Microscopic reservoir pores heterogeneity and its controlling factors of the Bashijiqike Formation in the Kuqa Depression

The microscopic pore systems in clastic reservoirs the margins of foreland basins are complex and heterogeneous, primarily controlled by the superimposition of burial diagenesis and tectonic diagenesis. These reservoirs have experienced not only vertical burial compaction but also intense lateral tectonic compression, accompanied by varying degrees of microfracture development and multiphase alteration by diverse diagenetic fluids. This study focuses on the Cretaceous Bashijiqike Formation in the Kushen area of the Kuqa Depression, Tarim Basin, which mainly consists of low-porosity and low-permeability to tight sandstone reservoirs deposited in a braided river delta environment. By systematically comparing burial depth, maximum paleo-stress, diagenesis, and microscopic pore characteristics across different tectonic positions, the study reveals how different reservoir-controlling factors combine to create different reservoir characteristics.

North to the Kelasu Fault, the reservoirs experienced relatively shallow burial (<4000 m) under strong tectonic stress. Diagenetic processes include compaction, calcite cementation, and meteoric water dissolution. Reservoir pores are dominated by primary pores with minor dissolution pores, accompanied by abundant microfractures. Between the Kelasu and Keshen Faults, reservoirs underwent deeper burial (6500-7000 m) under moderate tectonic stress. Diagenesis includes compaction, multi-type cementation (dolomite > quartz > albite > calcite > anhydrite), and multi-phase dissolution. Reservoir pores consist of mixed primary and dissolution pores, with dissolution pores exceeding primary pores in abundance, and local microfracture development. Between the Keshen and Baicheng Faults, reservoirs are characterized by ultra-deep burial (~7500 m) and low tectonic stress. Diagenesis includes compaction, multi-type cementation (dolomite > calcite > anhydrite > albite > quartz), and multi-phase dissolution. Reservoir pores are mainly primary pores with subordinate dissolution pores, and microfractures are rarely developed.

From north (foreland basin margin) to south (basin interior), increasing burial depth enhances vertical compaction, while decreasing tectonic stress reduces tectonic compaction and microfracture development. Concurrently, diagenetic fluids evolve from dominantly meteoric water in the north to multi-phase complex fluids including meteoric water, lagoon water, and organic acids in the south. These factors collectively control the diagenetic evolution and heterogeneity of microscopic reservoir pores in the study area.

Keywords: Tectonic stress; Diagenesis; Microscopic pore; Bashijiqike Formation; Kuqa Depression