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Influences of Temperature and Pressure on Diagenesis - Implications from Forward Physical Modeling

Abstract

Diagenesis study is critical to reservoir quality evaluation and prediction. As known to all, diagenesis is closely related to mineralogy, temperature (T), pressure (P), fluid and time. Given the lithology, T & P are the most important geological factors on diagenesis. However, previous works paid more attention to the relationship between diagenesis and reservoir quality than the quantitative evaluation of diagenesis response to T & P, which can not provide complete understanding of diagenesis. Therefore, investigation and discussion of the effects of key geological factors on diagenesis are necessary. Siliceous artificial sands are chosen as object of this study. The experiment equipment is diagenetic physical modeling system designed by CNPC Key Laboratory of O & G Reservoirs. The system is composed of six sample holders in which different T & P can be set. Two series with 12 samples are obtained. The first six samples are set at 300 centigrade degrees and different pressures from 55 MPa to 275 MPa. In the second series, three samples are set at 300 centigrade degrees and the other three samples are set at 400 centigrade degrees. The pressures are also from 55 MPa to 275 MPa. Subsequently, thin section, SEM, LSCM, CT, XRD and physical properties are analyzed on all the samples. The results indicate that the compaction of siliceous artificial sandstones increases with pressure and temperature. Temperature has the most significant effect on the crystallinity of authigenic minerals(quartz). In the modeling, the transformation from amorphous silica to crystalline quartz occurs under 400 centigrade degrees. Within a certain range of temperatures, there is a parabolic correlation between mineral crystallization and pressure. Best pressure for siliceous crystallization is around 137.5MPa. The pressure contributes to diagenesis in two ways, compaction and thermal effect, of which the former is more significant. During the process of diagenesis, longer reaction time may compensate lower T & P. Reaction time has more impact on chemical diagenesis than physical compaction. Response characteristics of diagenesis to T, P and time are quantitatively studied by forward physical modeling in this research. The relative results are of great reference value for diagenesis numerical simulation, quantitative study, and further research in mechanisms of diagenesis.