Mechanisms and Main Controlling Factors of Shale Oil Occurrence Differences: A Case Study of the Lianggaoshan Formation, Sichuan Basin

Understanding shale oil occurrence differences and their controlling factors is essential for evaluating shale oil resource potential and predicting favorable oil-bearing lithofacies. The Lianggaoshan Formation shales in the Sichuan Basin, China, are characterized by diverse lithologies and strong reservoir heterogeneity, resulting in significant variations in shale oil occurrence modes, distribution, and content. Previous studies have mainly focused on oil-bearing differences among lithofacies, whereas fine-scale characterization of shale oil occurrence and its controlling mechanisms at the lamina-to-layer scale remains limited. In this study, shale oil occurrence characteristics of the Lianggaoshan Formation were systematically investigated using X-ray diffraction (XRD), X-ray fluorescence (XRF), rock pyrolysis, nitrogen adsorption before and after extraction, nuclear magnetic resonance (NMR), laser scanning confocal microscopy, scanning electron microscopy (SEM), and multiple temperature step rock pyrolysis, combined with light hydrocarbon recovery and micro-drilling sampling. Results show that the Lianggaoshan Formation shales exhibit good oil-bearing capacity from the nanometer to centimeter scale. Shale oil mainly occurs as oil films, with no nearly spherical oil droplets observed. Light oil is preferentially enriched in interparticle pores and bedding fractures, whereas heavier components are mainly retained within interparticle pores. Five shale lithofacies are identified: moderately organic laminar siliceous-rich shale (MLSS); moderately organic laminar clay-rich shale (MLCS); highly organic laminar clay-rich shale (HLCS); moderately organic laminar mixed-composition shale (MLMS); and moderately organic beded clay-rich shale (MBCS). Among them, HLCS exhibits the highest contents of both free and adsorbed oil, with oil occurring across the entire pore-size spectrum and the oil-bearing volume peaking at a pore diameter of approximately 100 nm. In contrast, the other lithofacies contain oil mainly within the 3–100 nm pore-size range, with the lower limit for free oil occurrence at 3 nm.At the millimeter scale, different lithofacies are composed of various combinations of pure clay layers, clay–siliceous/calcareous interbeds, pure siliceous layers, and calcareous laminae or Beded structures. Clay-dominated layers and clay–siliceous/calcareous assemblages contain significantly higher free oil contents than pure siliceous or calcareous laminar/Beded structures. HLCS consists of interbedded clay layers and clay–siliceous/calcareous units, in which siliceous and calcareous laminae provide higher-quality reservoir space, accounting for its elevated free oil content. Free oil abundance is mainly controlled by total organic carbon (TOC), porosity, and pore volume, and shows weak correlation with siliceous mineral content. These results provide a scientific basis for screening favorable shale oil lithofacies in the Lianggaoshan Formation.