Shale oil migration across multiple scales: A review of characterization methods and different patterns

Oil migration in shale formations across multiple scales from nanometers to meters, a topic without receiving sufficient attention, is of great significance to the accumulation and production of shale oil. Compared with oil migration in conventional reservoirs usually with a long distance, shale oil migration is much shorter, which challenges the applicability of conventional characterization methods for shale formations. In addition, shale oil migration in different types of shale formations occurs at different scales, and their source-reservoir configuration, accumulation mechanism, and exploration plans are also different, greatly complicating a cost-effective and sustainable exploitation of shale oil resources. Accordingly, this review summarizes the experimental methods for characterizing the shale oil migration across multiple scales, and reveals the migration characteristics in three different types of shale oil formations of typical basins in China and USA. The shale oil migration distance in "pure shale-type" formation is the shortest and mainly in nanometer-microns scale. The migration characteristics of "pure shale-type" shale oil are difficult to reveal directly due to its ultra-short migration distance, which may be indicated by the microscale compositional changes at the surface of minerals being interacted with organic fluids and requires additional innovations on the shale oil migration characterization methods with a high accuracy. The shale oil migration distance in laminated-type shale oil formation is at the lamina (micron-millimeter) scale and the shale oil migrates from the organic matter-rich to -lean laminae. The lamina type (felsic, carbonate, or clayey) significantly affects the migration and enrichment of shale oil by providing different types of pore spaces and migration pathways. The shale oil "composition change" method effectively indicates the migration characteristics of shale oil among different laminae, while the "hydrocarbon expulsion efficiency" method is not accurate for the laminated-type shale oil. The migration distance of sandwich-type shale oil is at the millimeter-meter scale, which is the largest among three types of shale oil formations. The reservoir space is mainly intergranular pores and the shale oil migrates from the overlying or underlying organic matter-rich shale to the siltstone interlayer. Both "hydrocarbon expulsion efficiency" and "composition change" methods can be used to clarify the migration characteristics of sandwich-type shale oil. This review overall has a great significance for deeply understanding the characterization methods and migration characteristics of different types of shale oil formations at multiple scales.