An Improved Lacustrine Heterogeneous Shale Oil Sweet Spot Classification Evaluation Criterion Based on Micromigration Methods

Shale oil represents a significant unconventional hydrocarbon resource. The heterogeneity of lacustrine shale complicates the identification of sweet spots characterized by high mobility and enrichment. Utilizing total organic carbon (TOC), pyrolysis data, and light hydrocarbon calibration from the Shahejie Formation in the Dongpu Depression in China, we propose an improved lacustrine heterogeneous shale oil sweet spot classification evaluation criterion. This criterion considers the variation in oil content, the oil movable threshold, and micromigration effects. The detailed criterion establishment process is as follows: (1) The hydrocarbon expulsion threshold method is employed to determine the oil mobility threshold (OSI = 82 mg/g of TOC). (2) A cross-plot of TOC vs S1C is conducted to categorize shale oil into 4 resource types: 1) micromigration-enriched resource, 2) efficient resource; 3) inefficient resource, and 4) invalid resource. (3) The δQ proxy is introduced to further classify micromigrated shale oil resources into three categories: I) micromigration-enriched resource; II) retained hydrocarbon-enriched resource; and III) invalid resource. Based on the improved criterion, the well Wei 457 HF was divided into 5 Level I and 3 Level II sweet spots. Micromigration is a key factor controlling the differential enrichment of shale oil. High hydrocarbon generation potential and favorable permeability are essential for shale oil enrichment, while hydrocarbon pressurization is the charge force for oil micromigration. The sealing capacity ultimately determines the preservation of the enriched shale oil. Comparative analysis across various lacustrine shale types confirms the applicability of the improved criterion in different depressions or basins. This study offers valuable insights for accurate prediction of shale oil sweet spots.