Petrography

About: Petrography is a research topic. Over the lifetime, 7449 publications have been published within this topic receiving 102018 citations.

Clay mineral-carbonate relations in sedimentary rocks

Abstract: Mineralogical investigation of modern sediments off the coast of Peru and Chile reveals the presence of volcanic glass in many samples. The glass is commonly devitrified in sea water to fine aggregates of minerals, including calcite and kaolinite. The textural relations of these two phases suggest attainment of chemical equilibrium, and thus a thermo-dynamic analysis of their relationship is desirable. A 7-component system, including the phases dolomite, calcite, chlorite, kaolinite, and quartz, is chosen for study. This system may be analyzed graphically if quartz is present in excess and H 2 O and CO 2 are taken as "mobile" components. The expected phase associations under a given set of conditions are studied in detail and agree with existing petrographic data; the agreement shows that the hypothesis of chemical equilibrium among these phases in sedimentary rocks should be seriously entertained. The idea of incompatible relationship between calcite and kaolinite is reviewed and the evidences for it are not compelling. Possible complications due to other clay minerals and other components are briefly considered. The most likely sources of trouble are montmorillonite and the mixed-layer clays. Depending on its composition, the montmorillonite may change the geometry of the phase diagrams but will not affect the calcite-kaolinite binary join. Whether the mixed-layer clays will change the diagrams depends on whether such clays behave as mono- or polyphase systems, about which nothing is as yet known.

Petrophysical study of faults in sandstone using petrographic image analysis and X-ray computerized tomography

Abstract: Petrographic image analysis (PIA) and X-ray computerized tomography (CT) provide local determinations of porosity in sandstone. We have investigated small faults called deformation bands in porous sandstones using these techniques. Because the petrophysical properties of the fault rock vary at a small scale (mm scale), the ability of PIA and CT to determine porosity in small volumes of rock and to map porosity distribution in two and three dimensions is crucial. This information is used to recognize the processes involved in fault development and the different kinds of microstructures associated with dilatancy and compaction. The petrophysical study of fault rock in sandstone permits one to make predictions of the hydraulic properties of a fault and thereby evaluate the sealing or fluid transmitting characteristics of faulted reservoirs and aquifers. The results of this study indicate that faulting in sandstone alters the original porosity and permeability of the host rock: the porosity is reduced by an order of magnitude and the permeability is reduced by one to more than seven orders of magnitude for faults associated with compaction.