Distribution Law and Main Controlling Factors of Illite/Smectite Mixed Layer in Upper Wuerhe Formation, Northwestern Mahu Sag, Junggar Basin, Northwest China

Abstract

Exploring reservoir stratum sensitivity is an important basic work for oil and gas exploration and exploitation, in which the degree of water sensitivity damage is often affected by the clay mineral content and microscopic pore throat structure of illite/montmorillonite mixed clay minerals. To reveal the distribution law and the main controlling factors of illite/montmorillonite mixed layers, the diagenesis, regional distribution law, and main controlling factors of illite/montmorillonite mixed layers in the reservoir stratum of the Upper Wuerhe Formation in the study area were analyzed by high-resolution testing techniques such as scanning electron microscopy (SEM) and Advanced Mineral Identification and Characterization System. Diagenesis was the main cause of illite/montmorillonite mixed layers in the study area, and the occurrence state under SEM was mostly flaky, silky, and flocculent filling in intragranular dissolved pores or intergranular pores. The mass fraction of illite/montmorillonite mixed layers increased with the increase in depth in the longitudinal direction, which was closely related to the sedimentary microfacies and the hydrodynamic environment in the plane. Results show that potash feldspar is the material basis for the formation of illite/montmorillonite mixed layers, and the strength of the hydrodynamic environment is the main controlling factor for the quality distribution characteristics of such mixed layers. This study provides a new technical method and research idea for understanding the reservoir sensitivity characteristics, which has good guiding significance for the blocks with strong reservoir sensitivity. The original pore structure of the reservoir can also be protected against destruction by various sensitivity factors in the developmental stage caused by the unclear understanding of the spatial distribution characteristics of clay minerals affecting the yield in oil and gas field development.