Electrical images in boreholes are becoming increasingly important in interpreting the rock record. In addition to identifying fractures and faults, borehole imaging tools are used in a variety of other applications, such as horizontal drilling; environmental studies; stress-orientation-measurement studies; sequence stratigraphy; and paleotransport, facies, and diagenetic analyses. These tools produce electrical microconductivity images of the well bore, which are interpreted at an interactive graphics workstation, High-resolution (- 2.5-mm) and nearly complete borehole coverage can greatly increase the detail and precision of geological interpretations. Yet, to be fully useful, borehole images should be calibrated with core. This study provides the first comprehensive comparison of carbonate features in cores with a suite of all currently available electrical imaging logs. The Lower Ordovician Ellenburger Group, West Texas, serves as a model of a dolomitized, fractured, karsted, and brecciated carbonate reservoir.
Characteristic reservoir features, including fracture breccia, chaotic breccia, laminated mudstones, grainstones, and bioturbation, are identified both on electrical imaging logs and in cores. Electrical images provide more complete information than do cores in cavernous and highly fractured zones because cores either commonly show no recovery or occur as rubble in these zones, which are the most productive in the Ellenburger reservoir. Borehole imaging therefore not only provides in situ visualization of cavernous porosity, chaotic breccias or conglomerates, and highly fractured intervals, but also provides other key insights into karst stratigraphy.