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.
Sonic velocity and grain contact properties in reservoir sandstones
The purpose of this study was to examine the effect of grain contacts and quartz cementation with regard to acoustic wave propagation in sandstones. Grain contacts have been considered essential when relating acoustic velocities to physical rock properties, and a parameter numerically representing the contact area between individual grains (contact length) has been measured. The method used involves digital petrographic microscopy pictures analysed by image analysis software. Other parameters, such as grain size, number of grain-to-grain contacts, cracks, clay content and porosity, have also been closely examined. The results showed that the contact area between sand grains may be useful for explaining velocity variations during the initial stages of quartz cementation and grain framework stiffening. Continued increase in grain-contact area by chemical compaction will have less influence on the sonic velocity compared to variations in porosity. The Garn Fm. from the Norne Field (2.6–2.7 km burial depth), which is in the early phase of initial grain framework stiffening by quartz cementation, and the Garn Fm. from the Kristin Field (4.6–4.7 km burial depth), which is thoroughly quartz cemented, were chosen as sample materials. Log-derived velocities, in addition to some laboratory ultrasonic velocity measurements, were used in this study.
New approach to surface seawater palaeotemperatures using 18O/16O ratios in silica of diatom frustules
THE oxygen isotopic composition of calcium carbonate and silica deposited by organisms in water depends on the temperature and the isotopic composition of the water1.