Paleoenvironmental Reconstruction and Depositional Model of the No. 5 Coal Seam, North-central Ordos Basin: Insights from Coal Petrology and Geochemistry

The provenance and sedimentary environment of peat swamps control the material composition of coal and represent one of the key factors contributing to coal heterogeneity. To clarify the sedimentary environment, provenance characteristics, and coal-controlling mechanisms of the No. 5 Coal Seam in the north-central Ordos Basin, this study investigated the coal petrological, coal quality, geochemical, and coal-accumulating environmental characteristics of the seam via systematic drill core observation and sampling, maceral identification combined with proximate analysis, determination of total sulfur and sulfur forms, vitrinite reflectance measurement, and major-trace element testing. The results show that the macroscopic coal types of the No. 5 coal seam are dominated by bright and semibright coal with primary texture, belonging to low-volatile bituminous coal. Overall, it is characterized by low ash content, low sulfur content, and ultralow total moisture. Among the macerals in the coal, vitrinite is dominant, followed by inertinite, while the exinite content is extremely low; inorganic components mainly include clay minerals and pyrite. During the coal-accumulating period, the paleoclimate was mainly warm and humid, and the water medium environment of the peat swamp was significantly affected by seawater, characterized by a brackish-saline, dysoxic-anoxic reducing environment. The coal seam was formed in a humid, water-covered peat swamp environment dominated by herbaceous plants, featuring weak groundwater dynamics, poor fluidity, strong gelification, intense plant degradation, and high reducibility. The provenance supply of the No. 5 coal seam was stable and diverse, mainly derived from intermediate-felsic materials stably supplied by the Yinshan Paleoland in the northern basin. The coal-accumulation process of the No. 5 coal seam began with the transgression following the regression. The stable tectonic setting, steady provenance supply, and suitable paleoclimate and sedimentary environment all provided favorable conditions for organic matter accumulation in the peat swamp. During the coal-accumulating period of the No. 5 coal seam, short-term fluctuations in the sedimentary environment occurred due to periodic marine transgressions. Based on the North-South differentiation of seawater and paleotopographic relief characteristics, the peat swamp depositional environment was classified into four types: ombrotrophic moist interdistributary depression bog, minerotrophic moist interdistributary depression fen, ombrotrophic water-covered interdistributary bay bog, and minerotrophic water-covered interdistributary bay fen, each with unique coal petrological, coal quality, and sedimentary parameter characteristics. This study is of great significance for revealing the coal-forming mechanism and paleoenvironmental evolution and can provide theoretical support for the efficient and clean utilization of coal resources and the exploration and development of deep coalbed methane (CBM).