Hydrocarbon micro-migration and differential enrichment mechanism of shale oil in upper Cretaceous Qingshankou Formation in Songliao Basin

Shale reservoir has strong heterogeneity in mineral composition and oil content even at a short distance in the same interval. To better understand the accumulation mechanism of shale oil and hydrocarbon migration tendency in the interval, and explore the main influencing factors of distribution. This paper employed various methods, including thin-section observation, TOC (total organic matter (OM)) analysis, rock pyrolysis, Soxhlet extraction, group component separation, and GC–MS (gas chromatography mass spectrometry) analysis of saturated hydrocarbons. The shale oil samples were collected from an exploratory well in the Songliao Basin, and the distribution and enrichment characteristics of shale oil interlayers were analysed. The results show that the first member of Qingshankou Formation (K₂qn¹) could be divided into six small layers. From Q1 to Q6, the lithofacies could be divided into three types: high-frequency laminar shale, massive shale, and bioclastic shale. The lacustrine matrix shale of K₂qn¹ in Songliao Basin has medium TOC, and high oil contents, OM conversion rate. The TOC ranges from 1.5 to 4.2 wt%, S₁ varies between 1.4 and 4 mg/g, S₂ falls within the range of 4 to 10.6 mg/g, Tmax ranges from 351 to 497°C, and the OSI varies between 57 and 115.34 mg HC/g rock. The lower part of the K₂qn¹, Q1–Q4, characterized by high-frequency laminar shale, contained residual shale oil, while the upper part of the K₂qn¹, Q5 and Q6, characterized by bioclastic shale and massive mudstone, and are dominated by in-situ type shale oil. The crude oil in K₂qn¹ is homologous, and primarily undergoes micro-migration. The Q5–Q6 section has the highest light oil content, while other regions with notable light components are located in the middle of Q1, the upper sections of Q2, Q3, and Q4.Q1–Q4 are primarily composed of residual shale oil, whereas Q5–Q6 is primarily composed of in-situ shale oil, and the primary micro-migration direction of shale oil occurs laterally within Q1–Q4. The lateral migration of each section mainly occurs at the upper part of Q3 and Q4, the lower part of Q1 and Q2, the bottom of Q5, and the interface between Q6 and the second member of Qingshankou formation (K₂qn²). The block mudstone retains in-situ shale oil, especially light hydrocarbons. Within the high-frequency laminar shale, there is strong horizontal connectivity, and lateral hydrocarbon expulsion serves as the primary mechanism for micro-migration in Q1–Q4. This paper can provide reference values for K₂qn¹ Formation shale oil and the migration and differential reservoir formation of medium and high-maturity matrix shale oil.