Tectonic Factors Restricting Differences among Layers in Microstructure of Ultradeep Clastic Rock Reservoir: A Case Study on Cretaceous Bashijiqike Formation in Kuqa Depression

Abstract

Abstract The Cretaceous Bashijiqike Formation in the Kelasu structural belt of Kuqa Depression is an ultradeep reservoir with burial depth of more than 6 km. Due to the influence of strong thrust nappe from the South Tianshan in the north, the burial depth gradually deepened from north to south in the Kelasu tectonic belt. With the increase of the burial depth, the influence of tectonic compression relatively weakens, and the formation mechanism that affects the microstructure of ultradeep clastic reservoir is changed. Comprehensive analysis of thin section and quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), scanning electron microscope (SEM), and micro-CT scanning are used to investigate the characteristics of the Cretaceous Bashijiqike Formation at the depth of 8000 m (TVD) in various stress sections. The result shows that, for the ultradeep reservoir with burial depth up to 8000 m, its reservoir characteristics and interlayer differences are not completely restricted by the neutral surface effect. (1) The porosity and permeability of various stress sections are different, and the reservoir physical properties of the tensile section are higher than that in the compressive section. (2) The pore types, pore radius, and pore throat connectivity of various stress sections are different, and the development of pores and throats in the tensile section is better than that in the compressive section. (3) As an ultradeep clastic rock reservoir, the vertical fracture development characteristics of Well Bozi9 at the depth of 8000 m differ from that in Kela-Keshen area at the depth of 6000 m in the model of strain neutral surface. The interlayer difference in the microstructure of ultradeep clastic rock reservoir is clarified, which has a positive effect to evaluate and predict the distribution of favorable deep reservoir.