Reservoir characteristics and controlling factors of physical properties of Jurassic Toutunhe Formation in the eastern segment of the southern margin of Junggar Basin

The lower assemblage (from Jurassic to Cretaceous) on the southern margin of Junggar Basin is rich in oil and gas resources, and the Jurassic Toutunhe Formation in the eastern segment is an important horizon for reservoir development. However, no systematic study has been accomplished for its reservoir characteristics and controlling factors of physical properties. Based on the drilling and logging, microscopic observation of thin section, scanning electron microscopic observation, physical property and mercury injection test, vitrinite reflectance (R_o%) and other data including coring wells, combined with regional geological background and stratigraphic conditions, it was analyzed in this paper for the basic characteristics, diagenetic evolution and controlling factors of physical properties of Toutunhe Formation. The sedimentary hydrodynamic conditions of the Toutunhe Formation in the eastern segment of the southern Junggar Basin are relatively stronger, which makes the content of matrix inside the reservoirs relatively lower. It has the petrologic characteristics of low compositional maturity, high plastic debris content and medium structural maturity. The reservoir space is dominated by inter-granular pores, with an average porosity of 11.6% and an average permeability of 5.7×10^-3 μm². The whole reservoir belongs to low-porosity and low-permeability or medium-porosity and medium-permeability reservoir. Medium and large pores and coarse throats are dominant, relatively well sorted. Reservoir diagenesis is dominated by compaction, cementation and dissolution, but the content of cement is low. The dissolution effect is mainly acidic dissolution. The overall diagenesis degree of the reservoir is low, mainly in the early diagenetic stage A-B with some in the middle diagenetic stage A. Compaction effect, including both structural and burial ones, lead to the decrease of porosity in the Toutunhe Formation. In addition, the cementation of carbonate and clay minerals further reduces the physical properties of the reservoir. Although the effect of dissolution for porosity enhancement is relatively limited, the reservoir has begun to enter the middle diagenetic stage, the effect of formation acid fluid will strengthen in the deep layer, which will increase dissolution porosity. Due to the slow burial in the early stage, rapid deep burial and rapid uplift in the later stage, combined with the occurrence of formation overpressure and the continuous reduction of geothermal gradient, the physical properties of the Toutunhe Formation have been effectively preserved.