Differences in coal-accumulation regularity between northern and southern Yan'an Formation, Ordos Basin: Evidence from geochemistry

The significant heterogeneity in the development of coal-bearing successions across different regions of sedimentary basins constitutes a critical constraint on both exploration and subsequent exploitation of coal resources. This investigation focuses on the coal deposits of the northern and southern Yan'an Formation within the Ordos Basin. Biomarker identification and geochemical characterization of major, trace, and rare earth elements were conducted using gas chromatography (GC), gas chromatography-mass spectrometry (GC–MS), X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP–MS). These analyses aim to reconstruct paleoenvironmental variations during peat accumulation in the northern and southern parts of the Yan'an Formation, Ordos Basin. This investigation systematically examines the coal accumulation evolutionary processes within the Yan'an Formation to unravel the genetic mechanisms governing coal resource distribution patterns in sedimentary basin systems. The results show that: 1) Organic matter in both southern and northern coal deposits predominantly originates from terrestrial higher plants, with subordinate contributions from lower organisms. The southern coals have attained maturity, whereas northern counterparts remain in the immature to low-maturity range. 2) The coal-forming paleoenvironments of the southern and northern Yan'an Formation exhibit fundamental similarities, characterized by warm-humid climatic conditions and reducing freshwater systems. Nevertheless, the southern region exhibited higher paleoproductivity and a lower sedimentation rate, accompanied by evident hydrothermal depositional activity. 3) The southern part predominantly developed the coal seams within warm-humid, anoxic lakeshore swamp systems during the early depositional period of Yan'an Formation. The Yanshanian orogenies provided the geological precondition for early-stage peat accumulation, facilitating thick coal formation. Subsequent waterbody deepening and terminal erosional truncation resulted in stratigraphic absence of coal-bearing sequences of the middle–upper Yan'an Formation in this region. The northern part predominantly developed the middle–upper Yan'an Formation coal deposits under warm-humid climates within suboxic delta plain swamp systems. Stable tectonic subsidence facilitated the formation of thick coal seams characterized by substantial thickness and laterally persistent distribution during the middle–late depositional stage in the northern basin. This study provides critical insights into coal-accumulation regularity in sedimentary basins and enhances coal resource exploration strategies. • Organic geochemical constraints on the sources and thermal maturity of coals in the southern and northern basin. • Organic-inorganic geochemical constraints on paleoenvironmental contrasts between southern and northern coal-accumulating basins. • In-depth discussion on northern and southern coal-accumulation models and their differences based on the synergistic effects of tectonics, sedimentation, and paleoenvironment.