Systematic diagenetic changes in the grain-scale morphology and permeability of a quartz-cemented quartz arenite
The material properties of sedimentary rocks are controlled by a range of parameters, including grain size, sorting, and modification of the original sediment through the diagenetic processes of compaction and cementation. To isolate the effects of diagenesis and explore how they modify permeability, we quantified changes in grain and pore morphology accompanying progressive diagenesis of a simple system: a well-sorted, variably quartz-cemented quartz arenite of relatively uniform grain size. The most common type of authigenic cement in sandstones, quartz overgrowths, is responsible for significant porosity and permeability reduction. The distribution of overgrowths is controlled by available pore space and the crystallographic orientations of individual quartz grains. We show that progressive quartz cementation modifies the grain framework in consistent, predictable ways. Detailed microstructural characterization and multiple regression analyses demonstrate that both the number and length of grain contacts increase as the number of pores increases and the number of large well-connected pores decreases with progressive diagenesis. The aforementioned changes progressively alter pore shape and reduce pore-size variability and bulk permeability. These systematic variations in the pore network correlate with changes in permeability, such that we can use our data to calibrate the Kozeny-Carmen relation, demonstrating that it is possible to refine predictions of permeability based on knowledge of the sedimentary system.
A Combined Experimental and Numerical Approach to Understanding Quartz Cementation in Sandstones
John Farver, Adviser Charles Onasch, Co-Adviser Quartz cement formed during diagenesis is the principle cause of porosity and permeability reduction in sandstones and therefore greatly affects reservoir quality. Predictive quartz cement models have provided a basic ability to estimate reservoir quality for quartz-rich rocks under ideal diagenetic conditions. However, examination of more complex, specific diagenetic environments is required to improve the accuracy of predictive models applied to unconventional hydrocarbon reservoirs. Our experimental and numerical approach to this problem has examined quartz cementation processes by: 1) assessing the feasibility of relatively low temperature hydrothermal flow-through experiments for investigating silica dissolution, transport, and precipitation processes during diagenesis in active fluid flow environments, and 2) to assessing the effect of grain size and sorting on the rate of quartz cementation and corresponding pore space evolution in sandstones under static hydrothermal conditions. Hydrothermal flow-through results indicate that small amounts of new quartz cement can be generated in less than 10 days at relatively low temperatures when compared to previous experimental work. Results obtained from static hydrothermal experiments indicate that grain size and sorting effect both the rate of quartz cementation in our experiments and corresponding pore space evolution. The surface area normalized rate is observed to decrease more rapidly in fine grain size experiments when compared to coarse grain size experiments. Additionally, pore space analysis of both experimental and modeled results indicates that while all grain size fractions rapidly converge on similar total porosity values the 2D connectivity of porosity varies markedly as a function of grain size and sorting.
Regional strategies for the minerals industries of the states of the SADCC
The SADCC has been in existence for eleven years but little progress has been made in regional coordination of the minerals sector even though this sector is the most important exporter of the region. There is however scope for minerals development at the regional level.