gas in the western Ordos Basin.

Key words: Ordos Basin, Ningqing block, Benxi formation, coal rock, macrolithotype, sedimentary environment, coal facies