Influence of multi -stage diagenesis on the rock physical properties of deep carbonate rock-reservoirs : a case study of 4^th Member of Sinian Dengying Formation in Sichuan Basin

The deep-ultra deep carbonate reservoir in China, commonly subjected to modification of multi-stage diagenesis, has extremely high heterogeneity. Therefore, it is difficult to accurately identify the relationship between the rock physics variation pattern and reservoir characteristics via a rock physics analysis that fails to sufficiently incorporate the geological setting. In this research, the rock physics properties of the deep dolomite of the 4th Member of the Sinian Dengying Formation in the Hechuan area are experimentally measured, and subsequently, the regularity of the rock physics characteristic variation is investigated within the framework of the diagenetic processes, via the integration of the analysis of the rock physics and petrologic characteristics, multi -scale pore structure characterization, and identification of sedimentary environments. The results show that the differentiated diagenesis of the dolomite in the target interval results in different pore structure characteristics and micro-texture characteristics of the rock. The high-energy microbial dolomite of the algal mound-grain beach facies is subjected to the contemporaneous microbial dolomitization and seepage-reflux dolomitization, penecontemporaneous selective dissolution, burial dolomitization, and hydrothermal dolomitization, successively. The grain dolomites formed are mainly compact "welded" and compact dolomite crystal contact boundaries, and the dissolution pore is extensive development with the micro-cracks and pore throat as seepage path. The low-energy siliceous, muddy, and limy dolomite of the inter-beach sea environment mainly experiences the weak capillary concentration dolomitization, intensive mechanical compaction-induced densification, and burial dolomitization. Such crystalline dolomite is observed with four types of contact boundaries, namely the compact "welding" contact, clay contact, quartz cementation, and calcite contact boundaries. Pores are mainly composed of residual primary inter-granular or crystalline pores, and micro-crack are the main fluid seepage path. The macroscopic seismic elastic properties of the samples are controlled jointly by the crystal boundary property and pore structure. The samples with the same crystal boundary condition have consistent correlations between the compressional- and shear- wave velocities, and between the compressional-wave velocity and the velocity ratio. The varied effects of soft pores like micro-cracks on the compressional- and shear- wave velocity causes considerable changes in the relationships between acoustic velocity and effective pressure, compressional- and shear- wave velocities, compressional -wave velocity and velocity ratio, porosity and acoustic velocity, and the intensity of pore-scale fluid-related dispersion. This research is an attempt to demonstrate a novel method for investigating the petrophysical variation of rock during the geological process, and the obtained findings can provide the petrophysical basis for seismic prediction of the characteristics of deep carbonate reservoirs.