Differential Enrichment Mechanisms of Organic Matter in Lacustrine Shale Across Various Structural Units: Evidence From Petrology, Organic Geochemical Characteristics and Major and Trace Elements

ABSTRACT Shale oil exhibits strong heterogeneity, and the enrichment of organic matter also varies significantly. This is particularly prominent in the Yingxiongling Depression of the Qaidam Basin, which has been subjected to intense Himalayan tectonic movements. Unlike previous studies that treated lacustrine basins as a whole to investigate organic matter enrichment while overlooking the inherent heterogeneity caused by sedimentary, tectonic and other factors, this study conducted a multi‐technical characterisation of shale samples from various structural zones in the Yingxiongling Depression. The methodology integrated an integrated suite of techniques, including detailed core and thin‐section petrography, field‐emission scanning electron microscopy (FE‐SEM), X‐ray diffraction (XRD) for mineralogy, total organic carbon (TOC) and Rock–Eval pyrolysis for geochemistry, along with major and trace element analysis. Ultimately, this study reveals the mechanisms and variations of organic matter enrichment in different structural zones (the slope zone and deep lacustrine zone) and establishes two distinct organic matter enrichment models for these zones based on differences in tectonic evolution. Analysis confirms that paleoproductivity and paleoredox are primary controlling factors on organic enrichment; however, the operative mechanisms exhibit significant divergence between the various structural settings within the lacustrine basin. In the deep lacustrine zone, shales are predominantly composed of carbonate minerals. A coupled system where dual hydrocarbon precursors (algae and bacteria) serve as the foundation for high primary productivity, alongside a confined, high‐salinity anoxic water column that ensures effective preservation, collectively facilitates organic matter enrichment. In contrast, the slope zone is influenced by terrigenous input, leading to higher contents of clay and felsic minerals in the shales. Organic matter primarily originates from nutrients delivered by terrigenous input, supplemented by algae and bacteria as hydrocarbon‐generating materials. However, the preservation conditions are poorer (weakly reducing to oxidising environment). The coupling of terrigenous input, paleoproductivity and paleoredox conditions controls organic matter enrichment in this zone. The findings offer fundamental insights into the heterogeneous enrichment of organic matter controlled by tectonic zonation, thereby supplying essential theoretical support and practical guidance for the exploration and evaluation of high‐quality lacustrine source rocks.