The characteristics of unconformity surface at the bottom of the Paleogene and its significance in hydrocarbon migration in the Sikeshu Sag of the Junggar Basin, Northwest China

The unconformity surface at the bottom of the Paleogene, located in the Sikeshu Sag of the Junggar Basin, Northwest China, is one of the most important hydrocarbon migration pathways and characterized by 3 layers of upper coarse clastic rock, lower weathering crust and leached zone. The upper coarse clastic rock displays features of higher density, lower SDT and gamma-ray logging while the weathering crust in the lower part displays opposite features. The formation water is of NaHCO₃ type but at lower mineralization degree. The QGF indices are generally between 2.19 and 3.77 and the GOI parameters vary from 1% to 5%. From the southeast to the northwest of the sag, the content of saturated hydrocarbon increases from 30.81% to 53.74% while that of non-hydrocarbon and asphaltene decreases. The Pr/nC₁₇ decreases from 0.65 to 0.47 while the Ph/nC₁₈ decreases from 0.66 to 0.27, and the content of benzoc] carbazole declines while the benzoa] carbazole amount and (alkyl carbazole)/(alkyl+benzo carbazole) ratio both increase. These revealed that the hydrocarbons migrated from the sag to the ramp region along the unconformity surface.