Evolution mechanism of diagenesis of deep sag area in the central Fault basin—A case study of the Wuerhe Formation of the Dongdaohaizi sag in the Junggar basin, Northwest China

Abstract The Junggar basin is a typical terrestrial fault basin. The stratum in the central area is deeply buried and its sedimentary and rock types are diverse. Its diagenesis is complex and the evolution law of reservoir pores is challenging to characterize, which is not conducive to oil and gas exploration in this region. In this paper, the Dongdaohaizi sag in the central deep basin was taken as an example to be detailed on its sedimentary facies, diagenesis, and micropore structure, revealing the diagenesis sequence and pore evolution law of the deep buried reservoir. First, the sedimentary facies type of the focus stratum (Wuerhe Formation) is assigned to the fan delta, its source is from the northwest, and the rock type is conglomerate, pebbly sandstone, sandstone, and siltstone. The debris content is high among the clastic grains and can reach 40%, showing near-source transport characteristics. Second, the primary diagenesis types and evolution sequence of the Wuerhe Formation is cleared according to experiment data, and the influence degree of the compassion, cementation, and dissolution of the target reservoir rock is clarified. Finally, the sandstone pore evolution model is established, the diagenesis facies is classified, the contribution degree of the unstable clasts and cemented minerals to the secondary pore is determined, and the comprehensive factors with the greatest impact on the late densification of the reservoir are clarified. The diagenesis and pore evolution of the deep rocks in the deep central sag are revealed.