Stratigraphic records of volcanic activity and influence on the development of organic-rich shale in Chang 73 submember of the Yanchang Formation, Ordos Basin

The Chang 7 3 submember of the Yanchang Formation in the Ordos Basin exhibits multi-stage volcanic-shale interbedded sequences, with organic matter enrichment closely linked to surrounding volcanic activities. This study employs core observation, thin section identification, scanning electron microscopy, and micro-area X-ray fluorescence spectroscopy to systematically investigate tuff lithofacies types, volcanic activity periods, and their regulatory mechanisms influencing organic matter enrichment. Five lithofacies types were identified, namely vitric tuff, crystal-vitric tuff, crystal tuff, sedimentary tuff, and tuffaceous sandstone. These lithofacies exhibit northwestward distribution patterns, with thicknesses decreasing from 2.5 m in the south to 0.3 m in the north, suggesting that their provenance originated from the Qinling Orogenic Belt along the southern margin of Ordos Basin. Four vertical volcanic cycles were identified based on the coupling characteristics of grain size sequences and geochemical cycles of Hg/Al. Zircon U–Pb dating indicates that the volcanic activity during this period was driven by Late Triassic tectonic-magma events. Volcanic activity promotes organic matter enrichment via a “nutrient input–anoxia enhancement–deposition inhibition–closed coupling” mechanism. Volcanic ash releases essential biological elements, such as Fe and Si, triggering algal blooms. The post-eruption environment is characterized by a synergistic effect of enhanced paleoproductivity, intensified anoxia, and reduced paleosalinity. The “source-reservoir” configuration of shale overlying tuff forms a high-quality industrial standard layer and represents a promising new exploration target for continental shale oil and gas.