Abstract:

The Middle Jurassic Xishanyao formation in Yongjin oilfield is generally 5 600 m to 6 400 m deep in the hinterland of the Junggar basin, where the reservoirs are deep and ultra-deep ones with low porosity and low permeability. In order to reveal the relationship between the process of reservoir densification and hydrocarbon accumulation, the reservoir characteristics, diagenesis, diagenetic evolution sequence and hydrocarbon accumulation history of the Middle Jurassic Xishanyao formation were analyzed through core observation, casting thin section identification, fluid inclusion and asphalt fluorescent color analysis. The results show that the Xishanyao formation has undergone three stages of diagenesis-hydrocarbon charging, and each stage of diagenesis was terminated with hydrocarbon charging. The oil charged in the first stage was oxidized in the process of tectonic uplifting, and a large amount of carbonaceous asphalt was generated and occupied reservoir pores. At the end of the second stage of diagenesis, large-scale hydrocarbon charging took place, resulting in overpressure reservoirs. Hydrocarbon charging obviously inhibited reservoir cementation and pore reduction, dissolved the carbonaceous asphalt in the early pores and increased the reservoir space. That’s the key process of hydrocarbon accumulation and improvement and maintenance of reservoir physical properties. Reservoirs formed in the stage is the primary exploration target in Yongjin oilfield. In the third stage of diagenesis, rapid burying and compaction further worsened the reservoir physical properties, so that only a small amount of highly mature oil and gas accumulated in the stage.

Key words: Junggar basin, Yongjin oilfield, Xishanyao formation, diagenesis, diagenetic evolution, hydrocarbon accumulation sequence