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.
Insight into the Pore Structure of Tight Sandstones Using NMR and HPMI Measurements
Laboratory measurements including porosity, permeability, high-pressure mercury intrusion (HPMI), nuclear magnetic resonance (NMR) measurements, and microscopic analysis of thin sections and scanning electron microscopy (SEM) were performed to provide insights into the microscopic pore structure of the Xujiahe Formation tight sandstones in Sichuan Basin. The relationships between microscopic pore structure parameters, such as pore geometry, pore size distribution, pore network, and macroscopic consequences, such as permeability, reservoir quality index, Swanson parameter, fractal dimension as well as NMR parameters, have been investigated. The results show that the pore systems are dominated by secondary dissolution porosity with minor amounts of primary porosity and microfracture. NMR T2 pore size distributions are either uni- or multimodal. Long T2 components are not frequently present due to the lack of the macropores, whereas as shorter T2 components dominate the T2 spectrum. T2gm (the geometric mean ...
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.