Palynological Characteristics and Paleoenvironment of the Triassic Yan-Chang Group in the Southeastern Ordos Basin, North China

The Yanchang Group is the most important hydrocarbon-producing formation in the Ordos Basin, composed of dark mudstones and gray-green sandstones, and is further subdivided into 10 oil reservoir groups named Chang 1, Chang 2, …, Chang 10 in descending order. Palynological analysis of the Yanchang Group at the Hancheng section reveals two distinct assemblages. The lower Yanchang Group (Chang 8 to 10) yields a Todisporites– Verrucosisporites–Parataeniaesporites assemblage. Fossils are generally sparse, with pteridophyte spores dominating over gymnosperm pollen, characterized by low diversity, heavily dominated by Verrucosisporites. In contrast, the upper Yanchang Group (Chang 1-7) contain an abundant and diverse Angiopteridaspora–Aratrisporites–Cycadopites assemblage, gymnosperm pollen (notably Cycadopites) exceeds spores, with the latter showing high diversity and no extreme dominance. This significant shift between the two assemblages reflects a major paleoenvironmental transition. The Chang 8 lithology (sand-mud interbeds) indicates fluvial-deltaic conditions, while the overlying Chang 7 lacustrine mudstones mark a shift to deep lake deposition. More importantly, this palynofloral change is not local. It is widely documented across the Ordos Basin and correlates with a peak in pteridophyte spore diversity in Europe during the mid-Ladinian (Middle Triassic), pointing to a widespread regional paleoclimatic event. The transition indicates a major shift from an arid/semi-arid to a stable, warm, and humid climate in the Ordos Basin during the Ladinian. This climatic shift, broadly synchronous with major tectonic events of the Qinling Orogen, created favorable conditions for prolific biotic productivity. The resulting abundant organic matter was crucial for forming the high-quality Chang 7 hydrocarbon source rocks, underscoring the event's significance for petroleum resource assessment in the basin.