Multiscale Characterization and Mobility Evaluation of Storage Space in Laminated Shale Reservoirs: A Case Study of the Qingshankou Formation, Southern Songliao Basin.

Abstract

Mobility evaluation is critical for shale-oil exploration and development. To clarify the occurrence space and fluidity characteristics of laminated shale oil in the first member of the Qingshankou Formation (K₂qn¹) within the Changling Depression, southern Songliao Basin, we used analyses including total organic carbon (TOC), multitemperature-programmed pyrolysis, X-ray diffraction (XRD), thin-section petrography, fluorescence microscopy, scanning electron microscopy (SEM), nitrogen adsorption, high-pressure mercury intrusion (HPMI), and nuclear magnetic resonance (NMR). We performed qualitative and quantitative characterization of oil content, oil mobility, and controlling factors in laminated shale. The laminated shale contains primary intergranular, dissolution, organic, and intercrystalline pores; among these, dissolution pores and bedding-parallel fractures constitute the main occurrence space for shale oil, with pore diameters typically <100 nm. Four types of laminae: clay-rich, feldspar-rich, carbonate-rich, and siliceousare developed throughout the section. The laminated structure facilitates the expulsion of organic acids and thereby improves reservoir quality. 2D NMR effectively characterizes shale-oil content: total oil measured by NMR is 1.52 times that obtained from programmed pyrolysis, while free-oil and adsorbed-oil contents are 2.01 times and 1.19 times, respectively. The mean movable-oil content from 2D NMR is 0.85 mg/g (range: 0.12-2.99 mg/g), and the average movable-oil proportion is 20.73% (range: 2.28-54.26%). Petrophysical properties, organic-matter abundance, mineralogy, microscopic pore structure, and pore-scale heterogeneity jointly control shale-oil mobility. Specifically, higher TOC corresponds to a lower movable-oil proportion, whereas better reservoir quality, larger pore sizes, greater proportions of macropores and brittle minerals, lower clay-mineral content, and a smaller fractal dimension (D₂) are associated with enhanced mobility. These results provide a theoretical basis for refining sweet-spot evaluation of continental shale oil and for guiding the exploration and development of laminated shale oil.