Paleoenvironment and Provenance of the Lower Yan'an Formation, Jingbian Oilfield: Insights From Geochemistry

ABSTRACT The paleoenvironmental evolution and provenance characteristics of the Yan 9 and Yan 10 intervals in the lower Yan'an Group at Jingbian Oilfield remain poorly constrained, largely because previous studies have been confined to macroscopic sedimentary facies analysis without systematic geochemical investigation. As a result, differences in paleo‐water salinity, redox conditions, paleoclimate evolution, and sedimentation rates between these two intervals are still unclear. Additionally, precise identification of the tectonic setting and source rock composition of the provenance areas is lacking, which limits understanding of hydrocarbon accumulation mechanisms and favorable area prediction in the region. To address these key scientific issues, this study selected 27 samples of mudstone and silt‐rich mudstone from the lower Yan'an Group (Yan 9 and Yan 10 reservoirs) at Jingbian Oilfield, Ordos Basin. Using geochemical analytical techniques, we systematically measured major, trace, rare earth elements (REEs), and clay minerals to reconstruct the ancient depositional environment and clarify the provenance characteristics of the study area. The results indicate the following: Multiple indicators, including boron content, equivalent boron calculations, clay mineral assemblages (Couch method), and Sr/Ba and B/Ga ratios, consistently suggest that the sediments of the lower Yan'an Formation were deposited in a freshwater environment. Ratios of redox‐sensitive elements such as δ U, U/Th, and V/(V + Ni) reveal an oxygen‐rich water column. The index of compositional variability (ICV) and chemical index of alteration (CIA) indices indicate a tectonically stable provenance area that underwent intense chemical weathering. Combined with Sr/Cu and Al 2 O 3 /MgO ratios, these findings confirm a warm and humid paleoclimate, with the Yan 9 member being slightly drier than the Yan 10 member. The REE distribution patterns show significant fractionation between light and heavy REEs, whereas the (La/Yb) n ratio suggests a higher sedimentation rate in the Yan 9 member compared to the Yan 10 member. Integrated discrimination diagrams for tectonic settings reveal that the provenance is primarily associated with an active continental margin and continental island arc. The detrital materials are mainly derived from felsic upper crustal sources, with contributions from alkaline basalt and granite. This study demonstrates the effectiveness of geochemical methods in reconstructing depositional environments and analyzing provenance, providing important insights for hydrocarbon exploration in the study area.