Modal distribution of pore-throat size in sandy conglomerates from an alluvial fan environment: Lower Karamay Formation, Junggar Basin, West China

Abstract Alluvial fan exhibits various modal distribution of pore-throat size, which effectively control the oil displacement efficiency and fluid movement. However, the genetic mechanism of the modal distribution of the pore-throat size and its macroscopic distribution within alluvial fan is poorly understood. Based on core analysis, this paper, which take the sandy conglomerate reservoir within the alluvial fan in Junggar Basin as an example, aims to study the variations in modal distribution of pore-throat size and controlling mechanisms. The results reveal that (1) The modal distribution types of the pore-throat size are complicated, and can be classified into six subtypes, including unimodal type Ⅰ (Uni-Ⅰ), unimodal type Ⅱ (Uni-Ⅱ), unimodal type Ⅲ (Uni-Ⅲ), bimodal type Ⅰ (Bi-Ⅰ), bimodal type Ⅱ (Bi-Ⅱ) and multimodal type (Multi). Uni-Ⅰ, Uni-Ⅱ, and Uni-Ⅲ is dominated by intergranular pores, intragranular dissolved pores and micropores, respectively. Bi-Ⅰ, Bi-Ⅱ and Multi are characterized by the combination of several pore types. (2) Diagenesis, including compaction, cementation and dissolution, rock composition (the mudstone fragment content) and rock texture (the clay content) control the volume of each pore-throat type, which significantly influences the modal distribution of the pore-throat size. Lithofacies with low mudstone fragment and clay content and great grain sorting tend to have experienced weak compaction, thereby retaining a large volume of intergranular pores, and have been subjected to strong dissolution when adequate acidic ions occur in the porewater, resulting in a large volume of intragranular dissolved pores. Strong dissolution reduced the rock compressive strength, and further enhanced the degree of mechanical compaction during the burial process. Cementation occurred later than compaction and dissolution, and its intensity is controlled by the supply of CO2 and Ca2+ and compaction intensity, thus reducing the volume of intergranular pores. The content of mudstone fragment and clay varies among different lithofacies of different origins. For traction current deposits, lithofacies with smaller median grain size contain fewer mudstone fragments, resulting in fewer micropores. The clay content decreased first and then increased with decreasing median grain size, and lithofacies with lower clay content cause less volume of micropores. (3) Lithofacies types, combined with porewater condition for dissolution and lithofacies location, affect the diagenetic intensity, rock composition and rock texture variations, thus resulting in various modal distribution of pore-throat size within the alluvial fan. Moreover, the rock texture, especially grain sorting, is very important for compaction, and facilitates percolation of meteoric water and dissolution.