Abstract:

With the meanings such as core observation, casting thin section identification and scanning electron microscope imaging, and based on the analysis of sandstone in light of petrological characteristics, diagenesis, pore types and dissolution fluid sources, the trap formation mechanism was investigated. The results show that the traps in the sandstone reservoir of the first member of the Cretaceous Xiagou formation (K₁g₁) in the Changshaling structural zone, Jiuquan basin, are diagenetic traps, the reservoir space is dominated by secondary pores, and the barrier is tight sandstone cemented by calcite in the early diagenetic stage. Atmospheric fresh water carrying smectite particles infiltrates through faults and dissolves the calcite cements and feldspar particles in tight sandstone, creating secondary intergranular pores and intragranular pores and clay-rich reservoir forms. The tight sandstone which is far from faults and has non-contact with atmospheric fresh water becomes the barrier. The traps are elongated and distributed along the fault trend, with the characteristics of “large sand bodies and small traps”. For the K₁g₁ sandstone reservoir, the diagenetic traps controlled by faults should be preferentially explored and wells should be deployed in the areas close to faults.

Key words: Jiuquan basin, Changshaling structural zone, Cretaceous, Xiagou formation, trap characteristic, formation mechanism, secondary pore, sandstone