Archie's cementation factors for natural rocks; Measurements and insights from diagenetic perspectives

Yuan, X.*; Hu, Q. H.*; Fang, X.*; Wang, Q. M.*; Ma, Y.*; Tachi, Yukio  

Archie's cementation factor, m, is a critical parameter for petrophysical studies, and the value is influenced by several factors such as the shape, type, and size of grains, degrees of diagenesis, and associated pore structure. Using integrated experimental and theoretical approaches, the goal of this study is to obtain the cementation factor of rocks (both reservoir rock and caprock) and assess the impact of diagenesis processes on the values of the cementation factor. Thirteen samples of geologically diverse rocks (six mudstones, four fossiliferous limestones, two marbles, and one sandstone) were selected to achieve these research objectives. Two approaches, the diffusion of gas tracers and the Bosanquet formula calculation using pore-throat sizes from mercury intrusion porosimetry analyses, were used to derive the cementation factors of these rock samples. These rocks were categorized into two groups based on the correlation between average pore-throat diameter and diffusivity, and an exponential-law relationship between the cementation factor and porosity was determined for these sample groups. In addition, thin-section petrography and field emission-scanning electron microscopy observations were utilized to investigate diagenetic processes, with four diagenetic patterns being established: (1) strong compaction, strong cementation, and weak dissolution-diagenesis pattern; (2) weak compaction, medium cementation, and weak dissolution-diagenesis pattern; (3) weak compaction, medium cementation, and strong dissolution-diagenesis pattern; and (4) fracture-matrix pattern. The results indicated that diagenetic processes and microfractures contribute to the variability in the cementation factors in these rock samples.