Molecular compositions of crude oils with different occurrence states in shale reservoirs using high-resolution mass spectrometry: Implications for expulsion, micro-migration and accumulation of shale oil

In this study, sequence extraction method was used to obtain the shale oils with different occurrence states, i.e., free oils, adsorbed oils and residual oils in different pores. The Orbitrap MS was used to characterize the molecular compositions in different shale oils. The results show decreased contents of O 2 , O 3 and O 4 species and increased contents of N 1 species from free oils to adsorbed oils to residual oils, indicating a relatively increased maturity. Here, a conceptual modeling is proposed to show the micro-migration and accumulation scenarios for shale oils. In the first stage, the generated shale oils are relatively low mature with high contents of O 2 +O 3 +O 4 species and low contents of N 1 species and trapped into the organic pores. In the second stage, the oils, with higher maturity and higher contents of N 1 species and lower contents of O 2 +O 3 +O 4 species, filled to the organic pores and displaced early low mature oils to adsorbed pores. In the last stage, the oils are high mature and characterized by highest contents of N 1 species and lowest contents of O 2 +O 3 +O 4 species. With the large generation of oils, these oils are residual in the organic pores in kerogens and drive the oils formed in the second stage to adsorbed pores in the edge of kerogen and mineral, which leads to the migration of low mature oils in the first stage to mineral matrix pores to form free oils. This scenario can well interpret the reason of various compositions and maturity from free oils to adsorbed oils to residual oils and show the micro-migrations of oils in various pore types in shale play. Therefore, the studied results provide an insight for shale oils migration and accumulation from a molecular perspective.