Lamina combination mode and oil-bearing property of high-quality shale in the first member of Qingshankou Formation in Gulong Sag, Songliao Basin

Substantial advancements have been achieved in the exploration of Gulong shale oil in Songliao Basin. Pronounced disparities exist in oil-bearing capacity and productivity between the pure shale at the base of the Qingshankou Formation and the sandy interlayer at its top. However, the investigation of the microscopic characteristics underlying the oil-bearing differences in millimeter-scale assemblages of relatively high-yield lamina shale remains ambiguous, thereby fueling debates regarding the evaluation and genesis mechanisms of oil-bearing shale. This study integrates multiple analytical approaches, including conventional thin-section petrographic analysis, organic geochemical characterization, X-ray diffraction (XRD), argon ion polishing scanning electron microscopy observations, gas adsorption experiments, and thermal simulation experiments for hydrocarbon generation. The dark, organic-rich laminae and the bright, organic-poor laminae in the core of the Guye 3HC well section are analyzed to identify distinct lamina combination modes. The relationship between hydrocarbon generation, reservoir characteristics, and oil-bearing properties of these different lamina combinations during the hydrocarbon generation and storage processes is investigated. The results are obtained in four aspects. First, based on the total organic carbon (TOC) content and lamina types, the shale of the first member of Qingshankou Formation is categorized into two types: “organic-rich matter mixed” and “organic-poor matter felsic” binary lamina combination modes. Second, Thermal simulation experiments were performed on samples from both lamina combination types. During the hydrocarbon generation process, the organic-rich mixed lamina combination, which is characterized by a high TOC content, exhibited a higher oil-bearing evaluation index (S₁). This outcome of S₁ during the storage process was primarily influenced by clay minerals, specifically the illite-montmorillonite mixed layer. In contrast, the organic-poor matter felsic lamina combination mode demonstrated superior reservoir development potential. Third, the oil content reaches its peak when the vertical thickness ratio of organic-rich to organic-poor laminae in the Gulong shale is 2:1. At this ratio, the TOC content of the organic-rich laminae exceeds 2.15%, while the clay mineral component content exceeds 55%. This combination exhibits strong hydrocarbon generation potential but relatively poor reservoir capacity. Following hydrocarbon generation, the hydrocarbons migrate into the reservoir space provided by the organic-poor laminae. The “2:1 combination mode” represents the optimal source-to-reservoir ratio for developing “lamina-type” shale reservoirs. Fourth, the 2:1 combination model, defined at the millimeter scale, was identified by high gamma-ray (GR) values, TOC content exceeding 2.15%, and clay mineral content exceeding 55%. This model offers a novel approach for predicting sweet spots and efficiently developing shale oil from the Qingshankou Formation in the Gulong Sag.