Variation of Heteroatomic Compounds from the First Member of the Upper Cretaceous Qingshankou Formation in the Sanzhao Sag, Songliao Basin (NE China) using ESI FT-ICR MS and its Shale Oil Geological Significance

Abstract

Organic-rich mudstones and shales, which hold significant potential for shale oil resources, characterize the first member of the Upper Cretaceous Qingshankou Formation (K₂qn¹) in the Sanzhao sag of the Songliao Basin, NE China. Focusing on 30 core samples obtained from the first shale oil parameter well, named SYY3 in the study area, we systematically analyzed the composition and stratigraphic distribution of the K₂qn¹ heteroatomic compounds using electrospray ionization Fourier transform–ion cyclotron resonance mass spectrometry (ESI FT-ICR MS), to assess their geological relevance to shale oil. The findings indicate that in the negative ion mode, the heteroatomic compounds predominantly consist of N₁, N₁O₁–N₁O₈, O₁–O₈, O₁S₁–O₆S₁; contrastingly, in the positive ion mode, they are primarily composed of N₁–N₂, N₁O₁–N₁O₄, N₂O₁, O₁–O₄, O₁S₁–O₂S₁. Heteroatomic compound distributions vary significantly with depth in the negative ion mode, with minor variations in the positive ion mode. These distributions are categorized into three types based on the negative ion ratio ((N₁ + N₁O_x)/O_x): Type I (>1.5), Type II (0.8–1.5), and Type III (<0.8); types I and II generally exhibit a broader range of carbon numbers compared to Type III. The distribution of double bond equivalent (DBE) values across various sample types exhibits minimal variance, whereas that of carbon numbers shows substantial differences. Variations in heteroatomic compound compositions among the samples might have resulted from vertical sedimentary heterogeneity and differing biotic contributions. Type III samples show a decrease in total organic carbon (TOC) and free oil content (S₁) compared to types I and II, but an increased oil saturation index (OSI), indicating a lower content of free oil but a higher proportion of movable oil. The reduced content of N-containing compounds implies lower paleolake productivity during deposition, leading to a reduction in TOC and S₁. A lower TOC can enhance oil movability due to reduced oil adsorption, and the decreased presence of polar nitrogenous macromolecules with fewer high-C-number heteroatomic compounds further promote shale oil movability. Additionally, the negative ion ratios of N₁/N₁O₁ and O₂/O₁ exhibit positive and negative correlations with the values of TOC, S₁, and extractable organic matter (EOM), respectively, indicating that the salinity and redox conditions of the depositional water body are the primary controlling factors for both organic matter enrichment and shale oil accumulation.