1 INTRODUCTION
In quartzose rocks accessed only through small-diameter wellbores, analysis of quartz-lined microfractures (size: microns to millimeters) and textural relations among fractures and authigenic cement can be used to estimate attributes of subsurface fracture populations. Key to microstructure characterization in such rocks is photomultiplier-based electron beam-induced luminescence (scanned CL) imaging, which detects microfractures that are mostly not visible using standard petrographic techniques. Because microfractures are widespread and small samples can be used to get accurate fracture data, scanned CL observations of rocks that do not contain large (i.e., visible to unaided eye) fractures can be used to determine the timing of fracturing events relative to authigenesis, infer orientations of large fractures, and in concert with conventional paragenetic analysis estimate the degree to which fracture networks have been filled with authigenic cements.
Despite improvements in detection and characterization of large fractures with geophysical logging tools, subsurface fracture properties are often conjectural because large fractures rarely intersect wellbores where they can be observed. In shallowly dipping sedimentary rocks penetrated by petroleum exploration and development wells, the lack of adequate sampling results from the vertical attitude of most wells and fractures and the wide (~1 m) spacing of large fractures. Consequently, in exploration and development it is common that many fracture attributes that may critically affect hydraulic and mechanical properties of petroleum reservoirs are unknown. In many siliciclastic petroleum reservoirs, microfractures, with sizes of microns to millimeters, are more common than large fractures and can be sampled effectively even in small volumes of rock (Laubach 1989, Laubach & others 1995). Although some microfractures are due to drilling, core expansion/contraction, core handling, and sample preparation (e.g., Santarelli & Dusseault 1991), many are partly filled with authigenic mineral precipitates and represent a sample of subsurface fracture populations. In quartzose sandstones a widespread type of microfracture contains quartz that is in optical continuity with host-rock quartz. These microfractures are nearly invisible using conventional optical methods but can be imaged with photomultiplier-based cathodoluminescence (scanned CL) (Milliken 1994). Because fractures formed in association with quartz precipitation are prevalent in quartz- cemented siliciclastic petroleum reservoir rocks (Laubach & others 1995), scanned CL imaging of microfractures can potentially yield important information on subsurface fracture populations that have engineering and economic significance.
Cathodoluminescence imaging in scanning electron microscopes (SEMs) reveals subtle compositional and defect variations in silicate minerals more readily than conventional optical cold-cathode luminescence systems (Sipple 1968, Walker & Burley 1991, Milliken 1994). Quartz deposited from aqueous solution at relatively low temperatures generally has less intense CL than does quartz crystallized at high temperatures (e.g., Zinkernagel 1978), so micro fractures filled with dark-luminescent authigenic quartz are apparent where they cross bright- luminescent detrital grains (fig. 1). Under optical microscopy, these microfractures are either invisible or appear as indistinct curviplanar arrays of liquid- and/or gas-filled inclusions and mineral precipitates. In this study, images were produced using Oxford Instruments' photomultiplier-based CL detector Model CL302 installed on JEOL T330A SEM.
