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.
Construction of an intergranular volume compaction curve for evaluating and predicting compaction and porosity loss in rigid-grain sandstone reservoirs
To evaluate compaction as a factor in porosity evolution, a plot of intergranular volume vs. depth was constructed using data from relatively uncemented reservoir sandstones from a variety of depths, ages, and geographic locations. The resulting intergranular-volume-decline curve reveals that sands compact mechanically and intergranular volume declines rapidly, from about 40 to 42% at the surface, to about 28% at 1500 m. Between about 1500 and 2500 m, intergranular volume continues to decline slowly, until the framework stabilizes at around 26% (maximum potential porosity in the absence of cement or matrix). No further significant decrease in intergranular volume is observed to the depth limits of the data set at 6700 m. Comparison of intergranular volume and volume of quartz cement for different formations reveals no obvious balance between intergranular pressure solution (as monitored by intergranular volume) and quartz cementation. This indicates that grain-to-grain pressure solution and quartz cement precipitation do not proceed concomitantly on the thin-section scale. Moreover, grain compaction is limited (to about 26% intergranular volume) in rigid-grain sandstones, which suggests that the occurrence and distribution of deep porosity is a function of the volume of cement available to fill the intergranular pores. Therefore, deep, porous sandstones are relatively uncemented rather than undercompacted.
Methodological improvements for investigating silcrete formation: petrography, FT-IR and oxygen isotope ratio of silcrete quartz cement, Lake Eyre Basin (Australia)
The oxygen isotopic composition (δ18O) of the cement of a silcrete sample from the “Cordillo silcrete” (Late Eocene/Early Oligocene) of Lake Eyre Basin (Australia) was investigated. The massive structure of the silcrete outcrop and the absence of pedogenic features suggest that this silcrete formed by precipitation from groundwater. In order to avoid errors inherent in estimates of the cement δ18O value from isotopic analyses of bulk material and mass balance considerations, methodological improvements are described. The silcrete cement is physically separated from the detrital phases. Its purity is checked using cathodoluminescence (CL) spectroscopy. Optical microscopy, X-ray diffraction (XRD) analyses, and Fourier transform infrared (FT-IR) spectroscopy show the cement is made of three phases: (i) overgrowth; (ii) crypto-crystalline quartz; and (iii) micro-crystalline quartz. Thus, its δ18O value must be interpreted as that of a mixture, although the mixture consists mainly of crypto-crystalline quartz. As the amount of Si–OH-exchangeable oxygen in the cement would bias the results by an amount less than the analytical error (0.09‰), it is unnecessary to go through an equilibration procedure prior to isotopic analysis. The δ18O value of the cement sample is between 24.8‰ and 25.8‰ relative to V-SMOW. Assuming an Eocene/Oligocene temperature of 15–20 °C, in agreement with published regional temperature estimates, the inferred δ18O value of the quartz-forming solution was between −6.9‰ and −12.2‰. This is lower than the minimum δ18O value of −6.70‰ estimated for the modern groundwater near the sample site. A plausible paleoenvironmental scenario is that silcrete formed in a climate colder and wetter than the modern one, from shallow groundwater frequently recharged, in a setting of slow tectonic uplift.