Graded Evaluation and Controls on In Situ Oil Content within Lacustrine Shale of the Upper Cretaceous Qingshankou Formation, Songliao Basin, China

A pyrolysis parameter S1 is widely used to evaluate the oiliness of rocks. When the S1 (free hydrocarbon) is used to evaluate shale oil resources, there is often a phenomenon of light hydrocarbon loss and heavy hydrocarbon loss, resulting in underestimation of the amount of resources. To accurately estimate the oil content of shale, this study establishes a light hydrocarbon recovery coefficient curve based on routine rock pyrolysis and related geochemical tests, and calculates a heavy hydrocarbon correction coefficient. This study have ascertained the in situ oil content of the Qingshankou Formation shale in the northern part of the Songliao Basin, identified the controlling factors of in situ oil content, and quantitatively assessed the mobility and movable resource volume of shale oil. The research reveals that light hydrocarbon loss ranges from 0.01 to 8.69 mg HC/g Rock, heavy hydrocarbon loss varies between 0.04 and 4.05 mg HC/g Rock, and in situ oil content spans from 0.04 to 16.38 mg HC/g Rock. The latter accounts for roughly 3.9 to 6.3 times the measured S1, indicating that the district is mainly enriched and moderately riched with resources. Organic matter abundance, thermal evolution degree, and reservoir structure are the principal factors controlling the in situ oil content of shale. Movable oil content ranges from 0.02 to 11.30 mg HC/g Rock, and the movable resource quantity approximates 84.84 × 108 t, constituting about 59.45% of the total resources, suggesting large potential and high mobility. The results of this study can provide theoretical and practical guidance for shale oil resource evaluation and sweet spot optimization in the regional area.