Brittle Deformation in Sandstone Diagenesis as Revealed by Scanned Cathodoluminescence Imaging with Application to Characterization of Fractured Reservoirs

Abstract

The stable observing conditions, high magnifications, and sensitive light detection that are characteristic of scanning electron microscope-based cathodolu-minescence (scanned-CL) imaging overcome several of the disadvantages of conventional light microscope-based CL systems, allowing more routine application of this petrographic method for description of micron-scale textural relationships between detrital grains, cements, and fractures in sandstones. Scanned-CL imaging has great utility for documenting the interrelation between deformation and diagenesis at the micrometer scale in siliciclastic rocks. A survey of sandstone units of widely varying age, location, and burial history suggests that quartz-lined and quartz-filled microfractures are nearly ubiquitous in lithified quartzose sandstones. Because fractures formed in association with quartz precipitation are prevalent in quartz-cemented siliciclastic hydrocarbon reservoir rocks, scanned-CL imaging of microfractures can potentially yield important information on subsurface fracture populations that have economic significance. 2 Methods Images described here were produced using Oxford Instrument's photomulti-plier CL detectors CL302 and P2 installed on JEOL T330A and T300 SEMs. Light is collected with the parabolic mirror inserted about 1 mm above an epoxy-impregnated, carbon-coated, polished thin section. Panchromatic images are observed on the CRT of the SEM and recorded on Polaroid film. An accelerating voltage of 10 kV with sample current set near 90% of the maximum for the SEM provides adequate photon emission for examining the luminescence variations in authigenic (relatively dark luminescent) and detrital quartz (relatively bright luminescent). Many of the CL photomicrographs in our studies are taken after several minutes of beam exposure, thus taking advantage of the beam-induced enhancement of CL emission in quartz to achieve higher contrast in the image. Because this chapter reviews interpretations of deformation history as 9 M. Pagel et al. (eds.), Cathodoluminescence in Geosciences