Paleo‐Environment and Organic Matter Enrichment of Lacustrine Shale From Aershan Formation, E'rennaoer Sag, Erlian Basin

ABSTRACT Understanding the controls on organic matter enrichment in lacustrine shales is a fundamental issue in petroleum geology. Although organic matter enrichment in large, relatively stable lake basins has been well studied, the mechanisms controlling enrichment in small fault‐controlled basins remain poorly understood. To address this gap, this study investigates the fourth member of the Cretaceous Aershan Formation (FMAF) in the E'rennaoer Sag, Erlian Basin, as a representative case of a tectonically active lacustrine system. The FMAF in the E'rennaoer Sag is an important lacustrine shale source rock in the Erlian Basin. This study aims to reconstruct its depositional environment and clarify the mechanisms controlling organic matter enrichment. Comprehensive geochemical analyses, including total organic carbon (TOC), Rock–Eval pyrolysis, elemental ratios, and gas chromatography–mass spectrometry (GC–MS) biomarkers, were conducted. The results indicate that the FMAF was deposited under a semiarid to arid paleoclimate, with semi‐saline to saline water conditions, and predominantly anoxic bottom waters. Contrary to previous assumptions of overall shallow settings, the FMAF was mainly formed in a relatively deep lacustrine environment, with localized short‐term shallowing driven by tectonic and hydrological fluctuations. Organic matter is dominated by Types II 1 –II 2 kerogen derived from mixed terrestrial and algal inputs, and the generally low‐TOC content reflects limited primary productivity. Overall, organic matter enrichment in the FMAF was controlled primarily by preservation under reducing, stratified water‐column conditions, whereas productivity increases played only a secondary role. These findings enhance the understanding of shale oil accumulation mechanisms in fault‐controlled lacustrine basins.