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.
Microcracking and Healing in Granites: New Evidence from Cathodoluminescence
Quartz grains in granitic rocks usually have blue cathodoluminescence (CL). Within the blue-luminescing grains, there are often red-luminescing domains which are frequently impossible to detect without CL contrast. This finding suggests that the red-luminescing quartz is sealing preexisting mnicrocracks. The presence of these now-healed microcracks has important implications with respect to the role of pore fluid pressure and fluid transfer in metamorphism, the origih of granites, longperiod crustal deformation, earthquake mechanics, physical properties of rocks, and deep-seated geothermal energy.
History of burial diagenesis determined from isotopic geochemistry, Frio Formation, Brazoria County, Texas
Progressive burial diagenesis of the Oligocene Frio Formation in Brazoria County, Texas, has resulted in extensive reaction between pore fluids and sediment in a major shift toward water/rock equilibrium. Carbon and oxygen isotopic data, combined with fluid isotopic data from the literature, indicate that quartz cement formed at 75 to 80°C and kaolinite at approximately 100°C. The zone of most rapid albitization is near 150°C. Authigenic carbonates formed over a wide range of temperatures, and those within the peak zone of hydrocarbon generation are depleted in 13C. At depths shallower than approximately 2,600 m, quartz and carbonate cementation in primary intergranular pore spaces (passive diagenesis) dominated. Below 2,600 m, within the geopres ured zone, reaction of detrital components (active diagenesis) is the major process. Organic maturation, albitization, and the transition of smectite to illite are the processes that contribute most of the components required for precipitation of cements. Quartz cementation occurred quite early in the burial history of the Frio (beginning at approximately 1,500 m of burial), when rates of fluid expulsion were at a maximum and when little of the Frio sandstone section had reached the zone of albitization.